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Afzal N, du Bois de Dunilac S, Loutit AJ, Shea HO, Ulloa PM, Khamis H, Vickery RM, Wiertlewski M, Redmond SJ, Birznieks I. Role of arm reaching movement kinematics in friction perception at initial contact with smooth surfaces. J Physiol 2024; 602:2089-2106. [PMID: 38544437 DOI: 10.1113/jp286027] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/23/2023] [Accepted: 03/12/2024] [Indexed: 05/04/2024] Open
Abstract
When manipulating objects, humans begin adjusting their grip force to friction within 100 ms of contact. During motor adaptation, subjects become aware of the slipperiness of touched surfaces. Previously, we have demonstrated that humans cannot perceive frictional differences when surfaces are brought in contact with an immobilised finger, but can do so when there is submillimeter lateral displacement or subjects actively make the contact movement. Similarly, in, we investigated how humans perceive friction in the absence of intentional exploratory sliding or rubbing movements, to mimic object manipulation interactions. We used a two-alternative forced-choice paradigm in which subjects had to reach and touch one surface followed by another, and then indicate which felt more slippery. Subjects correctly identified the more slippery surface in 87 ± 8% of cases (mean ± SD; n = 12). Biomechanical analysis of finger pad skin displacement patterns revealed the presence of tiny (<1 mm) localised slips, known to be sufficient to perceive frictional differences. We tested whether these skin movements arise as a result of natural hand reaching kinematics. The task was repeated with the introduction of a hand support, eliminating the hand reaching movement and minimising fingertip movement deviations from a straight path. As a result, our subjects' performance significantly declined (66 ± 12% correct, mean ± SD; n = 12), suggesting that unrestricted reaching movement kinematics and factors such as physiological tremor, play a crucial role in enhancing or enabling friction perception upon initial contact. KEY POINTS: More slippery objects require a stronger grip to prevent them from slipping out of hands. Grip force adjustments to friction driven by tactile sensory signals are largely automatic and do not necessitate cognitive involvement; nevertheless, some associated awareness of grip surface slipperiness under such sensory conditions is present and helps to select a safe and appropriate movement plan. When gripping an object, tactile receptors provide frictional information without intentional rubbing or sliding fingers over the surface. However, we have discovered that submillimeter range lateral displacement might be required to enhance or enable friction sensing. The present study provides evidence that such small lateral movements causing localised partial slips arise and are an inherent part of natural reaching movement kinematics.
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Affiliation(s)
- Naqash Afzal
- School of Biomedical Sciences, UNSW Sydney, Sydney, NSW, Australia
- Neuroscience Research Australia, Sydney, NSW, Australia
- Department of Mechanical Engineering, Khalifa University, Abu Dhabi, United Arab Emirates
- Center for Autonomous Robotic Systems, Khalifa University, Abu Dhabi, United Arab Emirates
| | | | - Alastair J Loutit
- School of Biomedical Sciences, UNSW Sydney, Sydney, NSW, Australia
- Neuroscience Research Australia, Sydney, NSW, Australia
| | - Helen O Shea
- Department of Psychology, University of Limerick, Limerick, Ireland
| | - Pablo Martinez Ulloa
- School of Electrical and Electronic Engineering, University College Dublin, Dublin, Ireland
| | - Heba Khamis
- Graduate School of Biomedical Engineering, UNSW Sydney, Sydney, NSW, Australia
| | - Richard M Vickery
- School of Biomedical Sciences, UNSW Sydney, Sydney, NSW, Australia
- Neuroscience Research Australia, Sydney, NSW, Australia
- Bionics and Biorobotics, Tyree Foundation Institute of Health Engineering, UNSW Sydney, Sydney, NSW, Australia
| | - Michaël Wiertlewski
- Department of Cognitive Robotics, Delft University of Technology, Delft, The Netherlands
| | - Stephen J Redmond
- School of Electrical and Electronic Engineering, University College Dublin, Dublin, Ireland
| | - Ingvars Birznieks
- School of Biomedical Sciences, UNSW Sydney, Sydney, NSW, Australia
- Neuroscience Research Australia, Sydney, NSW, Australia
- Bionics and Biorobotics, Tyree Foundation Institute of Health Engineering, UNSW Sydney, Sydney, NSW, Australia
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2
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Loutit AJ, Wheat HE, Khamis H, Vickery RM, Macefield VG, Birznieks I. How Tactile Afferents in the Human Fingerpad Encode Tangential Torques Associated with Manipulation: Are Monkeys Better than Us? J Neurosci 2023; 43:4033-4046. [PMID: 37142429 PMCID: PMC10254986 DOI: 10.1523/jneurosci.1305-22.2023] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/23/2022] [Revised: 04/14/2023] [Accepted: 04/17/2023] [Indexed: 05/06/2023] Open
Abstract
Dexterous object manipulation depends critically on information about forces normal and tangential to the fingerpads, and also on torque associated with object orientation at grip surfaces. We investigated how torque information is encoded by human tactile afferents in the fingerpads and compared them to 97 afferents recorded in monkeys (n = 3; 2 females) in our previous study. Human data included slowly-adapting Type-II (SA-II) afferents, which are absent in the glabrous skin of monkeys. Torques of different magnitudes (3.5-7.5 mNm) were applied in clockwise and anticlockwise directions to a standard central site on the fingerpads of 34 human subjects (19 females). Torques were superimposed on a 2, 3, or 4 N background normal force. Unitary recordings were made from fast-adapting Type-I (FA-I, n = 39), and slowly-adapting Type-I (SA-I, n = 31) and Type-II (SA-II, n = 13) afferents supplying the fingerpads via microelectrodes inserted into the median nerve. All three afferent types encoded torque magnitude and direction, with torque sensitivity being higher with smaller normal forces. SA-I afferent responses to static torque were inferior to dynamic stimuli in humans, while in monkeys the opposite was true. In humans this might be compensated by the addition of sustained SA-II afferent input, and their capacity to increase or decrease firing rates with direction of rotation. We conclude that the discrimination capacity of individual afferents of each type was inferior in humans than monkeys which could be because of differences in fingertip tissue compliance and skin friction.SIGNIFICANCE STATEMENT We investigated how individual human tactile nerve fibers encode rotational forces (torques) and compared them to their monkey counterparts. Human hands, but not monkey hands, are innervated by a tactile neuron type (SA-II afferents) specialized to encode directional skin strain yet, so far, torque encoding has only been studied in monkeys. We find that human SA-I afferents were generally less sensitive and less able to discriminate torque magnitude and direction than their monkey counterparts, especially during the static phase of torque loading. However, this shortfall in humans could be compensated by SA-II afferent input. This indicates that variation in afferent types might complement each other signaling different stimulus features possibly providing computational advantage to discriminate stimuli.
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Affiliation(s)
- Alastair J Loutit
- Neuroscience Research Australia, Sydney, New South Wales 2031, Australia
- School of Biomedical Sciences, UNSW Sydney, Sydney, New South Wales 2031, Australia
| | - Heather E Wheat
- Department of Anatomy and Physiology, University of Melbourne, Melbourne, Victoria 3052, Australia
| | - Heba Khamis
- Neuroscience Research Australia, Sydney, New South Wales 2031, Australia
- Graduate School of Biomedical Engineering, UNSW Sydney, Sydney, New South Wales 2031, Australia
| | - Richard M Vickery
- Neuroscience Research Australia, Sydney, New South Wales 2031, Australia
- School of Biomedical Sciences, UNSW Sydney, Sydney, New South Wales 2031, Australia
- Bionics and Bio-robotics, Tyree Foundation Institute of Health Engineering, UNSW Sydney, Sydney, New South Wales 2031, Australia
| | - Vaughan G Macefield
- Baker Heart and Diabetes Institute, Melbourne, Victoria 3004, Australia
- Department of Anatomy and Physiology, University of Melbourne, Melbourne, Victoria 3052, Australia
- Department of Neuroscience, Monash University, Melbourne, Victoria 3052, Australia
| | - Ingvars Birznieks
- Neuroscience Research Australia, Sydney, New South Wales 2031, Australia
- School of Biomedical Sciences, UNSW Sydney, Sydney, New South Wales 2031, Australia
- Bionics and Bio-robotics, Tyree Foundation Institute of Health Engineering, UNSW Sydney, Sydney, New South Wales 2031, Australia
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Sharma D, Ng KKW, Birznieks I, Vickery RM. Perceived tactile intensity at a fixed primary afferent spike rate varies with the temporal pattern of spikes. J Neurophysiol 2022; 128:1074-1084. [DOI: 10.1152/jn.00284.2022] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The perceived intensity of a vibrotactile stimulus is thought to depend on single neuron firing rates (rate coding) and the number of active afferents (population coding). Unaddressed until now is whether the temporal relation of individual spikes also conveys information about tactile intensity. We used cutaneous electro-tactile stimulation to investigate how the temporal structure of a fixed number of spikes in a 1 s train influenced the perception of intensity. Four mean spike rates spanning the flutter and vibratory hum range (36 Hz, 60 Hz; 120 Hz, 180 Hz) were tested, with spikes grouped into a regular pattern, or bursts of 2-6 spikes spaced 3 ms apart. To link a putative neural code to perception, perceived intensity was assessed in sixteen human participants (aged 20-45; 4 females) using the psychophysical paradigm of magnitude estimation. Compound sensory nerve action potentials were recorded to assess any stimulus variation in afferent recruitment. The temporal structuring of a fixed number of spikes into periodic bursts of multiple spikes altered perceived intensity as a function of burst spike count. The largest increase was seen at 36 Hz, where the bursts of 6 spikes were rated 2.1x stronger than the regularly spaced spikes (95% CI: 1.9-2.3). The true increase is likely larger as temporal structuring of spikes into bursts had some negative effect on afferent recruitment. We conclude that the perceived intensity can be modulated by changing temporal features of afferent discharge even when normalized for the number of recruited afferents.
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Affiliation(s)
- Deepak Sharma
- School of Biomedical Sciences, UNSW Sydney,Sydney, Australia
- Neuroscience Research Australia, Sydney, Australia
| | - Kevin K. W. Ng
- Center for Social and Affective Neuroscience, Department of Biomedical and Clinical Sciences, Linköping University, Linköping, Sweden
| | - Ingvars Birznieks
- School of Biomedical Sciences, UNSW Sydney,Sydney, Australia
- Neuroscience Research Australia, Sydney, Australia
- Bionics and Bio‐robotics, Tyree Foundation Institute of Health Engineering, UNSW Sydney, Sydney, Australia
| | - Richard Martin Vickery
- School of Biomedical Sciences, UNSW Sydney,Sydney, Australia
- Neuroscience Research Australia, Sydney, Australia
- Bionics and Bio‐robotics, Tyree Foundation Institute of Health Engineering, UNSW Sydney, Sydney, Australia
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Sharma D, Ng KKW, Birznieks I, Vickery RM. Auditory clicks elicit equivalent temporal frequency perception to tactile pulses: A cross-modal psychophysical study. Front Neurosci 2022; 16:1006185. [PMID: 36161171 PMCID: PMC9500524 DOI: 10.3389/fnins.2022.1006185] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/29/2022] [Accepted: 08/24/2022] [Indexed: 12/02/2022] Open
Abstract
Both hearing and touch are sensitive to the frequency of mechanical oscillations—sound waves and tactile vibrations, respectively. The mounting evidence of parallels in temporal frequency processing between the two sensory systems led us to directly address the question of perceptual frequency equivalence between touch and hearing using stimuli of simple and more complex temporal features. In a cross-modal psychophysical paradigm, subjects compared the perceived frequency of pulsatile mechanical vibrations to that elicited by pulsatile acoustic (click) trains, and vice versa. Non-invasive pulsatile stimulation designed to excite a fixed population of afferents was used to induce desired temporal spike trains at frequencies spanning flutter up to vibratory hum (>50 Hz). The cross-modal perceived frequency for regular test pulse trains of either modality was a close match to the presented stimulus physical frequency up to 100 Hz. We then tested whether the recently discovered “burst gap” temporal code for frequency, that is shared by the two senses, renders an equivalent cross-modal frequency perception. When subjects compared trains comprising pairs of pulses (bursts) in one modality against regular trains in the other, the cross-sensory equivalent perceptual frequency best corresponded to the silent interval between the successive bursts in both auditory and tactile test stimuli. These findings suggest that identical acoustic and vibrotactile pulse trains, regardless of pattern, elicit equivalent frequencies, and imply analogous temporal frequency computation strategies in both modalities. This perceptual correspondence raises the possibility of employing a cross-modal comparison as a robust standard to overcome the prevailing methodological limitations in psychophysical investigations and strongly encourages cross-modal approaches for transmitting sensory information such as translating pitch into a similar pattern of vibration on the skin.
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Affiliation(s)
- Deepak Sharma
- School of Biomedical Sciences, The University of New South Wales (UNSW Sydney), Sydney, NSW, Australia
- Neuroscience Research Australia, Sydney, NSW, Australia
- *Correspondence: Deepak Sharma,
| | - Kevin K. W. Ng
- Center for Social and Affective Neuroscience, Department of Biomedical and Clinical Sciences, Linköping University, Linköping, Sweden
| | - Ingvars Birznieks
- School of Biomedical Sciences, The University of New South Wales (UNSW Sydney), Sydney, NSW, Australia
- Neuroscience Research Australia, Sydney, NSW, Australia
- Bionics and Bio-Robotics, Tyree Foundation Institute of Health Engineering, The University of New South Wales (UNSW Sydney), Sydney, NSW, Australia
| | - Richard M. Vickery
- School of Biomedical Sciences, The University of New South Wales (UNSW Sydney), Sydney, NSW, Australia
- Neuroscience Research Australia, Sydney, NSW, Australia
- Bionics and Bio-Robotics, Tyree Foundation Institute of Health Engineering, The University of New South Wales (UNSW Sydney), Sydney, NSW, Australia
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Afzal N, Stubbs E, Khamis H, Loutit AJ, Redmond SJ, Vickery RM, Wiertlewski M, Birznieks I. Submillimeter Lateral Displacement Enables Friction Sensing and Awareness of Surface Slipperiness. IEEE Trans Haptics 2022; 15:20-25. [PMID: 34982692 DOI: 10.1109/toh.2021.3139890] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/14/2023]
Abstract
Human tactile perception and motor control rely on the frictional estimates that stem from the deformation of the skin and slip events. However, it is not clear how exactly these mechanical events relate to the perception of friction. This study aims to quantify how minor lateral displacement and speed enables subjects to feel frictional differences. In a 2-alternative forced-choice protocol, an ultrasonic friction-reduction device was brought in contact perpendicular to the skin surface of an immobilized index finger; after reaching 1N normal force, the plate was moved laterally. A combination of four displacement magnitudes (0.2, 0.5, 1.2 and 2 mm), two levels of friction (high, low) and three displacement speeds (1, 5 and 10 mm/s) were tested. We found that the perception of frictional difference was enabled by submillimeter range lateral displacement. Friction discrimination thresholds were reached with lateral displacements ranging from 0.2 to 0.5 mm and surprisingly speed had only a marginal effect. These results demonstrate that partial slips are sufficient to cause awareness of surface slipperiness. These quantitative data are crucial for designing haptic devices that render slipperiness. The results also show the importance of subtle lateral finger movements present during dexterous manipulation tasks.
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Ng KKW, Tee X, Vickery RM, Birznieks I. The Relationship Between Tactile Intensity Perception and Afferent Spike Count is Moderated by a Function of Frequency. IEEE Trans Haptics 2022; 15:14-19. [PMID: 34990370 DOI: 10.1109/toh.2022.3140877] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/14/2023]
Abstract
It has been suggested that tactile intensity perception can be explained by a linear function of spike rate weighted by afferent type. Other than relying on mathematical models, verifying this experimentally is difficult due to the frequency tuning of different afferent types and changes in population recruitment patterns with vibrotactile frequency. To overcome these complexities, we used pulsatile mechanical stimuli which activate the same afferent population regardless of the repetition rate (frequency), generating one action potential per pulse. We used trains of different frequencies (20-200 Hz) to investigate perceived intensity. Subjects' magnitude ratings increased with pulse rate up to ∼100 Hz and plateaued beyond this frequency. This was true regardless of pulse amplitude, from small pulses that exclusively activated Pacinian (PC) afferents, to pulses large enough to activate other afferents including slowly adapting. Electrical stimulation, which activates afferents indiscriminately, plateaued at a similar frequency, although not in all subjects. As the plateauing did not depend on indentation magnitude and hence on afferent weights, we propose that the contribution of spike count to intensity perception is weighted by a function of frequency. This may explain why fine textures evoking high frequency vibrations of a small magnitude do not feel disproportionally intense.
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Jack BN, Chilver MR, Vickery RM, Birznieks I, Krstanoska-Blazeska K, Whitford TJ, Griffiths O. Movement Planning Determines Sensory Suppression: An Event-related Potential Study. J Cogn Neurosci 2021; 33:2427-2439. [PMID: 34424986 DOI: 10.1162/jocn_a_01747] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/04/2022]
Abstract
Sensory suppression refers to the phenomenon that sensory input generated by our own actions, such as moving a finger to press a button to hear a tone, elicits smaller neural responses than sensory input generated by external agents. This observation is usually explained via the internal forward model in which an efference copy of the motor command is used to compute a corollary discharge, which acts to suppress sensory input. However, because moving a finger to press a button is accompanied by neural processes involved in preparing and performing the action, it is unclear whether sensory suppression is the result of movement planning, movement execution, or both. To investigate this, in two experiments, we compared ERPs to self-generated tones that were produced by voluntary, semivoluntary, or involuntary button-presses, with externally generated tones that were produced by a computer. In Experiment 1, the semivoluntary and involuntary button-presses were initiated by the participant or experimenter, respectively, by electrically stimulating the median nerve in the participant's forearm, and in Experiment 2, by applying manual force to the participant's finger. We found that tones produced by voluntary button-presses elicited a smaller N1 component of the ERP than externally generated tones. This is known as N1-suppression. However, tones produced by semivoluntary and involuntary button-presses did not yield significant N1-suppression. We also found that the magnitude of N1-suppression linearly decreased across the voluntary, semivoluntary, and involuntary conditions. These results suggest that movement planning is a necessary condition for producing sensory suppression. We conclude that the most parsimonious account of sensory suppression is the internal forward model.
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Affiliation(s)
- Bradley N Jack
- University of New South Wales Sydney, Australia.,Australian National University, Canberra
| | - Miranda R Chilver
- University of New South Wales Sydney, Australia.,Neuroscience Research Australia, Sydney
| | - Richard M Vickery
- University of New South Wales Sydney, Australia.,Neuroscience Research Australia, Sydney
| | - Ingvars Birznieks
- University of New South Wales Sydney, Australia.,Neuroscience Research Australia, Sydney
| | | | | | - Oren Griffiths
- University of New South Wales Sydney, Australia.,Flinders University, Adelaide, Australia
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Khamis H, Afzal HMN, Sanchez J, Vickery R, Wiertlewski M, Redmond SJ, Birznieks I. Friction sensing mechanisms for perception and motor control: passive touch without sliding may not provide perceivable frictional information. J Neurophysiol 2021; 125:809-823. [PMID: 33439786 DOI: 10.1152/jn.00504.2020] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Perception of the frictional properties of a surface contributes to the multidimensional experience of exploring various materials; we slide our fingers over a surface to feel it. In contrast, during object manipulation, we grip objects without such intended exploratory movements. Given that we are aware of the slipperiness of objects or tools that are held in the hand, we investigated whether the initial contact between the fingertip skin and the surface of the object is sufficient to provide this consciously perceived frictional information. Using a two-alternative forced-choice protocol, we examined human capacity to detect frictional differences using touch, when two otherwise structurally identical surfaces were brought in contact with the immobilized finger perpendicularly or under an angle (20° or 30°) to the skin surface (passive touch). An ultrasonic friction reduction device was used to generate three different frictions over each of three flat surfaces with different surface structure: 1) smooth glass, 2) textured surface with dome-shaped features, and 3) surface with sharp asperities (sandpaper). Participants (n = 12) could not reliably indicate which of the two surfaces was more slippery under any of these conditions. In contrast, when slip was induced by moving the surface laterally by a total of 5 mm (passive slip), participants could clearly perceive frictional differences. Thus making contact with the surface, even with moderate tangential forces, was not enough to perceive frictional differences, instead conscious perception required a sufficient size slip.NEW & NOTEWORTHY This study contributes to understanding how frictional information is obtained and used by the brain. When the skin is contacting surfaces of identical topography but varying frictional properties, the deformation pattern is different; however, available sensory cues did not get translated into perception of frictional properties unless a sufficiently large lateral movement was present. These neurophysiological findings may inform how to design and operate haptic devices relying on friction modulation principles.
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Affiliation(s)
- Heba Khamis
- Graduate School of Biomedical Engineering, University of New South Wales (UNSW) Sydney, Sydney, New South Wales, Australia.,Neuroscience Research Australia, Randwick, New South Wales, Australia
| | - Hafiz Malik Naqash Afzal
- Neuroscience Research Australia, Randwick, New South Wales, Australia.,School of Medical Sciences, University of New South Wales (UNSW) Sydney, Sydney, New South Wales, Australia
| | - Jennifer Sanchez
- School of Medical Sciences, University of New South Wales (UNSW) Sydney, Sydney, New South Wales, Australia
| | - Richard Vickery
- School of Medical Sciences, University of New South Wales (UNSW) Sydney, Sydney, New South Wales, Australia
| | - Michaël Wiertlewski
- Cognitive Robotics Department, Delft University of Technology, Delft, The Netherlands
| | - Stephen J Redmond
- Graduate School of Biomedical Engineering, University of New South Wales (UNSW) Sydney, Sydney, New South Wales, Australia.,School of Electrical and Electronic Engineering, University College Dublin, Belfield, Ireland
| | - Ingvars Birznieks
- Neuroscience Research Australia, Randwick, New South Wales, Australia.,School of Medical Sciences, University of New South Wales (UNSW) Sydney, Sydney, New South Wales, Australia
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Ng KKW, Snow IN, Birznieks I, Vickery RM. Burst gap code predictions for tactile frequency are valid across the range of perceived frequencies attributed to two distinct tactile channels. J Neurophysiol 2021; 125:687-692. [PMID: 33439792 DOI: 10.1152/jn.00662.2020] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Perceived frequency of vibrotactile stimuli can be divided into two distinctive cutaneous sensations-flutter (<60 Hz) and vibratory hum (>60 Hz), mediated by two different tactile afferent types [fast adapting type I (FA1) and fast adapting type II (FA2), respectively]. We recently demonstrated a novel form of neural coding in the human tactile system, where frequency perception of stimulus pulses grouped into periodic bursts in the flutter range depended on the duration of the silent gap between bursts, rather than the periodicity or mean impulse rate. Here, we investigated whether this interburst interval could also explain the perceived frequency of electrocutaneous pulse patterns delivered at frequencies above the flutter range. At stimulus rates of 50 to 190 pulses/s, the burst gap model correctly predicted the perceived frequency. This shows that the burst gap code represents a general coding strategy that spans the range of frequencies traditionally attributed to two different tactile channels.NEW & NOTEWORTHY We present evidence for a generalized frequency processing strategy on tactile afferent inputs that is shared across a broad range of frequencies extending beyond the flutter range, supporting the notion that spike timing has an important role in shaping tactile perception.
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Affiliation(s)
- Kevin K W Ng
- School of Medical Sciences, UNSW Sydney, Sydney, Australia.,Neuroscience Research Australia, Sydney, Australia
| | - Ian N Snow
- School of Medical Sciences, UNSW Sydney, Sydney, Australia.,Neuroscience Research Australia, Sydney, Australia
| | - Ingvars Birznieks
- School of Medical Sciences, UNSW Sydney, Sydney, Australia.,Neuroscience Research Australia, Sydney, Australia
| | - Richard M Vickery
- School of Medical Sciences, UNSW Sydney, Sydney, Australia.,Neuroscience Research Australia, Sydney, Australia
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Ng KKW, Olausson C, Vickery RM, Birznieks I. Temporal patterns in electrical nerve stimulation: Burst gap code shapes tactile frequency perception. PLoS One 2020; 15:e0237440. [PMID: 32790784 PMCID: PMC7425972 DOI: 10.1371/journal.pone.0237440] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/15/2019] [Accepted: 07/27/2020] [Indexed: 12/25/2022] Open
Abstract
We have previously described a novel temporal encoding mechanism in the somatosensory system, where mechanical pulses grouped into periodic bursts create a perceived tactile frequency based on the duration of the silent gap between bursts, rather than the mean rate or the periodicity. This coding strategy may offer new opportunities for transmitting information to the brain using various sensory neural prostheses and haptic interfaces. However, it was not known whether the same coding mechanisms apply when using electrical stimulation, which recruits a different spectrum of afferents. Here, we demonstrate that the predictions of the burst gap coding model for frequency perception apply to burst stimuli delivered with electrical pulses, re-emphasising the importance of the temporal structure of spike patterns in neural processing and perception of tactile stimuli. Reciprocally, the electrical stimulation data confirm that the results observed with mechanical stimulation do indeed depend on neural processing mechanisms in the central nervous system, and are not due to skin mechanical factors and resulting patterns of afferent activation.
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Affiliation(s)
- Kevin K. W. Ng
- School of Medical Sciences, UNSW Sydney, Sydney, Australia
- Neuroscience Research Australia, Sydney, Australia
- * E-mail:
| | - Christoffer Olausson
- Neuroscience Research Australia, Sydney, Australia
- Faculty of Medicine and Health Sciences, Linköping University, Linköping, Sweden
| | - Richard M. Vickery
- School of Medical Sciences, UNSW Sydney, Sydney, Australia
- Neuroscience Research Australia, Sydney, Australia
| | - Ingvars Birznieks
- School of Medical Sciences, UNSW Sydney, Sydney, Australia
- Neuroscience Research Australia, Sydney, Australia
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Vickery RM, Ng KKW, Potas JR, Shivdasani MN, McIntyre S, Nagi SS, Birznieks I. Tapping Into the Language of Touch: Using Non-invasive Stimulation to Specify Tactile Afferent Firing Patterns. Front Neurosci 2020; 14:500. [PMID: 32508581 PMCID: PMC7248323 DOI: 10.3389/fnins.2020.00500] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/30/2019] [Accepted: 04/21/2020] [Indexed: 12/12/2022] Open
Abstract
The temporal pattern of action potentials can convey rich information in a variety of sensory systems. We describe a new non-invasive technique that enables precise, reliable generation of action potential patterns in tactile peripheral afferent neurons by brief taps on the skin. Using this technique, we demonstrate sophisticated coding of temporal information in the somatosensory system, that shows that perceived vibration frequency is not encoded in peripheral afferents as was expected by either their firing rate or the underlying periodicity of the stimulus. Instead, a burst gap or silent gap between trains of action potentials conveys frequency information. This opens the possibility of new encoding strategies that could be deployed to convey sensory information using mechanical or electrical stimulation in neural prostheses and brain-machine interfaces, and may extend to senses beyond artificial encoding of aspects of touch. We argue that a focus on appropriate use of effective temporal coding offers more prospects for rapid improvement in the function of these interfaces than attempts to scale-up existing devices.
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Affiliation(s)
- Richard M. Vickery
- School of Medical Sciences, UNSW Sydney, Sydney, NSW, Australia
- Neuroscience Research Australia, Sydney, NSW, Australia
| | - Kevin K. W. Ng
- School of Medical Sciences, UNSW Sydney, Sydney, NSW, Australia
- Neuroscience Research Australia, Sydney, NSW, Australia
| | - Jason R. Potas
- School of Medical Sciences, UNSW Sydney, Sydney, NSW, Australia
| | - Mohit N. Shivdasani
- Graduate School of Biomedical Engineering, UNSW Sydney, Sydney, NSW, Australia
| | - Sarah McIntyre
- Center for Social and Affective Neuroscience, Department of Biomedical and Clinical Sciences, Linköping University, Linköping, Sweden
| | - Saad S. Nagi
- Center for Social and Affective Neuroscience, Department of Biomedical and Clinical Sciences, Linköping University, Linköping, Sweden
| | - Ingvars Birznieks
- School of Medical Sciences, UNSW Sydney, Sydney, NSW, Australia
- Neuroscience Research Australia, Sydney, NSW, Australia
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Birznieks I, McIntyre S, Nilsson HM, Nagi SS, Macefield VG, Mahns DA, Vickery RM. Tactile sensory channels over-ruled by frequency decoding system that utilizes spike pattern regardless of receptor type. eLife 2019; 8:46510. [PMID: 31383258 PMCID: PMC6684274 DOI: 10.7554/elife.46510] [Citation(s) in RCA: 19] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/01/2019] [Accepted: 07/19/2019] [Indexed: 12/16/2022] Open
Abstract
The established view is that vibrotactile stimuli evoke two qualitatively distinctive cutaneous sensations, flutter (frequencies < 60 Hz) and vibratory hum (frequencies > 60 Hz), subserved by two distinct receptor types (Meissner’s and Pacinian corpuscle, respectively), which may engage different neural processing pathways or channels and fulfil quite different biological roles. In psychological and physiological literature, those two systems have been labelled as Pacinian and non-Pacinian channels. However, we present evidence that low-frequency spike trains in Pacinian afferents can readily induce a vibratory percept with the same low frequency attributes as sinusoidal stimuli of the same frequency, thus demonstrating a universal frequency decoding system. We achieved this using brief low-amplitude pulsatile mechanical stimuli to selectively activate Pacinian afferents. This indicates that spiking pattern, regardless of receptor type, determines vibrotactile frequency perception. This mechanism may underlie the constancy of vibrotactile frequency perception across different skin regions innervated by distinct afferent types.
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Affiliation(s)
- Ingvars Birznieks
- School of Medical Sciences, Faculty of Medicine, UNSW Sydney, Sydney, Australia.,Neuroscience Research Australia, Sydney, Australia.,Biomedical Engineering and Neuroscience, MARCS Institute, Western Sydney University, Sydney, Australia
| | - Sarah McIntyre
- Neuroscience Research Australia, Sydney, Australia.,Biomedical Engineering and Neuroscience, MARCS Institute, Western Sydney University, Sydney, Australia.,Linköping University, Linköping, Sweden
| | - Hanna Maria Nilsson
- Neuroscience Research Australia, Sydney, Australia.,Linköping University, Linköping, Sweden
| | - Saad S Nagi
- Linköping University, Linköping, Sweden.,School of Medicine, Western Sydney University, Sydney, Australia
| | - Vaughan G Macefield
- Neuroscience Research Australia, Sydney, Australia.,School of Medicine, Western Sydney University, Sydney, Australia.,The Baker Heart and Diabetes Institute, Melbourne, Australia
| | - David A Mahns
- School of Medicine, Western Sydney University, Sydney, Australia
| | - Richard M Vickery
- School of Medical Sciences, Faculty of Medicine, UNSW Sydney, Sydney, Australia.,Neuroscience Research Australia, Sydney, Australia
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13
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Loutit AJ, Shivdasani MN, Maddess T, Redmond SJ, Morley JW, Stuart GJ, Birznieks I, Vickery RM, Potas JR. Peripheral Nerve Activation Evokes Machine-Learnable Signals in the Dorsal Column Nuclei. Front Syst Neurosci 2019; 13:11. [PMID: 30983977 PMCID: PMC6448039 DOI: 10.3389/fnsys.2019.00011] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/03/2019] [Accepted: 02/26/2019] [Indexed: 02/04/2023] Open
Abstract
The brainstem dorsal column nuclei (DCN) are essential to inform the brain of tactile and proprioceptive events experienced by the body. However, little is known about how ascending somatosensory information is represented in the DCN. Our objective was to investigate the usefulness of high-frequency (HF) and low-frequency (LF) DCN signal features (SFs) in predicting the nerve from which signals were evoked. We also aimed to explore the robustness of DCN SFs and map their relative information content across the brainstem surface. DCN surface potentials were recorded from urethane-anesthetized Wistar rats during sural and peroneal nerve electrical stimulation. Five salient SFs were extracted from each recording electrode of a seven-electrode array. We used a machine learning approach to quantify and rank information content contained within DCN surface-potential signals following peripheral nerve activation. Machine-learning of SF and electrode position combinations was quantified to determine a hierarchy of information importance for resolving the peripheral origin of nerve activation. A supervised back-propagation artificial neural network (ANN) could predict the nerve from which a response was evoked with up to 96.8 ± 0.8% accuracy. Guided by feature-learnability, we maintained high prediction accuracy after reducing ANN algorithm inputs from 35 (5 SFs from 7 electrodes) to 6 (4 SFs from one electrode and 2 SFs from a second electrode). When the number of input features were reduced, the best performing input combinations included HF and LF features. Feature-learnability also revealed that signals recorded from the same midline electrode can be accurately classified when evoked from bilateral nerve pairs, suggesting DCN surface activity asymmetry. Here we demonstrate a novel method for mapping the information content of signal patterns across the DCN surface and show that DCN SFs are robust across a population. Finally, we also show that the DCN is functionally asymmetrically organized, which challenges our current understanding of somatotopic symmetry across the midline at sub-cortical levels.
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Affiliation(s)
- Alastair J Loutit
- School of Medical Sciences, UNSW Sydney, Sydney, NSW, Australia.,Eccles Institute of Neuroscience, The John Curtin School of Medical Research, Australian National University, Canberra, ACT, Australia
| | - Mohit N Shivdasani
- Graduate School of Biomedical Engineering, UNSW Sydney, Sydney, NSW, Australia.,Bionics Institute, East Melbourne, VIC, Australia
| | - Ted Maddess
- Eccles Institute of Neuroscience, The John Curtin School of Medical Research, Australian National University, Canberra, ACT, Australia
| | - Stephen J Redmond
- Graduate School of Biomedical Engineering, UNSW Sydney, Sydney, NSW, Australia.,UCD School of Electrical and Electronic Engineering, University College Dublin, Dublin, Ireland.,UCD Centre for Biomedical Engineering, University College Dublin, Dublin, Ireland
| | - John W Morley
- School of Medical Sciences, UNSW Sydney, Sydney, NSW, Australia.,School of Medicine, Western Sydney University, Sydney, NSW, Australia
| | - Greg J Stuart
- Eccles Institute of Neuroscience, The John Curtin School of Medical Research, Australian National University, Canberra, ACT, Australia
| | | | | | - Jason R Potas
- School of Medical Sciences, UNSW Sydney, Sydney, NSW, Australia.,Eccles Institute of Neuroscience, The John Curtin School of Medical Research, Australian National University, Canberra, ACT, Australia
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14
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Smith LJ, Macefield VG, Birznieks I, Burton AR. Effects of tonic muscle pain on fusimotor control of human muscle spindles during isometric ankle dorsiflexion. J Neurophysiol 2019; 121:1143-1149. [DOI: 10.1152/jn.00862.2018] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023] Open
Abstract
Studies on anesthetized animals have revealed that nociceptors can excite fusimotor neurons and thereby change the sensitivity of muscle spindles to stretch; such nociceptive reflexes have been suggested to underlie the mechanisms that lead to chronic musculoskeletal pain syndromes. However, the validity of the “vicious cycle” hypothesis in humans has yielded results contrasting with those found in animals. Given that spindle firing rates are much lower in humans than in animals, it is possible that some of the discrepancies between human experimental data and those obtained in animals could be explained by differences in background fusimotor drive when the leg muscles are relaxed. We examined the effects of tonic muscle pain during voluntary contractions of the ankle dorsiflexors. Unitary recordings were obtained from 10 fusimotor-driven muscle spindle afferents (6 primary, 4 secondary) supplying the ankle dorsiflexors via a microelectrode inserted percutaneously into the common peroneal nerve. A series of 1-min weak contractions was performed at rest and during 1 h of muscle pain induced by intramuscular infusion of 5% hypertonic saline into the tibialis anterior muscle. We did not observe any statistically significant increases in muscle spindle firing rates of six afferents followed during tonic muscle pain, although discharge variability increased slightly. Furthermore, a participant’s capacity to maintain a constant level of force, while relying on proprioceptive feedback in the absence of visual feedback, was not compromised during pain. We conclude that nociceptive inputs from contracting muscle do not excite fusimotor neurons during voluntary isometric contractions in humans. NEW & NOTEWORTHY Data obtained in the cat have shown that muscle pain causes a marked increase in the firing of muscle spindles, attributed to a nociceptor-driven fusimotor reflex. However, our studies of muscle spindles in relaxed leg muscles failed to find any effect on spindle discharge. Here we showed that experimental muscle pain failed to increase the firing of muscle spindle afferents during weak voluntary contractions, when fusimotor drive sufficient to increase their firing is present.
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Affiliation(s)
- Lyndon J. Smith
- School of Medicine, Western Sydney University, Sydney, New South Wales, Australia
| | - Vaughan G. Macefield
- School of Medicine, Western Sydney University, Sydney, New South Wales, Australia
- Neuroscience Research Australia, Sydney, New South Wales, Australia
- Baker Heart and Diabetes Institute, Melbourne, Victoria, Australia
| | - Ingvars Birznieks
- Neuroscience Research Australia, Sydney, New South Wales, Australia
- School of Medical Sciences, University of New South Wales, Sydney, New South Wales, Australia
- School of Science and Health, Western Sydney University, Sydney, New South Wales, Australia
| | - Alexander R. Burton
- School of Medicine, Western Sydney University, Sydney, New South Wales, Australia
- Neuroscience Research Australia, Sydney, New South Wales, Australia
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15
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Ng KKW, Birznieks I, Tse ITH, Andersen J, Nilsson S, Vickery RM. Perceived Frequency of Aperiodic Vibrotactile Stimuli Depends on Temporal Encoding. Haptics: Science, Technology, and Applications 2018. [DOI: 10.1007/978-3-319-93445-7_18] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
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16
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Steel KA, Mudie K, Sandoval R, Anderson D, Dogramaci S, Rehmanjan M, Birznieks I. Can Video Self-Modeling Improve Affected Limb Reach and Grasp Ability in Stroke Patients? J Mot Behav 2017; 50:117-126. [DOI: 10.1080/00222895.2017.1306480] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/19/2022]
Affiliation(s)
- Kylie Ann Steel
- School of Science and Health, Western Sydney University, Australia
- Brain, Behavior and Development, The MARCS Institute, Sydney, Australia
| | - Kurt Mudie
- School of Science and Health, Western Sydney University, Australia
- Institute of Sport, Exercise and Active Living (ISEAL), Victoria University, Melbourne, Australia
| | - Remi Sandoval
- School of Science and Health, Western Sydney University, Australia
| | - David Anderson
- Department of Kinesiology, San Francisco State University, California
| | - Sera Dogramaci
- New South Wales Institute of Sport, Sydney Markets, Australia
| | | | - Ingvars Birznieks
- School of Science and Health, Western Sydney University, Australia
- School of Medical Sciences, University of New South Wales, Sydney, Australia
- Neuroscience Research Australia, Randwick, Australia
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17
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Birznieks I, Vickery RM. Spike Timing Matters in Novel Neuronal Code Involved in Vibrotactile Frequency Perception. Curr Biol 2017; 27:1485-1490.e2. [PMID: 28479322 DOI: 10.1016/j.cub.2017.04.011] [Citation(s) in RCA: 26] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/18/2017] [Revised: 03/09/2017] [Accepted: 04/10/2017] [Indexed: 11/16/2022]
Abstract
Skin vibrations sensed by tactile receptors contribute significantly to the perception of object properties during tactile exploration [1-4] and to sensorimotor control during object manipulation [5]. Sustained low-frequency skin vibration (<60 Hz) evokes a distinct tactile sensation referred to as flutter whose frequency can be clearly perceived [6]. How afferent spiking activity translates into the perception of frequency is still unknown. Measures based on mean spike rates of neurons in the primary somatosensory cortex are sufficient to explain performance in some frequency discrimination tasks [7-11]; however, there is emerging evidence that stimuli can be distinguished based also on temporal features of neural activity [12, 13]. Our study's advance is to demonstrate that temporal features are fundamental for vibrotactile frequency perception. Pulsatile mechanical stimuli were used to elicit specified temporal spike train patterns in tactile afferents, and subsequently psychophysical methods were employed to characterize human frequency perception. Remarkably, the most salient temporal feature determining vibrotactile frequency was not the underlying periodicity but, rather, the duration of the silent gap between successive bursts of neural activity. This burst gap code for frequency represents a previously unknown form of neural coding in the tactile sensory system, which parallels auditory pitch perception mechanisms based on purely temporal information where longer inter-pulse intervals receive higher perceptual weights than short intervals [14]. Our study also demonstrates that human perception of stimuli can be determined exclusively by temporal features of spike trains independent of the mean spike rate and without contribution from population response factors.
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Affiliation(s)
- Ingvars Birznieks
- School of Medical Sciences, Faculty of Medicine, UNSW Sydney, Sydney, NSW 2052, Australia; Neuroscience Research Australia, Barker Street, Randwick, NSW 2031, Australia.
| | - Richard M Vickery
- School of Medical Sciences, Faculty of Medicine, UNSW Sydney, Sydney, NSW 2052, Australia; Neuroscience Research Australia, Barker Street, Randwick, NSW 2031, Australia
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18
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Day J, Bent LR, Birznieks I, Macefield VG, Cresswell AG. Muscle spindles in human tibialis anterior encode muscle fascicle length changes. J Neurophysiol 2017; 117:1489-1498. [PMID: 28077660 DOI: 10.1152/jn.00374.2016] [Citation(s) in RCA: 28] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/16/2016] [Revised: 12/23/2016] [Accepted: 01/05/2017] [Indexed: 11/22/2022] Open
Abstract
Muscle spindles provide exquisitely sensitive proprioceptive information regarding joint position and movement. Through passively driven length changes in the muscle-tendon unit (MTU), muscle spindles detect joint rotations because of their in-parallel mechanical linkage to muscle fascicles. In human microneurography studies, muscle fascicles are assumed to follow the MTU and, as such, fascicle length is not measured in such studies. However, under certain mechanical conditions, compliant structures can act to decouple the fascicles, and, therefore, the spindles, from the MTU. Such decoupling may reduce the fidelity by which muscle spindles encode joint position and movement. The aim of the present study was to measure, for the first time, both the changes in firing of single muscle spindle afferents and changes in muscle fascicle length in vivo from the tibialis anterior muscle (TA) during passive rotations about the ankle. Unitary recordings were made from 15 muscle spindle afferents supplying TA via a microelectrode inserted into the common peroneal nerve. Ultrasonography was used to measure the length of an individual fascicle of TA. We saw a strong correlation between fascicle length and firing rate during passive ankle rotations of varying rates (0.1-0.5 Hz) and amplitudes (1-9°). In particular, we saw responses observed at relatively small changes in muscle length that highlight the sensitivity of the TA muscle to small length changes. This study is the first to measure spindle firing and fascicle dynamics in vivo and provides an experimental basis for further understanding the link between fascicle length, MTU length, and spindle firing patterns.NEW & NOTEWORTHY Muscle spindles are exquisitely sensitive to changes in muscle length, but recordings from human muscle spindle afferents are usually correlated with joint angle rather than muscle fascicle length. In this study, we monitored both muscle fascicle length and spindle firing from the human tibialis anterior muscle in vivo. Our findings are the first to measure these signals in vivo and provide an experimental basis for exploring this link further.
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Affiliation(s)
- James Day
- School of Human Movement and Nutrition Sciences, University of Queensland, St. Lucia, Queensland, Australia
| | - Leah R Bent
- Department of Human Health and Nutritional Sciences, University of Guelph, Guelph, Ontario, Canada
| | - Ingvars Birznieks
- School of Medical Sciences, University of New South Wales, Sydney, New South Wales, Australia.,School of Science and Health, Western Sydney University, Sydney, Australia.,Neuroscience Research Australia, Sydney, Australia
| | - Vaughan G Macefield
- School of Medicine, Western Sydney University, Sydney, Australia; and.,Neuroscience Research Australia, Sydney, Australia
| | - Andrew G Cresswell
- School of Human Movement and Nutrition Sciences, University of Queensland, St. Lucia, Queensland, Australia;
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19
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McIntyre S, Birznieks I, Vickery RM, Holcombe AO, Seizova-Cajic T. The tactile motion aftereffect suggests an intensive code for speed in neurons sensitive to both speed and direction of motion. J Neurophysiol 2016; 115:1703-12. [PMID: 26823511 PMCID: PMC4808137 DOI: 10.1152/jn.00460.2015] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/11/2015] [Accepted: 01/13/2016] [Indexed: 11/22/2022] Open
Abstract
Neurophysiological studies in primates have found that direction-sensitive neurons in the primary somatosensory cortex (SI) generally increase their response rate with increasing speed of object motion across the skin and show little evidence of speed tuning. We employed psychophysics to determine whether human perception of motion direction could be explained by features of such neurons and whether evidence can be found for a speed-tuned process. After adaptation to motion across the skin, a subsequently presented dynamic test stimulus yields an impression of motion in the opposite direction. We measured the strength of this tactile motion aftereffect (tMAE) induced with different combinations of adapting and test speeds. Distal-to-proximal or proximal-to-distal adapting motion was applied to participants' index fingers using a tactile array, after which participants reported the perceived direction of a bidirectional test stimulus. An intensive code for speed, like that observed in SI neurons, predicts greater adaptation (and a stronger tMAE) the faster the adapting speed, regardless of the test speed. In contrast, speed tuning of direction-sensitive neurons predicts the greatest tMAE when the adapting and test stimuli have matching speeds. We found that the strength of the tMAE increased monotonically with adapting speed, regardless of the test speed, showing no evidence of speed tuning. Our data are consistent with neurophysiological findings that suggest an intensive code for speed along the motion processing pathways comprising neurons sensitive both to speed and direction of motion.
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Affiliation(s)
- S McIntyre
- School of Psychology, University of Sydney, Sydney, Australia; Neuroscience Research Australia, Sydney, Australia; Faculty of Health Sciences, University of Sydney, Sydney, Australia; MARCS Institute for Brain, Behaviour and Development, Western Sydney University, Sydney, Australia; and
| | - I Birznieks
- Neuroscience Research Australia, Sydney, Australia; MARCS Institute for Brain, Behaviour and Development, Western Sydney University, Sydney, Australia; and School of Medical Sciences, University of New South Wales, Australia, Sydney, Australia
| | - R M Vickery
- Neuroscience Research Australia, Sydney, Australia; School of Medical Sciences, University of New South Wales, Australia, Sydney, Australia
| | - A O Holcombe
- School of Psychology, University of Sydney, Sydney, Australia
| | - T Seizova-Cajic
- Faculty of Health Sciences, University of Sydney, Sydney, Australia
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20
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Abstract
Well-organized somatotopic representation of the hand is required to interpret input from cutaneous mechanoreceptors. Previous reports have identified patients with various distortions of somatotopic representation after stroke. Importantly, those patients were investigated years after the stroke, indicating that afferent signal regained access to the cortical circuits; however, further plastic changes, which would re-establish somatotopic order and ability to correctly localize tactile stimuli, did not follow. Thus, it was not known whether somatotopic organization could be restored in such patients and whether there is a potential for new rehabilitation strategies. This is the first case report demonstrating normalization of somatotopic representation.
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Affiliation(s)
- Ingvars Birznieks
- a School of Medical Sciences , UNSW Australia , Sydney , Australia.,b Neuroscience Research Australia , Sydney , Australia.,c School of Science and Health , University of Western Sydney , Sydney , Australia
| | - Inara Logina
- d Department of Neurology , Riga Stradins university , Riga , Latvia
| | - Gunnar Wasner
- b Neuroscience Research Australia , Sydney , Australia.,e Clinic for Neurology and Pain Medicine , Christian-Aöbrechts University Kiel , Kiel , Germany
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21
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Khamis HA, Redmond SJ, Macefield VG, Birznieks I. Tactile afferents encode grip safety before slip for different frictions. Annu Int Conf IEEE Eng Med Biol Soc 2015; 2014:4123-6. [PMID: 25570899 DOI: 10.1109/embc.2014.6944531] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
Adjustments to frictional forces are crucial to maintain a safe grip during precision object handling in both humans and robotic manipulators. The aim of this work was to investigate whether a population of human tactile afferents can provide information about the current tangential/normal force ratio expressed as the percentage of the critical load capacity - the tangential/normal force ratio at which the object would slip. A smooth stimulation surface was tested on the fingertip under three frictional conditions, with a 4 N normal force and a tangential force generated by motion in the ulnar or distal direction at a fixed speed. During stimulation, the responses of 29 afferents (12 SA-I, 2 SA-II, 12 FA-I, 3 FA-II) were recorded. A multiple regression model was trained and tested using cross-validation to estimate the percentage of the critical load capacity in real-time as the tangential force increased. The features for the model were the number of spikes from each afferent in windows of fixed length (50, 100 or 200 ms) around points spanning the range from 50% to 100% of the critical load capacity, in 5% increments. The mean regression estimate error was less than 1% of the critical load capacity with a standard deviation between 5% and 10%. A larger number of afferents is expected to improve the estimate error. This work is important for understanding human dexterous manipulation and inspiring improvements in robotic grippers and prostheses.
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22
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Nagi SS, Dunn JS, Birznieks I, Vickery RM, Mahns DA. The effects of preferential A- and C-fibre blocks and T-type calcium channel antagonist on detection of low-force monofilaments in healthy human participants. BMC Neurosci 2015; 16:52. [PMID: 26268809 PMCID: PMC4535530 DOI: 10.1186/s12868-015-0190-2] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/30/2015] [Accepted: 08/07/2015] [Indexed: 12/15/2022] Open
Abstract
Background A myriad of studies have argued that tactile sensibility is underpinned exclusively by large myelinated mechanoreceptors. However, the functional significance of their slow-conducting counterparts, termed C-low threshold mechanoreceptors (C-LTMRs), remains largely unexplored. We recently showed the emergence of brush- and vibration-evoked allodynia in human hairy and glabrous skin during background muscle pain. The allodynia persisted following the preferential blockade of myelinated fibres but was abolished by the preferential blockade of cutaneous C fibres, thereby suggesting a pathway involving hairy skin C-LTMRs and their functional counterparts in glabrous skin in this phenomenon. In the present study, we tested the effects of preferential A- and C-fibre conduction blocks and pharmacological blockade of T-type calcium channel Cav3.2 (expressed selectively on small-fibre LTMRs) on monofilament detection thresholds in healthy participants by compression, low-dose intradermal anaesthesia (xylocaine 0.25 %) and selective T-channel antagonist, TTA-A2. Results We found that all participants could detect monofilament contacts (as low as 1.6 mN) within the innocuous tactile range regardless of the preferential blockade of myelinated fibres. Furthermore, during the compression block no subject reported a switch in modality from touch to pain. That is, the low-force monofilament contacts were always perceived as non-painful. However, there was a small but significant elevation of monofilament thresholds (~2 mN) in the glabrous skin following the compression block. Importantly, no differences were found in the thresholds across hairy and glabrous regions while the myelinated fibres were conducting or not. The preferential blockade of C fibres in the glabrous skin (with myelinated fibres intact) also resulted in a small but significant elevation of tactile thresholds. Furthermore, the use of T-channel blocker in the glabrous skin during compression block of myelinated fibres resulted in complete abolition of monofilament sensibility within the innocuous tactile range (tested up to ~20 mN). Conclusions These observations suggest that C-LTMRs need not be regarded as a redundant tactile system, but appear to complement normal large-myelinated-fibre tactile function. Convergent findings in glabrous and hairy skin lend support for an underlying system of innocuous mechanoreception with Cav3.2-expressing unmyelinated fibres.
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Affiliation(s)
- Saad S Nagi
- School of Medicine, University of Western Sydney, Locked Bag 1797, Penrith, NSW, 2751, Australia.
| | - James S Dunn
- School of Medicine, University of Western Sydney, Locked Bag 1797, Penrith, NSW, 2751, Australia.
| | - Ingvars Birznieks
- Neuroscience Research Australia, PO Box 1165, Randwick, NSW, 2031, Australia. .,School of Medical Sciences, UNSW Australia, Sydney, NSW, 2052, Australia.
| | - Richard M Vickery
- School of Medical Sciences, UNSW Australia, Sydney, NSW, 2052, Australia.
| | - David A Mahns
- School of Medicine, University of Western Sydney, Locked Bag 1797, Penrith, NSW, 2751, Australia.
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23
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Hudson KM, Condon M, Ackerley R, McGlone F, Olausson H, Macefield VG, Birznieks I. Effects of changing skin mechanics on the differential sensitivity to surface compliance by tactile afferents in the human finger pad. J Neurophysiol 2015; 114:2249-57. [PMID: 26269550 DOI: 10.1152/jn.00176.2014] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/05/2014] [Accepted: 08/06/2015] [Indexed: 11/22/2022] Open
Abstract
It is not known how changes in skin mechanics affect the responses of cutaneous mechanoreceptors in the finger pads to compression forces. We used venous occlusion to change the stiffness of the fingers and investigated whether this influenced the firing of low-threshold mechanoreceptors to surfaces of differing stiffness. Unitary recordings were made from 10 slowly adapting type I (SAI), 10 fast adapting type I (FAI) and 9 slowly adapting type II (SAII) units via tungsten microelectrodes inserted into the median nerve at the wrist. A servo-controlled stimulator applied ramp-and-hold forces (1, 2, and 4 N) at a constant loading and unloading rate (2 N/s) via a flat 2.5-cm-diameter silicone disk over the center of the finger pad. Nine silicone disks (objects), varying in compliance, were used. Venous occlusion, produced by inflating a sphygmomanometer cuff around the upper arm to 40 ± 5 mmHg, was used to induce swelling of the fingers and increase the compliance of the finger pulp. Venous occlusion had no effect on the firing rates of the SAI afferents, nor on the slopes of the relationship between mean firing rate and object compliance at each amplitude, but did significantly reduce the slopes for the FAI afferents. Although the SAII afferents possess a poor capacity to encode changes in object compliance, mean firing rates were significantly lower during venous occlusion. The finding that venous occlusion had no effect on the firing properties of SAI afferents indicates that these afferents preserve their capacity to encode changes in object compliance, despite changes in skin mechanics.
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Affiliation(s)
- Kathryn M Hudson
- School of Medicine, Western Sydney University, Sydney, New South Wales, Australia; School of Science & Health, Western Sydney University, Sydney, New South Wales, Australia
| | - Melia Condon
- School of Medicine, Western Sydney University, Sydney, New South Wales, Australia; School of Science & Health, Western Sydney University, Sydney, New South Wales, Australia
| | - Rochelle Ackerley
- Department of Clinical Neurophysiology, Göteborg University, Gothenburg, Sweden
| | | | - Håkan Olausson
- Department of Clinical Neurophysiology, Göteborg University, Gothenburg, Sweden
| | - Vaughan G Macefield
- School of Medicine, Western Sydney University, Sydney, New South Wales, Australia; Neuroscience Research Australia, Sydney, New South Wales, Australia; and MARCS Institute, Western Sydney University, Sydney, New South Wales, Australia
| | - Ingvars Birznieks
- School of Science & Health, Western Sydney University, Sydney, New South Wales, Australia; Neuroscience Research Australia, Sydney, New South Wales, Australia; and MARCS Institute, Western Sydney University, Sydney, New South Wales, Australia
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24
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Khamis H, Birznieks I, Redmond SJ. Decoding tactile afferent activity to obtain an estimate of instantaneous force and torque applied to the fingerpad. J Neurophysiol 2015; 114:474-84. [PMID: 25948866 DOI: 10.1152/jn.00040.2015] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/15/2015] [Accepted: 05/04/2015] [Indexed: 11/22/2022] Open
Abstract
Dexterous manipulation is not possible without sensory information about object properties and manipulative forces. Fundamental neuroscience has been unable to demonstrate how information about multiple stimulus parameters may be continuously extracted, concurrently, from a population of tactile afferents. This is the first study to demonstrate this, using spike trains recorded from tactile afferents innervating the monkey fingerpad. A multiple-regression model, requiring no a priori knowledge of stimulus-onset times or stimulus combination, was developed to obtain continuous estimates of instantaneous force and torque. The stimuli consisted of a normal-force ramp (to a plateau of 1.8, 2.2, or 2.5 N), on top of which -3.5, -2.0, 0, +2.0, or +3.5 mNm torque was applied about the normal to the skin surface. The model inputs were sliding windows of binned spike counts recorded from each afferent. Models were trained and tested by 15-fold cross-validation to estimate instantaneous normal force and torque over the entire stimulation period. With the use of the spike trains from 58 slow-adapting type I and 25 fast-adapting type I afferents, the instantaneous normal force and torque could be estimated with small error. This study demonstrated that instantaneous force and torque parameters could be reliably extracted from a small number of tactile afferent responses in a real-time fashion with stimulus combinations that the model had not been exposed to during training. Analysis of the model weights may reveal how interactions between stimulus parameters could be disentangled for complex population responses and could be used to test neurophysiologically relevant hypotheses about encoding mechanisms.
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Affiliation(s)
- Heba Khamis
- Graduate School of Biomedical Engineering, University of New South Wales Australia, Sydney, Australia; Neuroscience Research Australia, Sydney, Australia; School of Science and Health, University of Western Sydney, Penrith, New South Wales, Australia
| | - Ingvars Birznieks
- Neuroscience Research Australia, Sydney, Australia; School of Medical Sciences, Medicine, University of New South Wales Australia, Sydney, Australia; and School of Science and Health, University of Western Sydney, Penrith, New South Wales, Australia
| | - Stephen J Redmond
- Graduate School of Biomedical Engineering, University of New South Wales Australia, Sydney, Australia
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Arabzadeh E, Clifford CWG, Harris JA, Mahns DA, Macefield VG, Birznieks I. Single tactile afferents outperform human subjects in a vibrotactile intensity discrimination task. J Neurophysiol 2014; 112:2382-7. [PMID: 25143540 DOI: 10.1152/jn.00482.2014] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
We simultaneously compared the sensitivity of single primary afferent neurons supplying the glabrous skin of the hand and the psychophysical amplitude discrimination thresholds in human subjects for a set of vibrotactile stimuli delivered to the receptive field. All recorded afferents had a dynamic range narrower than the range of amplitudes across which the subjects could discriminate. However, when the vibration amplitude was chosen to be within the steepest part of the afferent's stimulus-response function the response of single afferents, defined as the spike count over the vibration duration (500 ms), was often more sensitive in discriminating vibration amplitude than the perceptual judgment of the participants. We quantified how the neuronal performance depended on the integration window: for short windows the neuronal performance was inferior to the performance of the subject. The neuronal performance progressively improved with increasing spike count duration and reached a level significantly above that of the subjects when the integration window was 250 ms or longer. The superiority in performance of individual neurons over observers could reflect a nonoptimal integration window or be due to the presence of noise between the sensory periphery and the cortical decision stage. Additionally, it could indicate that the range of perceptual sensitivity comes at the cost of discrimination through pooling across neurons with different response functions.
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Affiliation(s)
- Ehsan Arabzadeh
- School of Psychology, University of Sydney, Sydney, Australia; Eccles Institute of Neuroscience, John Curtin School of Medical Research, Australian National University, Canberra, Australia; ARC Centre of Excellence for Integrative Brain Function, Australian National University Node, Canberra, Australia;
| | - Colin W G Clifford
- School of Psychology, University of Sydney, Sydney, Australia; School of Psychology, UNSW Australia, Sydney, Australia
| | - Justin A Harris
- School of Psychology, University of Sydney, Sydney, Australia
| | - David A Mahns
- School of Medicine, University of Western Sydney, Sydney, Australia
| | - Vaughan G Macefield
- School of Medicine, University of Western Sydney, Sydney, Australia; Neuroscience Research Australia, Sydney, Australia; and
| | - Ingvars Birznieks
- Neuroscience Research Australia, Sydney, Australia; and School of Science and Health, University of Western Sydney, Sydney, Australia
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Khamis H, Redmond SJ, Macefield V, Birznieks I. Classification of Texture and Frictional Condition at Initial Contact by Tactile Afferent Responses. Haptics: Neuroscience, Devices, Modeling, and Applications 2014. [DOI: 10.1007/978-3-662-44193-0_58] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
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Condon M, Birznieks I, Hudson K, Chelvanayagam DK, Mahns D, Olausson H, Macefield VG. Differential sensitivity to surface compliance by tactile afferents in the human finger pad. J Neurophysiol 2013; 111:1308-17. [PMID: 24371291 DOI: 10.1152/jn.00589.2013] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
We undertook a neurophysiological investigation of the responses of low-threshold mechanoreceptors in the human finger pad to surfaces of differing softness. Unitary recordings were made from 26 slowly adapting type I (SAI), 17 fast-adapting type I (FAI), and 9 slowly adapting type II (SAII) afferents via tungsten microelectrodes inserted into the median nerve at the wrist. A servo-controlled stimulator applied ramp-and-hold forces (1, 2, 4 N) at a constant loading and unloading rate (2 N/s) via a flat silicone disc over the center of the finger pad. Nine discs were used, which linearly increased in stiffness across the range. Population responses of the SAI afferents showed the greatest sensitivity to compliance, with a steep monotonic increase in mean firing rate with increasing stiffness (decreasing compliance) of the surface during the loading and plateau (but not unloading) phases. FAI afferents also showed a linear increase in firing during the loading but not unloading phase, although the slope was significantly lower than that of the SAI afferents at all amplitudes. Conversely, SAII afferents were influenced by object compliance only in certain conditions. Given their high density in the finger pads and their linear relationship between firing rate and object compliance during the loading and plateau phases, SAI afferents (together with FAI afferents during the loading phase) are ideally suited to contributing information on surface compliance to the overall estimation of softness, but the SAII afferents appear to play only a minor role.
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Affiliation(s)
- Melia Condon
- School of Medicine, University of Western Sydney, Sydney, Australia
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Rager DM, Alvares D, Birznieks I, Redmond SJ, Morley JW, Lovell NH, Vickery RM. Generating tactile afferent stimulation patterns for slip and touch feedback in neural prosthetics. Annu Int Conf IEEE Eng Med Biol Soc 2013; 2013:5922-5. [PMID: 24111087 DOI: 10.1109/embc.2013.6610900] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Abstract
Current prosthetic limbs are limited by a lack of tactile feedback. Slip feedback is particularly important to inform grip. Object slip is marked by both a change in the normal grip force applied and a change in force tangential to the fingertips. In this study, we demonstrate that a new multi-axial tactile sensor composed of gold nanoparticle strain gauges is able to record slip and reconstruct the X, Y, and Z forces incident on the sensor's surface due to a slipping object. We entered the X, Y, and Z force components generated by the slip event into a noisy leaky integrate and fire model to simulate the firing responses of SA1 and FA1 afferents. We also recorded a uniaxial normal force input representative of tactile contact. A single set of SA1 model and FA1 model parameters generated realistic firing patterns for both the slip and normal force recordings. These results suggest that canonical SA1 and FA1 afferent models could be used to generate biomimetic electrical stimulation patterns for both slip and touch stimuli. When used to activate the tactile afferents of an amputee, these electrical stimulation patterns could create natural and distinguishable slip and touch percepts for closed loop control of an upper limb neural prosthesis.
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Fazalbhoy A, Macefield VG, Birznieks I. Tonic muscle pain does not increase fusimotor drive to human leg muscles: implications for chronic muscle pain. Exp Physiol 2013; 98:1125-32. [DOI: 10.1113/expphysiol.2012.071670] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
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Fu J, Birznieks I, Goodwin AW, Khamis H, Redmond SJ. Decoding tactile sensation: multiple regression analysis of monkey fingertip afferent mechanoreceptor population responses. Annu Int Conf IEEE Eng Med Biol Soc 2013; 2012:4631-4. [PMID: 23366960 DOI: 10.1109/embc.2012.6346999] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
How complex tactile sensations are encoded by populations of afferent mechanoreceptors is currently not well understood. While much is known about how individual afferents respond to prescribed stimuli, their behavior as a population distributed across the fingertip has not been well described. In this study, tactile afferent mechanoreceptors in monkey fingertips were mechanically stimulated, using a flat disc shaped probe, with several magnitudes of normal force (1.8, 2.2 and 2.5 N) and torque (2.0 and 3.5 mNm), in clockwise and anticlockwise directions. Afferent nerve responses were acquired from 58 slowly-adapting (SA) type-I and 25 fast-adapting (FA) type-I isolated single cutaneous mechanoreceptive afferents, recorded from the median nerve. At 10 ms time intervals after the application of torque begins, a multiple regression model was trained and evaluated to estimate the magnitude of the applied normal force and torque. Averaged results over the 200 ms period after the torque reaches its maximum indicate that SA-I and FA-I afferents can both estimate the applied torque value. FA-I afferents gave the lowest estimation error mean and standard deviation of -0.051 ± 0.334 mNm for a target torque of 2.0 mNm, and 0.003 ± 0.414 mNm for a target torque of 3.5 mNm. However, while SA-I afferents could estimate normal force well, there was no significant difference (ANOVA, p=0.173) in the FA-I estimates of normal force, as this force had already been held constant for one second before the torque loading phase under analysis began.
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Affiliation(s)
- Joanne Fu
- Graduate School of Biomedical Engineering, The University of New South Wales, Sydney, Australia
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Birznieks I, Logina I, Wasner G. Somatotopic mismatch following stroke: a pathophysiological condition escaping detection. BMJ Case Rep 2012; 2012:bcr-2012-006304. [PMID: 23045439 DOI: 10.1136/bcr-2012-006304] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/04/2022] Open
Abstract
Clinical evaluation of somatosensory deficits in stroke patients is very limited and usually does not include testing of somatotopic organisation, which is a prerequisite for meaningful interpretation of sensory input and sensorimotor control. Detailed tactile testing of the left hand of a 54-year-old patient suffering from sensory deficit and central pain after a right-sided stroke revealed severe distortion of somatotopic sensory maps as evidenced by incorrect localisation of the point stimuli. Unlike previously reported gross somatotopic remapping taking place within reduced representational space after lesion, this is the first case report revealing chaotic scrambled somatosensory maps. While the incidence of such scrambled somatotopic representation of tactile input is not yet known in stroke patients, current observations indicate that in-depth investigations of somatotopic organisation of affected area may reveal the underlying cause for various functional deficits including central pain. Thus, new rehabilitation strategies may need to be developed specifically for such patients.
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Affiliation(s)
- Ingvars Birznieks
- School of Science and Health, University of Western Sydney, Penrith, New South Wales, Australia.
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McIntyre S, Holcombe AO, Birznieks I, Seizova-Cajic T. Tactile motion adaptation reduces perceived speed but shows no evidence of direction sensitivity. PLoS One 2012; 7:e45438. [PMID: 23029010 PMCID: PMC3454433 DOI: 10.1371/journal.pone.0045438] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/04/2012] [Accepted: 08/20/2012] [Indexed: 12/03/2022] Open
Abstract
INTRODUCTION While the directionality of tactile motion processing has been studied extensively, tactile speed processing and its relationship to direction is little-researched and poorly understood. We investigated this relationship in humans using the 'tactile speed aftereffect' (tSAE), in which the speed of motion appears slower following prolonged exposure to a moving surface. METHOD We used psychophysical methods to test whether the tSAE is direction sensitive. After adapting to a ridged moving surface with one hand, participants compared the speed of test stimuli on the adapted and unadapted hands. We varied the direction of the adapting stimulus relative to the test stimulus. RESULTS Perceived speed of the surface moving at 81 mms(-1) was reduced by about 30% regardless of the direction of the adapting stimulus (when adapted in the same direction, Mean reduction = 23 mms(-1), SD = 11; with opposite direction, Mean reduction = 26 mms(-1), SD = 9). In addition to a large reduction in perceived speed due to adaptation, we also report that this effect is not direction sensitive. CONCLUSIONS Tactile motion is susceptible to speed adaptation. This result complements previous reports of reliable direction aftereffects when using a dynamic test stimulus as together they describe how perception of a moving stimulus in touch depends on the immediate history of stimulation. Given that the tSAE is not direction sensitive, we argue that peripheral adaptation does not explain it, because primary afferents are direction sensitive with friction-creating stimuli like ours (thus motion in their preferred direction should result in greater adaptation, and if perceived speed were critically dependent on these afferents' response intensity, the tSAE should be direction sensitive). The adaptation that reduces perceived speed therefore seems to be of central origin.
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Affiliation(s)
- Sarah McIntyre
- Faculty of Health Sciences, University of Sydney, Sydney, Australia.
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Fazalbhoy A, Birznieks I, Macefield VG. Individual differences in the cardiovascular responses to tonic muscle pain: parallel increases or decreases in muscle sympathetic nerve activity, blood pressure and heart rate. Exp Physiol 2012; 97:1084-92. [DOI: 10.1113/expphysiol.2012.066191] [Citation(s) in RCA: 41] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
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Hall SC, Fazalbhoy A, Birznieks I, Macefield VG. Biphasic effects of tonic stimulation of muscle nociceptors on skin sympathetic nerve activity in human subjects. Exp Brain Res 2012; 221:107-14. [PMID: 22766847 DOI: 10.1007/s00221-012-3156-y] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/02/2012] [Accepted: 06/18/2012] [Indexed: 11/30/2022]
Abstract
Skin sympathetic nerve activity (SSNA) controls skin blood flow and sweat release, and acute noxious stimulation of skin has been shown to cause a decrease in SSNA in the anaesthetised or spinal cat. In awake human subjects, acute muscle pain causes a transient rise in SSNA, but the impact of long-lasting (tonic) stimulation of muscle nociceptors on skin sympathetic outflow, blood flow and sweat release is unknown. We tested the hypothesis that tonic stimulation of muscle nociceptors causes a sustained increase in sympathetic outflow to the skin. SSNA was recorded from the common peroneal nerve of 10 awake human subjects. Tonic muscle pain was induced by infusing hypertonic saline (7 %) into the tibialis anterior muscle over ~40 min, titrated to achieve a constant level of muscle pain. SSNA initially increased following the onset of the infusion, reaching a peak of 164 % of baseline within 5 min, but then showed a prolonged and sustained decrease, reaching a nadir of 77 % in 20 min. Conversely, skin blood flow (and vascular conductance) initially decreased, followed by a progressive increase; there were no consistent changes in sweat release. In 9 of 10 subjects, SSNA and skin blood flow were inversely related. We conclude that sympathetic outflow to the skin exhibits a biphasic response to long-lasting stimulation of muscle nociceptors: an initial increase presumably related to the 'arousal' or 'alerting' component of pain, characterised by increased SSNA and decreased skin blood flow, followed by a prolonged decrease in SSNA and increased skin blood flow. The latter may be a purposeful response that contributes to wound healing.
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Affiliation(s)
- Samuel C Hall
- School of Medicine, University of Western Sydney, Locked Bag 1797, Penrith, Sydney, NSW 2751, Australia
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Birznieks I, Boonstra TW, Macefield VG. Modulation of human muscle spindle discharge by arterial pulsations--functional effects and consequences. PLoS One 2012; 7:e35091. [PMID: 22529975 PMCID: PMC3328488 DOI: 10.1371/journal.pone.0035091] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/04/2011] [Accepted: 03/13/2012] [Indexed: 11/18/2022] Open
Abstract
Arterial pulsations are known to modulate muscle spindle firing; however, the physiological significance of such synchronised modulation has not been investigated. Unitary recordings were made from 75 human muscle spindle afferents innervating the pretibial muscles. The modulation of muscle spindle discharge by arterial pulsations was evaluated by R-wave triggered averaging and power spectral analysis. We describe various effects arterial pulsations may have on muscle spindle afferent discharge. Afferents could be "driven" by arterial pulsations, e.g., showing no other spontaneous activity than spikes generated with cardiac rhythmicity. Among afferents showing ongoing discharge that was not primarily related to cardiac rhythmicity we illustrate several mechanisms by which individual spikes may become phase-locked. However, in the majority of afferents the discharge rate was modulated by the pulse wave without spikes being phase locked. Then we assessed whether these influences changed in two physiological conditions in which a sustained increase in muscle sympathetic nerve activity was observed without activation of fusimotor neurones: a maximal inspiratory breath-hold, which causes a fall in systolic pressure, and acute muscle pain, which causes an increase in systolic pressure. The majority of primary muscle spindle afferents displayed pulse-wave modulation, but neither apnoea nor pain had any significant effect on the strength of this modulation, suggesting that the physiological noise injected by the arterial pulsations is robust and relatively insensitive to fluctuations in blood pressure. Within the afferent population there was a similar number of muscle spindles that were inhibited and that were excited by the arterial pulse wave, indicating that after signal integration at the population level, arterial pulsations of opposite polarity would cancel each other out. We speculate that with close-to-threshold stimuli the arterial pulsations may serve as an endogenous noise source that may synchronise the sporadic discharge within the afferent population and thus facilitate the detection of weak stimuli.
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Affiliation(s)
- Ingvars Birznieks
- School of Science and Health, University of Western Sydney, Sydney, New South Wales, Australia.
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Chan GSH, Fazalbhoy A, Birznieks I, Macefield VG, Middleton PM, Lovell NH. Spontaneous fluctuations in the peripheral photoplethysmographic waveform: roles of arterial pressure and muscle sympathetic nerve activity. Am J Physiol Heart Circ Physiol 2011; 302:H826-36. [PMID: 22114133 DOI: 10.1152/ajpheart.00970.2011] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Assessment of spontaneous slow waves in the peripheral blood volume using the photoplethysmogram (PPG) has shown potential clinical value, but the physiological correlates of these fluctuations have not been fully elucidated. This study addressed the contribution of arterial pressure and muscle sympathetic nerve activity (MSNA) in beat-to-beat PPG variability in resting humans under spontaneous breathing conditions. Peripheral PPG waveforms were measured from the fingertip, earlobe, and toe in young and healthy individuals (n = 13), together with the arterial pressure waveform, electrocardiogram, respiration, and direct measurement of MSNA by microneurography. Cross-spectral coherence analysis revealed that among the PPG waveforms, low-frequency fluctuations (0.04-0.15 Hz) in the ear PPG had the highest coherence with arterial pressure (0.71 ± 0.15) and MSNA (0.44 ± 0.18, with a peak of 0.71 ± 0.16 at 0.10 ± 0.03 Hz). The normalized midfrequency powers (0.08-0.15 Hz), with an emphasis on the 0.1-Hz region, were positively correlated between MSNA and the ear PPG (r = 0.77, P = 0.002). Finger and toe PPGs had lower coherence with arterial pressure (0.35 ± 0.10 and 0.30 ± 0.11, respectively) and MSNA (0.33 ± 0.10 and 0.26 ± 0.10, respectively) in the LF band but displayed higher coherence between themselves (0.54 ± 0.09) compared with the ear (P < 0.001), which may suggest the dominance of regional vasomotor activities and a common sympathetic influence in the glabrous skin. These findings highlight the differential mechanisms governing PPG waveform fluctuations across different body sites. Spontaneous PPG variability in the ear includes a major contribution from arterial pressure and MSNA, which may provide a rationale for its clinical utility.
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Affiliation(s)
- Gregory S H Chan
- 1School of Electrical Engineering and Telecommunications, University of New South Wales, Sydney, New South Wales
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Birznieks I, Wheat HE, Redmond SJ, Salo LM, Lovell NH, Goodwin AW. Encoding of tangential torque in responses of tactile afferent fibres innervating the fingerpad of the monkey. J Physiol 2010; 588:1057-72. [PMID: 20142274 DOI: 10.1113/jphysiol.2009.185314] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
Torsional loads are ubiquitous during everyday dextrous manipulations. We examined how information about torque is provided to the sensorimotor control system by populations of tactile afferents. Torsional loads of different magnitudes were applied in clockwise and anticlockwise directions to a standard central site on the fingertip. Three different background levels of contact (grip) force were used. The median nerve was exposed in anaesthetized monkeys and single unit responses recorded from 66 slowly adapting type-I (SA-I) and 31 fast adapting type-I (FA-I) afferents innervating the distal segments of the fingertips. Most afferents were excited by torque but some were suppressed. Responses of the majority of both afferent types were scaled by torque magnitude applied in one or other direction, with the majority of FA-I afferent responses and about half of SA-I afferent responses scaled in both directions. Torque direction affected responses in both afferent types, but more so for the SA-I afferents. Latencies of the first spike in FA-I afferent responses depended on the parameters of the torque. We used a Parzen window classifier to assess the capacity of the SA-I and FA-I afferent populations to discriminate, concurrently and in real-time, the three stimulus parameters, namely background normal force, torque magnitude and direction. Despite the potentially confounding interactions between stimulus parameters, both the SA-I and the FA-I populations could extract torque magnitude accurately. The FA-I afferents signalled torque magnitude earlier than did the SA-I afferents, but torque direction was extracted more rapidly and more accurately by the SA-I afferent population.
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Affiliation(s)
- Ingvars Birznieks
- Prince of Wales Medical Research Institute, Randwick, NSW 2031, Sydney, Australia.
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Burton AR, Birznieks I, Spaak J, Henderson LA, Macefield VG. Effects of deep and superficial experimentally induced acute pain on skin sympathetic nerve activity in human subjects. Exp Brain Res 2009; 195:317-24. [DOI: 10.1007/s00221-009-1790-9] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/14/2009] [Accepted: 03/27/2009] [Indexed: 10/20/2022]
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Burton AR, Birznieks I, Bolton PS, Henderson LA, Macefield VG. Effects of deep and superficial experimentally induced acute pain on muscle sympathetic nerve activity in human subjects. J Physiol 2009; 587:183-93. [PMID: 19015194 PMCID: PMC2670032 DOI: 10.1113/jphysiol.2008.162230] [Citation(s) in RCA: 62] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/24/2008] [Accepted: 11/12/2008] [Indexed: 11/08/2022] Open
Abstract
Human studies conducted more than half a century ago have suggested that superficial pain induces excitatory effects on the sympathetic nervous system, resulting in increases in blood pressure (BP) and heart rate (HR), whereas deep pain is believed to cause vasodepression. To date, no studies have addressed whether deep or superficial pain produces such differential effects on muscle sympathetic nerve activity (MSNA). Using microneurography we recorded spontaneous MSNA from the common peroneal nerve in 13 awake subjects. Continuous blood pressure was recorded by radial arterial tonometry. Deep pain was induced by intramuscular injection of 0.5 ml hypertonic saline (5%) into the tibialis anterior muscle, superficial pain by subcutaneous injection of 0.2 ml hypertonic saline into the overlying skin. Muscle pain, with a mean rating of 4.9 +/- 0.8 (S.E.M.) on a 0-10 visual analog scale (VAS) and lasting on average 358 +/- 32 s, caused significant increases in MSNA (43.9 +/- 10.0%), BP (5.4 +/- 1.1%) and HR (7.0 +/- 2.0%) - not the expected decreases. Skin pain, rated at 4.9 +/- 0.6 and lasting 464 +/- 54 s, also caused significant increases in MSNA (38.2 +/- 12.8%), BP (5.1 +/- 2.1%) and HR (5.6 +/- 2.0%). The high-frequency (HF) to low-frequency (LF) ratio of heart rate variability (HRV) increased from 1.54 +/- 0.25 to 2.90 +/- 0.45 for muscle pain and 2.80 +/- 0.52 for skin pain. Despite the different qualities of deep (dull and diffuse) and superficial (burning and well-localized) pain, we conclude that pain originating in muscle and skin does not exert a differential effect on muscle sympathetic nerve activity, both causing an increase in MSNA and an increase in the LF:HF ratio of HRV. Whether this holds true for longer lasting experimental pain remains to be seen.
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Affiliation(s)
- A R Burton
- Prince of Wales Medical Research Institute, Sydney, NSW 2031, Australia
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Birznieks I, Burton AR, Macefield VG. The effects of experimental muscle and skin pain on the static stretch sensitivity of human muscle spindles in relaxed leg muscles. J Physiol 2008; 586:2713-23. [PMID: 18403422 DOI: 10.1113/jphysiol.2008.151746] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
Animal studies have shown that noxious inputs onto gamma-motoneurons can cause an increase in the activity of muscle spindles, and it has been proposed that this causes a fusimotor-driven increase in muscle stiffness that is believed to underlie many chronic pain syndromes. To test whether experimental pain also acts on the fusimotor system in humans, unitary recordings were made from 19 spindle afferents (12 Ia, 7 II) located in the ankle and toe extensors or peronei muscles of awake human subjects. Muscle pain was induced by bolus intramuscular injection of 0.5 ml 5% hypertonic saline into tibialis anterior (TA); skin pain was induced by 0.2 ml injection into the overlying skin. Changes in fusimotor drive to the muscle spindles were inferred from changes in the mean discharge frequency and discharge variability of spindle endings in relaxed muscle. During muscle pain no afferents increased their discharge activity: seven afferents (5 Ia, 2 II) showed a decrease and six (4 Ia, 2 II) afferents were not affected. During skin pain of 13 afferents discharge rate increased in one (Ia) and decreased in two (1 Ia, 1 II). On average, the overall discharge rate decreased during muscle pain by 6.1% (P < 0.05; Wilcoxon), but remained essentially the same during skin pain. There was no detectable correlation between subjective pain level and the small change in discharge rate of muscle spindles. Irrespective of the type of pain, discharge variability parameters were not influenced (P > 0.05; Wilcoxon). We conclude that, contrary to the 'vicious cycle' hypothesis, acute activation of muscle or skin nociceptors does not cause a reflex increase in fusimotor drive in humans. Rather, our results are more aligned with the pain adaptation model, based on clinical studies predicting pain-induced reductions of agonist muscle activity.
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Affiliation(s)
- Ingvars Birznieks
- Prince of Wales Medical Research Institute, Barker Street, Randwick, NSW 2031, Sydney, Australia.
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Johansson RS, Birznieks I. First spikes in ensembles of human tactile afferents code complex spatial fingertip events. Nat Neurosci 2004; 7:170-7. [PMID: 14730306 DOI: 10.1038/nn1177] [Citation(s) in RCA: 312] [Impact Index Per Article: 15.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/06/2003] [Accepted: 12/03/2003] [Indexed: 11/09/2022]
Abstract
It is generally assumed that primary sensory neurons transmit information by their firing rates. However, during natural object manipulations, tactile information from the fingertips is used faster than can be readily explained by rate codes. Here we show that the relative timing of the first impulses elicited in individual units of ensembles of afferents reliably conveys information about the direction of fingertip force and the shape of the surface contacting the fingertip. The sequence in which different afferents initially discharge in response to mechanical fingertip events provides information about these events faster than the fastest possible rate code and fast enough to account for the use of tactile signals in natural manipulation.
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Affiliation(s)
- Roland S Johansson
- Department of Integrative Medical Biology, Physiology Section, Umeå University, SE-901 87 Umeå, Sweden.
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Jenmalm P, Birznieks I, Goodwin AW, Johansson RS. Influence of object shape on responses of human tactile afferents under conditions characteristic of manipulation. Eur J Neurosci 2003; 18:164-76. [PMID: 12859350 DOI: 10.1046/j.1460-9568.2003.02721.x] [Citation(s) in RCA: 95] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Most objects that we grasp, lift and further manipulate are curved, with curvatures of the same order of magnitude as those of the fingertips. Tactile information pertaining to such 'gross' geometrical features of objects are used in the automatic control of fingertip actions. We analyzed responses from 172 human tactile afferents distributed over the entire terminal phalanx when spherically shaped surfaces were applied to a standard site on the fingertip; the curvatures and force magnitudes and directions used were representative of everyday manipulations. Nearly all SA-I, SA-II and FA-I afferents responded, and for more than 80% of these afferents the response intensity was correlated with curvature. The correlation was positive for approximately half the afferents and negative for the other half, resulting in a curvature contrast signal within the populations of tactile afferents; afferents terminating at the sides and end of the fingertip tended to show negative correlations. For nearly all afferents, curvature and force direction had interactive effects. Changing the direction of force affected an afferent's sensitivity to curvature and vice versa. We conclude that recognition of such shapes takes advantage of signals originating from tactile afferents distributed over the entire terminal phalanx, and that both the direction of fingertip forces and the curvatures of objects contacted during natural manipulations influence the afferents' responses. Consequently, if humans are able to perceive independently curvature and force direction from signals in tactile afferents, then the CNS must possess mechanisms that disentangle interactions between these and other parameters of stimuli on the fingertips.
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Affiliation(s)
- Per Jenmalm
- Department of Integrative Medical Biology, Physiology Section, Umeå University, SE-90187 Umeå, Sweden
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Birznieks I, Jenmalm P, Goodwin AW, Johansson RS. Encoding of direction of fingertip forces by human tactile afferents. J Neurosci 2001; 21:8222-37. [PMID: 11588194 PMCID: PMC6763843] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/21/2023] Open
Abstract
In most manipulations, we use our fingertips to apply time-varying forces to the target object in controlled directions. Here we used microneurography to assess how single tactile afferents encode the direction of fingertip forces at magnitudes, rates, and directions comparable to those arising in everyday manipulations. Using a flat stimulus surface, we applied forces to a standard site on the fingertip while recording impulse activity in 196 tactile afferents with receptive fields distributed over the entire terminal phalanx. Forces were applied in one of five directions: normal force and forces at a 20 degrees angle from the normal in the radial, distal, ulnar, or proximal directions. Nearly all afferents responded, and the responses in most slowly adapting (SA)-I, SA-II, and fast adapting (FA)-I afferents were broadly tuned to a preferred direction of force. Among afferents of each type, the preferred directions were distributed in all angular directions with reference to the stimulation site, but not uniformly. The SA-I population was biased for tangential force components in the distal direction, the SA-II population was biased in the proximal direction, and the FA-I population was biased in the proximal and radial directions. Anisotropic mechanical properties of the fingertip and the spatial relationship between the receptive field center of the afferent and the stimulus site appeared to influence the preferred direction in a manner dependent on afferent type. We conclude that tactile afferents from the whole terminal phalanx potentially contribute to the encoding of direction of fingertip forces similar to those that occur when subjects manipulate objects under natural conditions.
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Affiliation(s)
- I Birznieks
- Department of Integrative Medical Biology, Physiology Section, Umeå University, SE-901 87 Umeå, Sweden.
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Jenmalm P, Birznieks I, Goodwin A, Johansson R. Differential responses in populations of fingertip tactile afferents to objects' surface curvatures. Acta Physiol Scand 1999; 167:A24-A25. [PMID: 10571585 DOI: 10.1046/j.1365-201x.1999.600af.x] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Affiliation(s)
- P Jenmalm
- Address of presenting author: Per Jenmalm, Department of Integrative Medical Biology, Section of Physiology, Umeå University SE-90187 Umeå, Sweden Telephone: +46 90 7865186-15; Fax: +46 90 7866683
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Affiliation(s)
- I Birznieks
- Address of presenting author: Ingvars Birznieks, Department of Integrative Medical Biology, Section of Physiology Umeå University SE-90187 Umeå, Sweden Telephone: +46 90 7865186; Fax: +46 90 7866683
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Birznieks I, Burstedt MK, Edin BB, Johansson RS. Mechanisms for force adjustments to unpredictable frictional changes at individual digits during two-fingered manipulation. J Neurophysiol 1998; 80:1989-2002. [PMID: 9772255 DOI: 10.1152/jn.1998.80.4.1989] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Previous studies on adaptation of fingertip forces to local friction at individual digit-object interfaces largely focused on static phases of manipulative tasks in which humans could rely on anticipatory control based on the friction in previous trials. Here we instead analyze mechanisms underlying this adaptation after unpredictable changes in local friction between consecutive trials. With the tips of the right index and middle fingers or the right and left index fingers, subjects restrained a manipulandum whose horizontal contact surfaces were located side by side. At unpredictable moments a tangential force was applied to the contact surfaces in the distal direction at 16 N/s to a plateau at 4 N. The subjects were free to use any combination of normal and tangential forces at the two fingers, but the sum of the tangential forces had to counterbalance the imposed load. The contact surface of the right index finger was fine-grained sandpaper, whereas that of the cooperating finger was changed between sandpaper and the more slippery rayon. The load increase automatically triggered normal force responses at both fingers. When a finger contacted rayon, subjects allowed slips to occur at this finger during the load force increase instead of elevating the normal force. These slips accounted for a partitioning of the load force between the digits that resulted in an adequate adjustment of the normal:tangential force ratios to the local friction at each digit. This mechanism required a fine control of the normal forces. Although the normal force at the more slippery surface had to be comparatively low to allow slippage, the normal forces applied by the nonslipping digit at the same time had to be high enough to prevent loss of the manipulandum. The frictional changes influenced the normal forces applied before the load ramp as well as the size of the triggered normal force responses similarly at both fingers, that is, with rayon at one contact surface the normal forces increased at both fingers. Thus to independently adapt fingertip forces to the local friction the normal forces were controlled at an interdigital level by using sensory information from both engaged digits. Furthermore, subjects used both short- and long-term anticipatory mechanisms in a manner consistent with the notion that the central nervous system (CNS) entertains internal models of relevant object and task properties during manipulation.
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Affiliation(s)
- I Birznieks
- Department of Physiology, Umeâ University, SE-901 87 Umeâ, Sweden
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Abstract
We investigated the coordination of fingertip forces in subjects who used the tips of two fingers to restrain an instrumented manipulandum with horizontally oriented grip surfaces. The grip surfaces were subjected to tangential pulling forces in the distal direction in relation to the fingers. The subjects used either the right index and middle fingers (unimanual grasp) or both index fingers (bimanual grasp) to restrain the manipulandum. To change the frictional condition at the digit-object interfaces, either both grip surfaces were covered with sandpaper or one was covered with sandpaper and the other with rayon. The forces applied normally and tangentially to the grip surfaces were measured separately at each plate along with the position of the plates. Subjects could have performed the present task successfully with many different force distributions between the digits. However, they partitioned the load in a manner that reflected the frictional condition at the local digit-object interfaces. When both digits contacted sandpaper, they typically partitioned the load symmetrically, but when one digit made contact with rayon and the other with sandpaper, the digit contacting the less slippery material (sandpaper) took up a larger part of the load. The normal forces were also influenced by the frictional condition, but they reflected the average friction at the two contact sites rather than the local friction. That is, when friction was low at one of the digit-object interfaces, only the applied normal forces increased at both digits. Thus sensory information related to the local frictional condition at the respective digit-object interfaces controlled the normal force at both digits. The normal:tangential force ratio at each digit appeared to be a controlled variable. It was adjusted independently at each digit to the minimum ratio required to prevent frictional slippage, keeping an adequate safety margin against slippage. This was accomplished by the scaling of the normal forces to the average friction and by partitioning of the load according to frictional differences between the digit-object interfaces. In conclusion, by adjusting the normal:tangential force ratios to the local frictional condition, subjects avoided excessive normal forces at the individual digit-object interfaces, and by partitioning the load according the frictional difference, subjects avoided high normal forces. Thus the local frictional condition at the separate digit-object interfaces is one factor that can strongly influence the distribution of forces across digits engaged in a manipulative act.
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Affiliation(s)
- M K Burstedt
- Department of Physiology, Umeå University, Sweden
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