Importance of cutaneous feedback in maintaining a secure grip during manipulation of hand-held objects

Dexterous manipulation: differential sensitivity of manipulation and grasp forces to task requirements

How humans coordinate digit forces to perform dexterous manipulation is not well understood. This gap is due to the use of tasks devoid of dexterity requirements and/or the use of analytical techniques that cannot isolate the roles that digit forces play in preventing object slip and controlling object position and orientation (pose). In our recent work, we used a dexterous manipulation task and decomposed digit forces into FG, the internal force that prevents object slip, and FM, the force responsible for object pose control. Unlike FG, FM was modulated from object lift onset to hold, suggesting their different sensitivity to sensory feedback acquired during object lift. However, the extent to which FG and FM can be controlled independently remains to be determined. Importantly, how FG and FM change as a function of object property is mathematically indeterminate and therefore requires active modulation. To address this gap, we systematically changed either object mass or external torque. The FM normal component responsible for object orientation control was modulated to changes in object torque but not mass. In contrast, FG was distinctly modulated to changes in object mass and torque. These findings point to a differential sensitivity of FG and FM to task requirements and provide novel insights into the neural control of dexterous manipulation. Importantly, our results indicate that the proposed digit force decomposition has the potential to capture important differences in how sensory inputs are processed and integrated to simultaneously ensure grasp stability and dexterous object pose control.NEW & NOTEWORTHY Successful dexterous object manipulation requires simultaneous prevention of object slip and object pose control. How these two task goals are attained can be investigated by decomposing digit forces into grasp and manipulation forces, respectively. We found that these forces were characterized by differential sensitivity to changes in object properties (mass and torque). This finding suggests the involvement of distinct sensorimotor mechanisms that, combined, simultaneously ensure grasp stability and dexterous control of object pose.

PVA/PANI-DBSA Nanomesh Tactile Sensor for Force Feedback

Touch serves as an important medium for human-environment interaction. The piezoresistive tactile sensor has attracted much attention due to its convenient technology, simple principle, and convenient signal acquisition and analysis. In this paper, conductive beads-on-string polyvinyl alcohol (PVA)/polyaniline doped with dodecyl benzene sulfonic acid (PANI-DBSA) nanofibers were fabricated via the electrospinning technique. Due to the special nanostructure of PVA-coated PANI-DBSA, the tactile sensor presented a wide measuring range of 12 Pa-121 kPa and appreciable sensitivity of 8.576 kPa-1 at 12 Pa~484 Pa. In addition, the response time and recovery time of the sensor were approximately 500 ms, demonstrating promising prospects in the field of tactile sensing for active upper limb prostheses.

Fast grip force adaptation to friction relies on localized fingerpad strains

During object manipulation, humans adjust the grip force to friction, such that slippery objects are squeezed more firmly than sticky ones. This essential mechanism to keep a stable grasp relies on feedback from tactile afferents innervating the fingertips, that are sensitive to local skin strains. To test if this feedback originates from the skin-object interface, we asked participants to perform a grip-lift task with an instrumented object able to monitor skin strains at the contact through transparent plates of different frictions. We observed that, following an unbeknown change in plate across trials, participants adapted their grip force to friction. After switching from high to low friction, we found a significant increase in strain inside the contact arising ~100 ms before the modulation of grip force, suggesting that differences in strain patterns before lift-off are used by the nervous system to quickly adjust the force to the frictional properties of manipulated objects.

Open-Source Instrumented Object to Study Dexterous Object Manipulation

Humans use tactile feedback to perform skillful manipulation. When tactile sensory feedback is unavailable, for instance, if the fingers are anesthetized, dexterity is severely impaired. Imaging the deformation of the finger pad skin when in contact with a transparent plate provides information about the tactile feedback received by the central nervous system. Indeed, skin deformations are transduced into neural signals by the mechanoreceptors of the finger pad skin. Understanding how this feedback is used for active object manipulation would improve our understanding of human dexterity. In this paper, we present a new device for imaging the skin of the finger pad of one finger during manipulation performed with a precision grip. The device's mass (300 g) makes it easy to use during unconstrained dexterous manipulation. Using this device, we reproduced the experiment performed in Delhaye et al. (2021) We extracted the strains aligned with the object's movement, i.e., the vertical strains in the ulnar and radial parts of the fingerpad, to see how correlated they were with the grip force (GF) adaptation. Interestingly, parts of our results differed from those in Delhaye et al. (2021) due to weight and inertia differences between the devices, with average GF across participants differing significantly. Our results highlight a large variability in the behavior of the skin across participants, with generally low correlations between strain and GF adjustments, suggesting that skin deformations are not the primary driver of GF adaptation in this manipulation scenario.

The effect of type 2 diabetes and diabetic peripheral neuropathy on predictive grip force control

This study investigated the impact of type 2 diabetes and diabetic peripheral neuropathy on grip force control during object manipulation. The study included three age-matched groups: type 2 diabetes alone (n = 11), type 2 diabetes with neuropathy (n = 13), and healthy controls (n = 12). Grip force control variables derived from lifting and holding an experimental cup were the ratio between grip force and load forces during lifting (GFR), latency 1 and latency 2, which represented the time between the object's grip and its lift-off from the table, and the period between object's lift-off and the grip force peak, respectively; time lag, which denoted the time difference between the grip and load force peaks during the lifting phase, and finally static force, which was the grip force average during the holding phase. Grip force control variables were compared between groups using one-way ANOVA and Kruskal-Wallis test. Post-hoc analysis was used to compare differences between groups. GFR and latency 1 showed significant differences between groups; the type 2 diabetes with neuropathy group showed larger GFR than the type 2 diabetes alone and healthy control groups. The latency 1was longer for the group with neuropathy in comparison with the health control group. There were no significant differences between groups for latency 2, time lag, and static force. Our results showed impaired GFR and latency 1 in participants with type 2 diabetes with neuropathy while the time lag was preserved. People with type 2 diabetes alone might not have any deficits in grip force control. Higher grip forces might expose people with type 2 diabetes and diabetic peripheral neuropathy to the risk of fatigue and injuring their hands. Future studies should investigate strategies to help people with type 2 diabetes with neuropathy adjust grip forces during object manipulation.

Changes in EEG alpha-band power during prehension indicates neural motor drive inhibition

Changes in alpha band activity (8-12 Hz) indicate the downregulation of brain regions during cognitive tasks, reflecting real-time cognitive load. Despite this, its feasibility to be used in a more dynamic environment with ongoing motor corrections has not been studied. This research used electroencephalography (EEG) to explore how different brain regions are engaged during a simple grasp and lift task where unexpected changes to the object's weight or surface friction are introduced. The results suggest that alpha activity changes related to motor error correction occur only in motor-related areas (i.e. central areas) but not in error processing areas (i.e., frontoparietal network) during unexpected weight changes. This suggests that oscillations over motor areas reflect the reduction of motor drive related to motor error correction, thus, being a potential cortical electrophysiological biomarker for the process and not solely as a proxy for cognitive demands. This observation is particularly relevant in scenarios where these signals are used to evaluate high cognitive demands co-occurring with high levels of motor errors and corrections, such as prosthesis use. The establishment of electrophysiological biomarkers of mental resource allocation during movement and cognition can help identify indicators of mental workload and motor drive, which may be useful for improving brain-machine interfaces.NEW & NOTEWORTHY We demonstrated that alpha suppression, an EEG phenomenon with high temporal resolution, occurs over the primary sensorimotor area during error correction during lift movements. Interpretations of alpha activity are often attributed to high cognitive demands, thus recognizing that it is also influenced by motor processes is important in situations where cognitive demands are paired with movement errors. This could further have application as a biomarker for error correction in human-machine interfaces, such as neuroprostheses.

An evaluation of 3D printable elastics for post stroke dynamic hand bracing: a pilot study

The rise of 3D printing allows unprecedented customization of rehabilitation devices, and with an ever-expanding library of 3D printable (3DP) materials, the spectrum of attenable rehabilitation devices is likewise expanding. The current pilot study explores feasibility of using 3DP elastic materials to create dynamic hand orthoses for stroke survivors. A dynamic orthosis featuring a replaceable finger component was fabricated using 3DP elastic materials. Duplicates of the finger component were printed using different materials ranging from low stiffness (low elastic modulus) to relatively high stiffness (high elastic modulus). Five stroke survivors with predominantly moderate hand impairment were recruited to evaluate usability and impact of orthoses on upper extremity function and biomechanics. No significant differences in usability were found between 3D-printed orthoses and a commercial orthosis. Increases in stiffness of the 3DP material reduced pincer force (p = .0041) and the BBT score (p = .043). In comparison, the commercial orthosis did not reduce pincer force but may reduce BBT score to a degree that is clinically significant (p = .0002). While preliminary, these findings suggest that a dynamic orthosis is a feasible clinical application of 3DP elastic materials, and future study is warranted.

A haptic illusion created by gravity

Human dexterity requires very fine and efficient control of fingertip forces, which relies on the integration of cutaneous and proprioceptive feedback. Here, we examined the influence of gravity on isometric force control. We trained participants to reproduce isometric vertical forces on a dynamometer held between the thumb and the index finger in normal gravity and tested them during parabolic flight creating phases of microgravity and hypergravity, thereby strongly influencing the motor commands and the proprioceptive feedback. We found that gravity creates the illusion that upward forces are larger than downward forces of the same magnitude. The illusion increased under hypergravity and was abolished under microgravity. Gravity also affected the control of the grip force employed to secure the grasp. These findings suggest that gravity biases the haptic estimation of forces, which has implications for the design of haptic devices to be used during flight or space activities.

Restoration of sensory information via bionic hands

Individuals who have lost the use of their hands because of amputation or spinal cord injury can use prosthetic hands to restore their independence. A dexterous prosthesis requires the acquisition of control signals that drive the movements of the robotic hand, and the transmission of sensory signals to convey information to the user about the consequences of these movements. In this Review, we describe non-invasive and invasive technologies for conveying artificial sensory feedback through bionic hands, and evaluate the technologies' long-term prospects.

Temporal and spatial characteristics of bone conduction as non-invasive haptic sensory feedback for upper-limb prosthesis

Bone conduction is a promising haptic feedback modality for upper-limb prosthesis users, however, its potential and characteristics as a non-invasive feedback modality have not been thoroughly investigated. This study aimed to establish the temporal and spatial characteristics of non-invasive bone conduction as a sensory feedback interface for upper-limb prostheses. Psychometric human-subject experiments were conducted on three bony landmarks of the elbow, with a vibrotactile transducer affixed to each to provide the stimulus. The study characterized the temporal domain by testing perception threshold and resolution in amplitude and frequency. The spatial domain was evaluated by assessing the ability of subjects to detect the number of simultaneous active stimulation sites. The experiment was conducted with ten able-bodied subjects and compared to two subjects with trans-radial amputation. The psychometric evaluation of the proposed non-invasive bone conduction feedback showed results comparable to invasive methods. The experimental results demonstrated similar amplitude and frequency resolution of the interface for all three stimulation sites for both able-bodied subjects and subjects with trans-radial amputation, highlighting its potential as a non-invasive feedback modality for upper-limb prostheses.

Mass-Manufacturable 3D Magnetic Force Sensor for Robotic Grasping and Slip Detection

The manipulation of delicate objects remains a key challenge in the development of industrial robotic grippers. Magnetic force sensing solutions, which provide the required sense of touch, have been demonstrated in previous work. The sensors feature a magnet embedded within a deformable elastomer, which is mounted on top of a magnetometer chip. A key drawback of these sensors lies in the manufacturing process, which relies on the manual assembly of the magnet-elastomer transducer, impacting both the repeatability of measurements across sensors and the potential for a cost-effective solution through mass-manufacturing. In this paper, a magnetic force sensor solution is presented with an optimized manufacturing process that will facilitate mass production. The elastomer-magnet transducer was fabricated using injection molding, and the assembly of the transducer unit, on top of the magnetometer chip, was achieved using semiconductor manufacturing techniques. The sensor enables robust differential 3D force sensing within a compact footprint (5 mm × 4.4 mm × 4.6 mm). The measurement repeatability of these sensors was characterized over multiple samples and 300,000 loading cycles. This paper also showcases how the 3D high-speed sensing capabilities of these sensors can enable slip detection in industrial grippers.

Performing a shortened version of the Action Research Arm Test in immersive virtual reality to assess post-stroke upper limb activity

BACKGROUND

To plan treatment and measure post-stroke recovery, frequent and time-bounded functional assessments are recommended. With increasing needs for neurorehabilitation advances, new technology based methods, such as virtual reality (VR) have emerged. Here, we developed an immersive VR version of the Action Research Arm Test (ARAT-VR) to complement neurorehabilitation.

OBJECTIVE

This study aimed to assess the validity, usability and test-retest reliability of the ARAT-VR among individuals with stroke, healthcare professionals and healthy control subjects (HCS).

METHODS

Among the 19 items of the ARAT, 13 items were selected and developed in immersive VR. 11 healthcare professionals, 30 individuals with stroke, and 25 HCS were recruited. Content validity was assessed by asking healthcare professionals to rate the difficulty of performing each item of the ARAT-VR in comparison to the classical Action Research Arm Test (ARAT-19). Concurrent validity was first measured using correlation (Spearman tests) between the ARAT-VR and ARAT-19 scores for the individuals with stroke, and second through correlation and comparison between the scores of the ARAT-VR and the reduced version of the ARAT (ARAT-13) for both individuals with stroke and HCS (Wilcoxon signed rank tests and Bland-Altman plots). Usability was measured using the System Usability Scale. A part of individuals with stroke and HCS were re-tested following a convenient delay to measure test-retest reliability (Intra-class correlation and Wilcoxon tests).

RESULTS

Regarding the content validity, median difficulty of the 13 ARAT-VR items (0[0 to - 1] to 0[0-1]) evaluated by healthcare professionals was rated as equivalent to the classical ARAT for all tasks except those involving the marbles. For these, the difficulty was rated as superior to the real tasks (1[0-1] when pinching with the thumb-index and thumb-middle fingers, and 1[0-2] when pinching with thumb-ring finger). Regarding the concurrent validity, for paretic hand scores, there were strong correlations between the ARAT-VR and ARAT-13 (r = 0.84), and between the ARAT-VR and ARAT-19 (r = 0.83). Usability (SUS = 82.5[75-90]) and test-retest reliability (ICC = 0.99; p < 0.001) were excellent.

CONCLUSION

The ARAT-VR is a valid, usable and reliable tool that can be used to assess upper limb activity among individuals with stroke, providing potential to increase assessment frequency, remote evaluation, and improve neurorehabilitation. Trial registration https://clinicaltrials.gov/ct2/show/NCT04694833 ; Unique identifier: NCT04694833, Date of registration: 11/24/2020.

Influence of emotion on precision grip force control: A comparison of pleasant and neutral emotion

Objective

The present study aimed to investigate the impact of emotion on force steadiness of isometric precision pinch grip that is not direction-specific.

Methods

Thirty-two healthy volunteer subjects participated in the present study. Subjects were divided into two experimental groups: pleasant image group and neutral image group. The isometric precision pinch grip task was performed for three times. Specifically, the first task was performed before pleasant or neutral picture viewing, the second task was performed immediately after picture viewing, further the third task was performed 30 seconds after the second task. During the isometric precision pinch grip task, participants were asked to exert pinch grip force at 10% of maximal voluntary contraction with visual feedback. The coefficient of variation of force production and normalized root mean square value of electromyography activity were calculated.

Results

After pleasant picture viewing, coefficient of variation of pinch force production and normalized root mean square value of electromyography was decreased. While, in the neutral image condition, theses variables were not altered. More important, compared to the neutral image condition, pleasant emotion led to lower coefficient of variation of pinch grip force production.

Conclusion

These findings indicate that pleasant emotion improves force control of isometric precision pinch grip. Therefore, in clinical settings, the emotional state of patients may affect the effectiveness of rehabilitation and should be taken into consideration.

Detection and discrimination of electrical stimuli from an upper limb cuff electrode inM. Mulatta

Objective.Peripheral nerve interfaces seek to restore nervous system function through electrical stimulation of peripheral nerves. In clinical use, these devices should function reliably for years or decades. In this study, we assessed evoked sensations from multi-channel cuff electrode stimulation in macaque monkeys up to 711 d post-implantation.Approach.Three trained macaque monkeys received multi-channel cuff electrode implants at the median or ulnar nerves in the upper arm. Electrical stimuli from the cuff interfaces evoked sensations, which we measured via standard psychophysical tasks. We adjusted pulse amplitude or pulse width for each block with various electrode channel configurations to examine the effects of stimulus parameterization on sensation. We measured detection thresholds and just-noticeable differences (JNDs) at irregular, near-daily intervals for several months using Bayesian inferencing from trial data. We examined data trends using classical models such as Weber's Law and the strength-duration relationship using linear regression.Main results.Detection thresholds were similar between blocks with pulse width modulation and blocks with pulse amplitude modulation when represented as charge per pulse, the product of the amplitude and the pulse width. Conversely, Weber fractions-calculated as the slope of the regression between JND charge values and reference stimulus charge-were significantly different between pulse width and pulse amplitude modulation blocks for the discrimination task.Significance.Weber fractions were lower in blocks with amplitude modulation than in blocks with pulse width modulation, suggesting that pulse amplitude modulation allows finer resolution of sensory encoding above threshold. Consequently, amplitude modulation may enable a greater dynamic range for sensory perception with neuroprosthetic devices.

Peripheral neurostimulation for encoding artificial somatosensations

The direct neural stimulation of peripheral or central nervous systems has been shown as an effective tool to treat neurological conditions. The electrical activation of the nervous sensory pathway can be adopted to restore the artificial sense of touch and proprioception in people suffering from sensory-motor disorders. The modulation of the neural stimulation parameters has a direct effect on the electrically induced sensations, both when targeting the somatosensory cortex and the peripheral somatic nerves. The properties of the artificial sensations perceived, as their location, quality and intensity are strongly dependent on the direct modulation of pulse width, amplitude and frequency of the neural stimulation. Different sensory encoding schemes have been tested in patients showing distinct effects and outcomes according to their impact on the neural activation. Here, I reported the most adopted neural stimulation strategies to artificially encode somatosensation into the peripheral nervous system. The real-time implementation of these strategies in bionic devices is crucial to exploit the artificial sensory feedback in prosthetics. Thus, neural stimulation becomes a tool to directly communicate with the human nervous system. Given the importance of adding artificial sensory information to neuroprosthetic devices to improve their control and functionality, the choice of an optimal neural stimulation paradigm could increase the impact of prosthetic devices on the quality of life of people with sensorimotor disabilities.

The neural basis of tactile texture perception

Running our fingers across a textured surface gives rise to two types of skin deformations, each transduced by different tactile nerve fibers. Coarse features produce large-scale skin deformations whose spatial configuration is reflected in the spatial pattern of activation of some tactile fibers. Scanning a finely textured surface elicits vibrations in the skin, which in turn evoked temporally patterned responses in other fibers. These two neural codes-spatial and temporal-drive a spectrum of neural response properties in somatosensory cortex: At one extreme, neurons are sensitive to spatial patterns and encode coarse features; at the other extreme, neurons are sensitive to vibrations and encode fine features. While the texture responses of nerve fibers are dependent on scanning speed, those of cortical neurons are less so, giving rise to a speed invariant texture percept. Neurons in high-level somatosensory cortices combine information about texture with information about task variables.

Efficient tactile encoding of object slippage

When grasping objects, we rely on our sense of touch to adjust our grip and react against external perturbations. Less than 200 ms after an unexpected event, the sensorimotor system is able to process tactile information to deduce the frictional strength of the contact and to react accordingly. Given that roughly 1,300 afferents innervate the fingertips, it is unclear how the nervous system can process such a large influx of data in a sufficiently short time span. In this study, we measured the deformation of the skin during the initial stages of incipient sliding for a wide range of frictional conditions. We show that the dominant patterns of deformation are sufficient to estimate the distance between the frictional force and the frictional strength of the contact. From these stereotypical patterns, a classifier can predict if an object is about to slide during the initial stages of incipient slip. The prediction is robust to the actual value of the interfacial friction, showing sensory invariance. These results suggest the existence of a possible compact set of bases that we call Eigenstrains. These Eigenstrains are a potential mechanism to rapidly decode the margin from full slip from the tactile information contained in the deformation of the skin. Our findings suggest that only 6 of these Eigenstrains are necessary to classify whether the object is firmly stuck to the fingers or is close to slipping away. These findings give clues about the tactile regulation of grasp and the insights are directly applicable to the design of robotic grippers and prosthetics that rapidly react to external perturbations.

Restoration of Proprioceptive and Cutaneous Sensation Using Regenerative Peripheral Nerve Interfaces in Humans with Upper Limb Amputations

SUMMARY

Without meaningful and intuitive sensory feedback, even the most advanced prosthetic limbs remain insensate and impose an enormous cognitive burden during use. The regenerative peripheral nerve interface can serve as a novel bidirectional motor and sensory neuroprosthetic interface. In previous human studies, regenerative peripheral nerve interfaces demonstrated stable high-amplitude motor electromyography signals with excellent signal-to-noise ratio for prosthetic control. In addition, they can treat and prevent postamputation pain by mitigating neuroma formation. In this study, the authors investigated whether electrical stimulation applied to regenerative peripheral nerve interfaces could produce appreciable proprioceptive and/or tactile sensations in two participants with upper limb amputations. Stimulation of the interfaces resulted in both participants reporting proprioceptive sensations in the phantom hand. Specifically, stimulation of participant 1's median nerve regenerative peripheral nerve interface activated a flexion sensation in the thumb or index finger, whereas stimulation of the ulnar nerve interface evoked a flexion sensation of the ring or small finger. Likewise, stimulation of one of participant 2's ulnar nerve interfaces produced a sensation of flexion at the ring finger distal interphalangeal joint. In addition, stimulation of participant 2's other ulnar nerve interface and the median nerve interface resulted in perceived cutaneous sensations that corresponded to each nerve's respective dermatome. These results suggest that regenerative peripheral nerve interfaces have the potential to restore proprioceptive and cutaneous sensory feedback that could significantly improve prosthesis use and embodiment.

Normal and tangential forces combine to convey contact pressure during dynamic tactile stimulation

Humans need to accurately process the contact forces that arise as they perform everyday haptic interactions such as sliding the fingers along a surface to feel for bumps, sticky regions, or other irregularities. Several different mechanisms are possible for how the forces on the skin could be represented and integrated in such interactions. In this study, we used a force-controlled robotic platform and simultaneous ultrasonic modulation of the finger-surface friction to independently manipulate the normal and tangential forces during passive haptic stimulation by a flat surface. To assess whether the contact pressure on their finger had briefly increased or decreased during individual trials in this broad stimulus set, participants did not rely solely on either the normal force or the tangential force. Instead, they integrated tactile cues induced by both components. Support-vector-machine analysis classified physical trial data with up to 75% accuracy and suggested a linear perceptual mechanism. In addition, the change in the amplitude of the force vector predicted participants' responses better than the change of the coefficient of dynamic friction, suggesting that intensive tactile cues are meaningful in this task. These results provide novel insights about how normal and tangential forces shape the perception of tactile contact.

Spatial Instability during Precision Grip–Lift in Children with Poor Manual Dexterity

Although children with developmental coordination disorder (DCD) show impaired precision grip control due to a sensory-motor integration deficit, their spatial instability (such as changes in force direction and object roll during a precision grip task) is unclear. Herein, we investigated the spatial instability in the precision grip force control of children with poor manual dexterity. We divided 66 school-aged children who performed a precision gripping and lifting of heavy- or lightweight objects into those with low manual dexterity (n = 11) and those with high manual dexterity (n = 55) as revealed by the Movement Assessment Battery for Children (2nd edition). The group and weight effects were then determined. The results revealed that the total trajectory lengths of the center of pressure (COP) were longer in the lightweight object data of the children in the low-manual-dexterity group and were related to the children’s grip force. The low-manual-dexterity group also showed a shifted COP position from the center of the object in the medial–lateral direction and in the object roll regardless of the object’s weight; these were closely related in both weights’ tests. These results demonstrated that children with poor manual dexterity show spatial instability and different adaptations to the weight of objects during a precision grip task. Further studies are needed to determine whether these findings would be replicated in children with a diagnosis of DCD.

The science and engineering behind sensitized brain-controlled bionic hands

Advances in our understanding of brain function, along with the development of neural interfaces that allow for the monitoring and activation of neurons, have paved the way for brain-machine interfaces (BMIs), which harness neural signals to reanimate the limbs via electrical activation of the muscles or to control extracorporeal devices, thereby bypassing the muscles and senses altogether. BMIs consist of reading out motor intent from the neuronal responses monitored in motor regions of the brain and executing intended movements with bionic limbs, reanimated limbs, or exoskeletons. BMIs also allow for the restoration of the sense of touch by electrically activating neurons in somatosensory regions of the brain, thereby evoking vivid tactile sensations and conveying feedback about object interactions. In this review, we discuss the neural mechanisms of motor control and somatosensation in able-bodied individuals and describe approaches to use neuronal responses as control signals for movement restoration and to activate residual sensory pathways to restore touch. Although the focus of the review is on intracortical approaches, we also describe alternative signal sources for control and noninvasive strategies for sensory restoration.

Grip Force is Adjusted at a Level That Maintains an Upper Bound on Partial Slip Across Friction Conditions During Object Manipulation

Dexterous manipulation of objects heavily relies on the feedback provided by the tactile afferents innervating the fingertips. Previous studies have suggested that humans might take advantage of partial slip, localized loss of grip between the skin and the object, to gauge the stability of a contact and react appropriately when it is compromised, that is, when slippage is about to happen. To test this hypothesis, we asked participants to perform point-to-point movements using a manipulandum. Through optical imaging, the device monitored partial slip at the contact interface, and at the same time, the forces exerted by the fingers. The level of friction of the contact material was changed every five trials. We found that the level of grip force was systematically adjusted to the level of friction, and thus partial slip was limited to an amount similar across friction conditions. We suggest that partial slip is a key signal for dexterous manipulation and that the grip force is regulated to continuously maintain an upper bound on partial slip across friction conditions.

Decoupled Control of Grasp and Rotation Constraints During Prehension of Weightless Objects

Gravity provides critical information for the adjustment of body movement or manipulation of the handheld object. Indeed, the changes in gravity modify the mechanical constraints of prehensile actions, which may be accompanied by the changes in control strategies. The current study examined the effect of the gravitational force of a handheld object on the control strategies for subactions of multidigit prehension. A total of eight subjects performed prehensile tasks while grasping and lifting the handle by about 250 mm along the vertical direction. The experiment consisted of two conditions: lifting gravity-induced (1g) and weightless (0g) handheld objects. The weightless object condition was implemented utilizing a robot arm that produced a constant antigravitational force of the handle. The current analysis was limited to the two-dimensional grasping plane, and the notion of the virtual finger was employed to formulate the cause–effect chain of elemental variables during the prehensile action. The results of correlation analyses confirmed that decoupled organization of two subsets of mechanical variables was observed in both 1g and 0g conditions. While lifting the handle, the two subsets of variables were assumed to contribute to the grasping and rotational equilibrium, respectively. Notably, the normal forces of the thumb and virtual finger had strong positive correlations. In contrast, the normal forces had no significant relationship with the variables as to the moment of force. We conclude that the gravitational force had no detrimental effect on adjustments of the mechanical variables for the rotational action and its decoupling from the grasping equilibrium.

Aging and Bimanual Effects on Finger Center of Pressure during Precision Grip: Different Strategies for Spatial Stability

The purpose of this study was to examine aging and bimanual effects on finger spatial stability during precision grip. Twenty-one older and 21 younger adults performed precision grip tasks consisting of a single task (grip and lift an object with the thumb and index finger) and a dual task (the grip-lifting task with one hand and a peg board task with the other hand). The center of pressure (COP) trajectory and the grip force were evaluated using a pressure sensor with a high spatial resolution. In the COP trajectory, the main effects of age for the thumb (F_1,140 = 46.17, p < 0.01) and index finger (F_1,140 = 22.14, p < 0.01) and task difficulty for the thumb (F_1,140 = 6.47, p = 0.01) were significant based on ANCOVA. The COP trajectory was statistically decreased in the older adults. The COP trajectory was also decreased in the dual task, regardless of age. The results suggest the existence of a safety strategy to prioritize the spatial stability in the elderly group and in the dual task. This study provides new insights into the interpretation of the COP trajectory.

Initial contact shapes the perception of friction

Humans have the remarkable ability to manipulate a large variety of objects, regardless of how fragile, heavy, or slippery they are. To correctly scale the grip forces, the nervous system gauges the slipperiness of the surface. This information is present at the instant we first touch an object, even before any lateral force develops. However, how friction could be estimated without slippage only from the fingertip skin deformation is not understood, either in neuroscience or engineering disciplines. This study demonstrates that a radial tensile strain of the skin is involved in the perception of slipperiness during this initial contact. These findings can inform the design of advanced tactile sensors for robotics or prosthetics and for improving haptic human–machine interactions.

Humans efficiently estimate the grip force necessary to lift a variety of objects, including slippery ones. The regulation of grip force starts with the initial contact and takes into account the surface properties, such as friction. This estimation of the frictional strength has been shown to depend critically on cutaneous information. However, the physical and perceptual mechanism that provides such early tactile information remains elusive. In this study, we developed a friction-modulation apparatus to elucidate the effects of the frictional properties of objects during initial contact. We found a correlation between participants’ conscious perception of friction and radial strain patterns of skin deformation. The results provide insights into the tactile cues made available by contact mechanics to the sensorimotor regulation of grip, as well as to the conscious perception of the frictional properties of an object.

The neural mechanisms of manual dexterity

The hand endows us with unparalleled precision and versatility in our interactions with objects, from mundane activities such as grasping to extraordinary ones such as virtuoso pianism. The complex anatomy of the human hand combined with expansive and specialized neuronal control circuits allows a wide range of precise manual behaviours. To support these behaviours, an exquisite sensory apparatus, spanning the modalities of touch and proprioception, conveys detailed and timely information about our interactions with objects and about the objects themselves. The study of manual dexterity provides a unique lens into the sensorimotor mechanisms that endow the nervous system with the ability to flexibly generate complex behaviour.

Measuring fingerpad deformation during active object manipulation

During active object manipulation, the finger-object interactions give rise to complex fingertip skin deformations. These deformations are in turn encoded by the local tactile afferents and provide rich and behaviorally relevant information to the central nervous system. Most of the work studying the mechanical response of the finger to dynamic loading has been performed under a passive setup, thereby precisely controlling the kinematics or the dynamics of the loading. However, to identify aspects of the deformations that are relevant to online control during object manipulation, it is desirable to measure the skin response in an active setup. To that end, we developed a device that allows us to monitor finger forces, skin deformations, and kinematics during fine manipulation. We describe the device in detail and test it to precisely describe how the fingertip skin in contact with the object deforms during a simple vertical oscillation task. We show that the level of grip force directly influences the fingerpad skin strains and that the strain rates are substantial during active manipulation (norm up to 100%/s). The developed setup will enable us to causally relate sensory information, i.e. skin deformation, to online control, i.e. grip force adjustment, in future studies.NEW & NOTEWORTHY We present a novel device, a manipulandum, that enables to image the contact between the finger and the contact surface during active manipulation of the device. The device is tested in a simple vertical oscillation task involving 18 participants. We demonstrate that substantial surface skin strains take place at the finger-object interface and argue that those deformations provide essential information for grasp stability during object manipulation.

Evolution of touch and proprioception of the limbs: Insights from fish and humans

The function of the hands is inextricably linked to cutaneous mechanosensation, both in touch and in how hand movement and posture (proprioception) are controlled. The structure and behavior of hands and distal forelimbs of other vertebrates have been evolutionarily shaped by these mechanosensory functions. The distal forelimb of tetrapod vertebrates is homologous to the pectoral fin rays and membrane of fishes. Fish fins demonstrate similar mechanosensory abilities to hands and other distal tetrapod forelimbs in touch and proprioception. These results indicate that vertebrates were using the core mechanosensory inputs, such as fast adapting and slow adapting nerve responses, to inform fin and limb function and behavior before their diversification in fish and tetrapod lineages.

Testing silicone digit extensions as a way to suppress natural sensation to evaluate supplementary tactile feedback

Dexterous use of the hands depends critically on sensory feedback, so it is generally agreed that functional supplementary feedback would greatly improve the use of hand prostheses. Much research still focuses on improving non-invasive feedback that could potentially become available to all prosthesis users. However, few studies on supplementary tactile feedback for hand prostheses demonstrated a functional benefit. We suggest that confounding factors impede accurate assessment of feedback, e.g., testing non-amputee participants that inevitably focus intently on learning EMG control, the EMG’s susceptibility to noise and delays, and the limited dexterity of hand prostheses. In an attempt to assess the effect of feedback free from these constraints, we used silicone digit extensions to suppress natural tactile feedback from the fingertips and thus used the tactile feedback-deprived human hand as an approximation of an ideal feed-forward tool. Our non-amputee participants wore the extensions and performed a simple pick-and-lift task with known weight, followed by a more difficult pick-and-lift task with changing weight. They then repeated these tasks with one of three kinds of audio feedback. The tests were repeated over three days. We also conducted a similar experiment on a person with severe sensory neuropathy to test the feedback without the extensions. Furthermore, we used a questionnaire based on the NASA Task Load Index to gauge the subjective experience. Unexpectedly, we did not find any meaningful differences between the feedback groups, neither in the objective nor the subjective measurements. It is possible that the digit extensions did not fully suppress sensation, but since the participant with impaired sensation also did not improve with the supplementary feedback, we conclude that the feedback failed to provide relevant grasping information in our experiments. The study highlights the complex interaction between task, feedback variable, feedback delivery, and control, which seemingly rendered even rich, high-bandwidth acoustic feedback redundant, despite substantial sensory impairment.

Neurophysiology of slip sensation and grip reaction: insights for hand prosthesis control of slippage

Sensory feedback is pivotal for a proficient dexterity of the hand. By modulating the grip force in function of the quick and not completely predictable change of the load force, grabbed objects are prevented to slip from the hand. Slippage control is an enabling achievement to all manipulation abilities. However, in hand prosthetics, the performance of even the most innovative research solutions proposed so far to control slippage remain distant from the human physiology. Indeed, slippage control involves parallel and compensatory activation of multiple mechanoceptors, spinal and supraspinal reflexes, and higher-order voluntary behavioral adjustments. In this work, we reviewed the literature on physiological correlates of slippage to propose a three-phases model for the slip sensation and reaction. Furthermore, we discuss the main strategies employed so far in the research studies that tried to restore slippage control in amputees. In the light of the proposed three-phase slippage model and from the weaknesses of already implemented solutions, we proposed several physiology-inspired solutions for slippage control to be implemented in the future hand prostheses. Understanding the physiological basis of slip detection and perception and implementing them in novel hand feedback system would make prosthesis manipulation more efficient and would boost its perceived naturalness, fostering the sense of agency for the hand movements.

Fine adaptive precision grip control without maximum pinch strength changes after upper limb neurodynamic mobilization

Before and immediately after passive upper limb neurodynamic mobilizations targeting the median nerve, grip (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$G_F$$\end{document}GF) and load (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$L_F$$\end{document}LF) forces applied by the thumb, index and major fingers (three-jaw chuck pinch) were collected using a manipulandum during three different grip precision tasks: grip-lift-hold-replace (GLHR), vertical oscillations (OSC), and vertical oscillations with up and down collisions (OSC/COLL/u, OSC/COLL/d). Several parameters were collected or computed from \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$G_F$$\end{document}GF and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$L_F$$\end{document}LF. Maximum pinch strength and fingertips pressure sensation threshold were also examined. After the mobilizations, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$L_F$$\end{document}LF max changes from 3.2 ± 0.4 to 3.4 ± 0.4 N (p = 0.014), d\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$G_F$$\end{document}GF from 89.0 ± 66.6 to 102.2 ± 59.6 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$N~\text{s}^{-1}$$\end{document}Ns-1 (p = 0.009), and d\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$L_F$$\end{document}LF from 43.6 ± 17.0 to 56.0 ± 17.9 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$N~\text{s}^{-1}$$\end{document}Ns-1 (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$p<$$\end{document}p<0.001) during GLHR. \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$L_F$$\end{document}LF SD changes from 0.9 ± 0.3 to 1.0 ± 0.2 N (p = 0.004) during OSC. \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$L_F$$\end{document}LF peak changes from 17.4 ± 8.3 to 15.1 ± 7.5 N (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$p<$$\end{document}p<0.001), \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$G_F$$\end{document}GF from 12.4 ± 6.7 to 11.3 ± 6.8 N (p = 0.033), and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$L_F$$\end{document}LF from 2.9 ± 0.4 to 3.00 ± 0.4 N (p = 0.018) during OSC/COLL/u. \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$G_F$$\end{document}GF peak changes from 13.5 ± 7.4 to 12.3 ± 7.7 N (p = 0.030) and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$L_F$$\end{document}LF from 14.5 ± 6.0 to 13.6 ± 5.5 N (p = 0.018) during OSC/COLL/d. Sensation thresholds at index and thumb were reduced (p = 0.001, p = 0.008). Precision grip adaptations observed after the mobilizations could be partly explained by changes in cutaneous median-nerve pressure afferents from the thumb and index fingertips.

Anticipation and compensation for somatosensory deficits in object handling: evidence from a patient with large fiber sensory neuropathy

The purpose of this study was to determine the contributions of feedforward and feedback processes on grip force regulation and object orientation during functional manipulation tasks. One patient with massive somatosensory loss resulting from large fiber sensory neuropathy and 10 control participants were recruited. Three experiments were conducted: 1) perturbation to static holding; 2) discrete vertical movement; and 3) functional grasp and place. The availability of visual feedback was also manipulated to assess the nature of compensatory mechanisms. Results from experiment 1 indicated that both the deafferented patient and controls used anticipatory grip force adjustments before self-induced perturbation to static holding. The patient exhibited increased grip response time, but the magnitude of grip force adjustments remained correlated with perturbation forces in the self-induced and external perturbation conditions. In experiment 2, the patient applied peak grip force substantially in advance of maximum load force. Unlike controls, the patient's ability to regulate object orientation was impaired without visual feedback. In experiment 3, the duration of unloading, transport, and release phases were longer for the patient, with increased deviation of object orientation at phase transitions. These findings show that the deafferented patient uses distinct modes of anticipatory control according to task constraints and that responses to perturbations are mediated by alternative afferent information. The loss of somatosensory feedback thus appears to impair control of object orientation, whereas variation in the temporal organization of functional tasks may reflect strategies to mitigate object instability associated with changes in movement dynamics.NEW & NOTEWORTHY This study evaluates the effects of sensory neuropathy on the scaling and timing of grip force adjustments across different object handling tasks (i.e., holding, vertical movement, grasping, and placement). In particular, these results illustrate how novel anticipatory and online control processes emerge to compensate for the loss of somatosensory feedback. In addition, we provide new evidence on the role of somatosensory feedback for regulating object orientation during functional prehensile movement.

Grip Force Control During Virtual Interaction With Deformable and Rigid Objects Via a Haptic Gripper

Most studies of grip force control focus on the manipulation of rigid objects. However, in virtual and teleoperation applications, objects are often elastic in the pinch degree-of-freedom, and are manipulated using a handle that presents haptic feedback to the user. When designing controllers for haptic grippers, it is crucial to understand how grip force is controlled when manipulating rigid and deformable objects. Here, we used a virtual teleoperation setup with a haptic gripper interface to investigate grip force control in virtual environments. Ten participants lifted virtual objects and performed vertical, cyclic motions using a haptic gripper. We manipulated the control signal to the virtual interface, the haptic properties of the gripper, and the visual properties of the virtual objects to test their effect on the grip force control. We found that participants modulated their grip force as a function of the anticipated load force in all of the experimental conditions. The control signal and properties of the haptic gripper, but not the visual properties of the objects, affected the baseline and the extent of the grip force modulation. These results can provide design guidelines for haptic grippers that elicit natural grasping in virtual and bilateral teleoperation applications.

Recent advances in wearable biosensing gloves and sensory feedback biosystems for enhancing rehabilitation, prostheses, healthcare, and virtual reality

Wearable sensing gloves and sensory feedback devices that record and enhance the sensations of the hand are used in healthcare, prosthetics, robotics, and virtual reality. Recent technological advancements in soft actuators, flexible bioelectronics, and wireless data acquisition systems have enabled the development of ergonomic, lightweight, and low-cost wearable devices. This review article includes the most up-to-date materials, sensors, actuators, and system-packaging technologies to develop wearable sensing gloves and sensory feedback devices. Furthermore, this review contemplates the use of wearable sensing gloves and sensory feedback devices together to advance their capabilities as assistive devices for people with prostheses and sensory impaired limbs. This review is divided into two sections: one detailing the technologies used to develop strain, pressure, and temperature sensors integrated with a multifunctional wearable sensing glove, and the other reviewing the devices and methods used for wearable sensory displays. We discuss the limitations of the current methods and technologies along with the future direction of the field. Overall, this paper presents an all-inclusive review of the technologies used to develop wearable sensing gloves and sensory feedback devices.

Grasping Embodiment: Haptic Feedback for Artificial Limbs

Upper-limb prostheses are subject to high rates of abandonment. Prosthesis abandonment is related to a reduced sense of embodiment, the sense of self-location, agency, and ownership that humans feel in relation to their bodies and body parts. If a prosthesis does not evoke a sense of embodiment, users are less likely to view them as useful and integrated with their bodies. Currently, visual feedback is the only option for most prosthesis users to account for their augmented activities. However, for activities of daily living, such as grasping actions, haptic feedback is critically important and may improve sense of embodiment. Therefore, we are investigating how converting natural haptic feedback from the prosthetic fingertips into vibrotactile feedback administered to another location on the body may allow participants to experience haptic feedback and if and how this experience affects embodiment. While we found no differences between our experimental manipulations of feedback type, we found evidence that embodiment was not negatively impacted when switching from natural feedback to proximal vibrotactile feedback. Proximal vibrotactile feedback should be further studied and considered when designing prostheses.

High-resolution imaging of skin deformation shows that afferents from human fingertips signal slip onset

Human tactile afferents provide essential feedback for grasp stability during dexterous object manipulation. Interacting forces between an object and the fingers induce slip events that are thought to provide information about grasp stability. To gain insight into this phenomenon, we made a transparent surface slip against a fixed fingerpad while monitoring skin deformation at the contact. Using microneurography, we simultaneously recorded the activity of single tactile afferents innervating the fingertips. This unique combination allowed us to describe how afferents respond to slip events and to relate their responses to surface deformations taking place inside their receptive fields. We found that all afferents were sensitive to slip events, but fast-adapting type I (FA-I) afferents in particular faithfully encoded compressive strain rates resulting from those slips. Given the high density of FA-I afferents in fingerpads, they are well suited to detect incipient slips and to provide essential information for the control of grip force during manipulation.

Each fingertip hosts thousands of nerve fibers that allow us to handle objects with great dexterity. These fibers relay the amount of friction between the skin and the item, and the brain uses this sensory feedback to adjust the grip as necessary. Yet, exactly how tactile nerve fibers encode information about friction remains largely unknown.

Previous research has suggested that friction might not be recorded per se in nerve signals to the brain. Instead, fibers in the finger pad might be responding to localized ‘partial slips’ that indicate an impending loss of grip. Indeed, when lifting an object, fingertips are loaded with a tangential force that puts strain on the skin, resulting in subtle local deformations. Nerve fibers might be able to detect these skin changes, prompting the brain to adjust an insecure grip before entirely losing grasp of an object.

However, technical challenges have made studying the way tactile nerve fibers respond to slippage and skin strain difficult. For the first time, Delhaye et al. have now investigated how these fibers respond to and encode information about the strain placed on fingertips as they are loaded tangentially. A custom-made imaging apparatus was paired with standard electrodes to record the activity of four different kinds of tactile nerve fibers in participants who had a fingertip placed against a plate of glass. The imaging focused on revealing changes in skin surface as tangential force was applied; the electrodes measured impulses from individual nerve fibers from the fingertip. While all the fibers responded during partial slips, fast-adapting type 1 nerves generated strong responses that signal a local loss of grip. Recordings showed that these fibers consistently encoded changes in the skin strain patterns, and were more sensitive to skin compressions related to slippage than to stretch.

These results show how tactile nerve fibers encode the subtle skin compressions created when fingers handle objects. The methods developed by Delhaye et al. could further be used to explore the response properties of tactile nerve fibers, sensory feedback and grip.

Skin wetness detection thresholds and wetness magnitude estimations of the human index fingerpad and their modulation by moisture temperature

Humans often experience wet stimuli using their hands, yet we know little on how sensitive our fingers are to wetness and the mechanisms underlying this sensory function. We therefore aimed to quantify the minimum amount of water required to detect wetness on the human index fingerpad, the wetness detection threshold, and assess its modulation by temperature. Eight blinded participants (24.0 ± 5.2 yr; 23.3 ± 3.5 body mass index) used their index fingerpad to statically touch stimuli varying in volume (0, 10, 20, 30, 40, or 50 mL) and temperature (25, 29, 33, or 37°C). During and after contact, participants rated wetness and thermal sensations using a modified yes/no task and a visual analog scale. The wetness detection threshold at a moisture temperature akin to human skin (33°C) was 24.7 ± 3.48 mL. This threshold shifted depending on moisture temperature (R = 0.746), with cooler temperatures reducing (18.7 ± 3.94 mL at 29°C) and warmer temperatures increasing (27.0 ± 3.04 mL at 37°C) thresholds. When normalized over contact area, the wetness detection threshold at 33°C corresponded to 1.926 × 10⁻⁴ mL·mm⁻² [95% confidence interval (CI): 1.873 × 10⁻⁴, 1.979 × 10⁻⁴ mL·mm⁻²]. Threshold differences were reflected by magnitude estimation data, which were analyzed using linear regression to show that both volume and moisture temperature can predict magnitude estimations of wetness (R = 0.949; R = 0.179). Our results indicate high sensitivity to wetness in the human index fingerpad, which can be modulated by moisture temperature. These findings are relevant for the design of products with wetness management properties.

NEW & NOTEWORTHY The perception of wetness is a fundamental sensory experience which underpins many aspects of life, from homeostasis to enjoyable experiences. Although previous research has highlighted the importance of cold sensations in human wetness perception, the maximum sensitivity of our wetness sensing system remains to be established. This research presents a novel methodology, which for the first time, has quantified the high sensitivity of the human index fingerpad to wetness and its modulation by moisture temperature.

A Novel Grip Force Measurement Concept for Tactile Stimulation Mechanisms - Design, Validation, and User Study

In this article, we developed a new grip force measurement concept that allows for embedding tactile stimulation mechanisms in a gripper. This concept is based on a single force sensor to measure the force applied on each side of the gripper, and substantially reduces tactor motion artifacts on force measurement. To test the feasibility of this new concept, we built a device that measures control of grip force in response to a tactile stimulation from a moving tactor. We calibrated and validated our device with a testing setup with a second force sensor over a range of 0 to 20 N without movement of the tactors. We tested the effect of tactor movement on the measured grip force, and measured artifacts of 1% of the measured force. We demonstrated that during the application of dynamically changing grip forces, the average errors were 2.9% and 3.7% for the left and right sides of the gripper, respectively. We characterized the bandwidth, backlash, and noise of our tactile stimulation mechanism. Finally, we conducted a user study and found that in response to tactor movement, participants increased their grip force, the increase was larger for a smaller target force, and depended on the amount of tactile stimulation.

Grip force anticipation of nonlinear, underactuated load force

Feedforward internal model-based control enabled by efference copies of motor commands is the prevailing theoretical account of motor anticipation. Grip force control during object manipulation-a paradigmatic example of motor anticipation-is a key line of evidence for that account. However, the internal model approach has not addressed the computational challenges faced by the act of manipulating mechanically complex objects with nonlinear, underactuated degrees of freedom. These objects exhibit complex and unpredictable load force dynamics which cannot be encoded by efference copies of underlying motor commands, leading to the prediction from the perspective of an efference copy-enabled feedforward control scheme that grip force should either lag or fail to coordinate with changes in load force. In contrast to that prediction, we found evidence for strong, precise, anticipatory grip force control during manipulations of a complex object. The results are therefore inconsistent with the internal forward model approach and suggest that efference copies of motor commands are not necessary to enable anticipatory control during active object manipulation.NEW & NOTEWORTHY From the perspective of feedforward internal model-based control, precise, anticipatory grip force (GF) control when manipulating a complex object should not be possible as the object's changing load forces (LFs) cannot be encoded by efference copies of the underlying movements. However, we observed that GF exhibited strong, precise, anticipatory coupling with LF during extended manipulations of a complex object. These findings suggest that an alternative theoretical framework is needed to account for anticipatory GF control.

Influence of Stochastic Resonance on Manual Dexterity in Children With Developmental Coordination Disorder: A Double-Blind Interventional Study

Background: There is increasing evidence that the stochastic resonance (SR) phenomenon provided by subthreshold mechanical noise stimulation improves the sensory-motor system. However, the effect of SR on children with developmental coordination disorder (DCD) is unclear. The purpose of this study was to assess whether SR activated by subthreshold vibrotactile noise stimulation of the wrist influences manual dexterity in children with DCD.

Methods: A double-blind interventional study was conducted. Participants were 30 children (age: 9.3 ± 1.44 years, range 6–11 years; 27 male, three female; 25 right-handed, five left-handed) meeting DCD diagnostic criteria in DSM-5. The manual dexterity test was administered the day before SR intervention (baseline-data). SR was elicited using subthreshold vibrotactile noise stimulation at 60% of the vibrotactile threshold measured at the wrist. SR was delivered two times and the manual dexterity test was administered during each SR stimulation block (SR-on condition) and after each SR stimulation block (SR-off), for a total of four measurements. Target outcomes were the component score, the standard score, and the percentile score of the manual dexterity test.

Results: The manual dexterity test scores in the SR-on condition were significantly improved compared to scores at the baseline and in the SR-off condition (p < 0.001).

Conclusions: The present study showed that subthreshold noise stimulation eliciting SR significantly improved manual dexterity outcomes in children with DCD during stimulation but not after stimulation. Future studies will need to investigate the carry-over effects of SR stimulation.

Anatomical bases of fast parietal grasp control in humans: A diffusion-MRI tractography study

The dorso-posterior parietal cortex (DPPC) is a major node of the grasp/manipulation control network. It is assumed to act as an optimal forward estimator that continuously integrates efferent outflows and afferent inflows to modulate the ongoing motor command. In agreement with this view, a recent per-operative study, in humans, identified functional sites within DPPC that: (i) instantly disrupt hand movements when electrically stimulated; (ii) receive short-latency somatosensory afferences from intrinsic hand muscles. Based on these results, it was speculated that DPPC is part of a rapid grasp control loop that receives direct inputs from the hand-territory of the primary somatosensory cortex (S1) and sends direct projections to the hand-territory of the primary motor cortex (M1). However, evidence supporting this hypothesis is weak and partial. To date, projections from DPPC to M1 grasp zone have been identified in monkeys and have been postulated to exist in humans based on clinical and transcranial magnetic studies. This work uses diffusion-MRI tractography in two samples of right- (n = 50) and left-handed (n = 25) subjects randomly selected from the Human Connectome Project. It aims to determine whether direct connections exist between DPPC and the hand control sectors of the primary sensorimotor regions. The parietal region of interest, related to hand control (hereafter designated DPPC_hand), was defined permissively as the 95% confidence area of the parietal sites that were found to disrupt hand movements in the previously evoked per-operative study. In both hemispheres, irrespective of handedness, we found dense ipsilateral connections between a restricted part of DPPC_hand and focal sectors within the pre and postcentral gyrus. These sectors, corresponding to the hand territories of M1 and S1, targeted the same parietal zone (spatial overlap > 92%). As a sensitivity control, we searched for potential connections between the angular gyrus (AG) and the pre and postcentral regions. No robust pathways were found. Streamline densities identified using AG as the starting seed represented less than 5 % of the streamline densities identified from DPPC_hand. Together, these results support the existence of a direct sensory-parietal-motor loop suited for fast manual control and more generally, for any task requiring rapid integration of distal sensorimotor signals.

Distinct adaptation patterns between grip dynamics and arm kinematics when the body is upside-down

In humans, practically all movements are learnt and performed in a constant gravitational field. Yet, studies on arm movements and object manipulation in parabolic flight have highlighted very fast sensorimotor adaptations to altered gravity environments. Here, we wondered if the motor adjustments observed in those altered gravity environments could also be observed on Earth in a situation where the body is upside-down. To address this question, we asked participants to perform rhythmic arm movements in two different body postures (right-side-up and upside-down) while holding an object in precision grip. Analyses of grip-load force coordination and of movement kinematics revealed distinct adaptation patterns between grip and arm control. Grip force and load force were tightly synchronized from the first movements performed in upside-down posture, reflecting a malleable allocentric grip control. In contrast, velocity profiles showed a more progressive adaptation to the upside-down posture and reflected an egocentric planning of arm kinematics. In addition to suggesting distinct mechanisms between grip dynamics and arm kinematics for adaptation to novel contexts, these results also suggest the existence of general mechanisms underlying gravity-dependent motor adaptation that can be used for fast sensorimotor coordination across different postures on Earth and, incidentally, across different gravitational conditions in parabolic flights, in human centrifuges, or in Space.NEW & NOTEWORTHY During rhythmic arm movements performed in an upside-down posture, grip control adapted very quickly, but kinematics adaptation was more progressive. Our results suggest that grip control and movement kinematics planning might operate in different reference frames. Moreover, by comparing our results with previous results from parabolic flight studies, we propose that a common mechanism underlies adaptation to unfamiliar body postures and adaptation to altered gravity.

Role of Tactile Noise in the Control of Digit Normal Force

Whenever we grasp and lift an object, our tactile system provides important information on the contact location and the force exerted on our skin. The human brain integrates signals from multiple sites for a coherent representation of object shape, inertia, weight, and other material properties. It is still an open question whether the control of grasp force occurs at the level of individual fingers or whether it is also influenced by the control and the signals from the other fingers of the same hand. In this work, we approached this question by asking participants to lift, transport, and replace a sensorized object, using three- and four-digit grasp. Tactile input was altered by covering participant's fingertips with a rubber thimble, which reduced the reliability of the tactile sensory input. In different experimental conditions, we covered between one and three fingers opposing the thumb. Normal forces at each finger and the thumb were recorded while grasping and holding the object, with and without the thimble. Consistently with previous studies, reducing tactile sensitivity increased the overall grasping force. The gasping force increased in the covered finger, whereas it did not change from baseline in the remaining bare fingers (except the thumb for equilibrium constraints). Digit placement and object tilt were not systematically affected by rubber thimble conditions. Our results suggest that, in each finger opposing thumb, digit normal force is controlled locally in response to the applied tactile perturbation.

A review of the neurobiomechanical processes underlying secure gripping in object manipulation

O'SHEA, H. and S. J. Redmond. A review of the neurobiomechanical processes underlying secure gripping in object manipulation. NEUROSCI BIOBEHAV REV 286-300, 2021. Humans display skilful control over the objects they manipulate, so much so that biomimetic systems have yet to emulate this remarkable behaviour. Two key control processes are assumed to facilitate such dexterity: predictive cognitive-motor processes that guide manipulation procedures by anticipating action outcomes; and reactive sensorimotor processes that provide important error-based information for movement adaptation. Notwithstanding increased interdisciplinary research interest in object manipulation behaviour, the complexity of the perceptual-sensorimotor-cognitive processes involved and the theoretical divide regarding the fundamentality of control mean that the essential mechanisms underlying manipulative action remain undetermined. In this paper, following a detailed discussion of the theoretical and empirical bases for understanding human dexterous movement, we emphasise the role of tactile-related sensory events in secure object handling, and consider the contribution of certain biophysical and biomechanical phenomena. We aim to provide an integrated account of the current state-of-art in skilled human-object interaction that bridges the literature in neuroscience, cognitive psychology, and biophysics. We also propose novel directions for future research exploration in this area.

Neural and Behavioral Evidence for Vibrotactile Beat Perception and Bimodal Enhancement

The ability to synchronize movements to a rhythmic stimulus, referred to as sensorimotor synchronization (SMS), is a behavioral measure of beat perception. Although SMS is generally superior when rhythms are presented in the auditory modality, recent research has demonstrated near-equivalent SMS for vibrotactile presentations of isochronous rhythms [Ammirante, P., Patel, A. D., & Russo, F. A. Synchronizing to auditory and tactile metronomes: A test of the auditory-motor enhancement hypothesis. Psychonomic Bulletin & Review, 23, 1882-1890, 2016]. The current study aimed to replicate and extend this study by incorporating a neural measure of beat perception. Nonmusicians were asked to tap to rhythms or to listen passively while EEG data were collected. Rhythmic complexity (isochronous, nonisochronous) and presentation modality (auditory, vibrotactile, bimodal) were fully crossed. Tapping data were consistent with those observed by Ammirante et al. (2016), revealing near-equivalent SMS for isochronous rhythms across modality conditions and a drop-off in SMS for nonisochronous rhythms, especially in the vibrotactile condition. EEG data revealed a greater degree of neural entrainment for isochronous compared to nonisochronous trials as well as for auditory and bimodal compared to vibrotactile trials. These findings led us to three main conclusions. First, isochronous rhythms lead to higher levels of beat perception than nonisochronous rhythms across modalities. Second, beat perception is generally enhanced for auditory presentations of rhythm but still possible under vibrotactile presentation conditions. Finally, exploratory analysis of neural entrainment at harmonic frequencies suggests that beat perception may be enhanced for bimodal presentations of rhythm.

Static Weight Perception Through Skin Stretch and Kinesthetic Information: Detection Thresholds, JNDs, and PSEs

We examined the contributions of kinesthetic and skin stretch cues to static weight perception. In three psychophysical experiments, several aspects of static weight perception were assessed by asking participants either to detect on which hand a weight was presented or to compare between two weight cues. Two closed-loop controlled haptic devices were used to present cutaneous and kinesthetic weights, in isolation and together, with a precision of 0.05 g. Our results show that combining skin stretch and kinesthetic information leads to better weight detection thresholds than presenting uni-sensory cues does. For supra-threshold stimuli, Weber fractions were 22-44%. Kinesthetic information was less reliable for lighter weights, while both sources of information were equally reliable for weights up to 300 g. Weight was perceived as equally heavy regardless of whether skin stretch and kinesthetic cues were presented together or alone. Data for lighter weights complied with an Optimal Integration model, while for heavier weights, measurements were closer to predictions from a Sensory Capture model. The presence of correlated noise might explain this discrepancy, since that would shift predictions from the Optimal Integration model towards our measurements. Our experiments provide device-independent perceptual measures, and can be used to inform, for instance, skin stretch device design.

Increased visual bias in children with developmental coordination disorder: Evidence from a visual-tactile temporal order judgment task

BACKGROUND

Previous studies have suggested that children with developmental coordination disorder (DCD) rely heavily on vision to perform movements, which may contribute to their clumsy movements. However, few studies have objectively and quantitatively investigated the perceptual biases of children with DCD.

METHODS

A visual-tactile temporal order judgment (TOJ) task was used to measure and compare the perceptual biases of 19 children with DCD and 19 age- and sex-matched typically developing children. The point of subjective equality, which demonstrates when "visual first" and "tactile first" judgment probabilities are equal (50%), obtained by analyzing the results of the visual-tactile TOJ task, was used as an indicator of perceptual biases. Further, variables (age and manual dexterity in all participants; motor function, autism spectrum disorder and attention-deficit hyperactivity disorder traits, and depressive symptoms in children with DCD) associated with perceptual biases were examined with correlation analysis.

RESULTS

Children with DCD had significantly stronger visual bias than typically developing children. Overall correlation analysis showed that increased visual bias was significantly correlated with poor manual dexterity.

CONCLUSION

Children with DCD had a strong visual bias, which was associated with poor manual dexterity.

Information about contact force and surface texture is mixed in the firing rates of cutaneous afferent neurons

Cutaneous mechanoreceptors in our hands gather information about the objects we handle. Tactile fibers encode mixed information about contact events and object properties. Neural coding in tactile afferents is typically studied by varying a single aspect of tactile stimuli, avoiding the confounds of real-world haptic interactions. We instead record responses of small populations of dorsal root ganglia (DRG) neurons to variable tactile stimuli and find that neurons primarily respond to force, though some texture information can be detected. Tactile nerve fibers convey information about many features of haptic interactions, including the force and speed of contact, as well as the texture and shape of the objects being handled. How we perceive these object features is relatively unaffected by the forces and movements we use when interacting with the object. Because signals related to contact events and object properties are mixed in the responses of tactile fibers, our ability to disentangle these different components of our tactile experience implies that they are demultiplexed as they propagate along the neuraxis. To understand how texture and contact mechanics are encoded together by tactile fibers, we studied the activity of multiple neurons recorded simultaneously in the cervical DRG of two anesthetized rhesus monkeys while textured surfaces were applied to the glabrous skin of the fingers and palm using a handheld probe. A transducer at the tip of the textured probe measured contact forces as tactile stimuli were applied at different locations on the finger-pads and palm. We examined how a sample population of DRG neurons encode force and texture and found that firing rates of individual neurons are modulated by both force and texture. In particular, slowly adapting (SA) neurons were more responsive to force than texture, and rapidly adapting (RA) neurons were more responsive to texture than force. Although force could be decoded accurately throughout the entire contact interval, texture signals were most salient during onset and offset phases of the contact interval.NEW & NOTEWORTHY Cutaneous mechanoreceptors in our hands gather information about the objects we handle. Tactile fibers encode mixed information about contact events and object properties. Neural coding in tactile afferents is typically studied by varying a single aspect of tactile stimuli, avoiding the confounds of real-world haptic interactions. We instead record responses of small populations of DRG neurons to variable tactile stimuli and find that neurons primarily respond to force, though some texture information can be detected.

Sensory-to-Motor Overflow: Cooling Foot Soles Impedes Squat Jump Performance

Evidence from recent studies on animals and humans suggest that neural overflow from the primary sensory cortex (S1) to the primary motor cortex (M1) may play a critical role in motor control. However, it is unclear if whole-body maximal motor tasks are also governed by this mechanism. Maximum vertical squat jumps were performed by 15 young adults before cooling, then immediately following a 15-min cooling period using an ice-water bath for the foot soles, and finally immediately following a 15-min period of natural recovery from cooling. Jump heights were, on average, 3.1 cm lower immediately following cooling compared to before cooling (p = 3.39 × 10⁻⁸) and 1.9 cm lower following natural recovery from cooling (p = 0.00124). The average vertical ground reaction force (vGRF) was also lower by 78.2 N in the condition immediately following cooling compared to before cooling (p = 8.1 × 10⁻⁵) and 56.7N lower following natural recovery from cooling (p = 0.0043). The current study supports the S1-to-M1 overflow mechanism in a whole-body dynamic jump.

Impacts of different fabric scissor designs on physical demands and performance in simulated fabric cutting tasks

While there is wide evidence that the occupational use of hand tools increases the risk of musculoskeletal disorder, evidence is limited regarding manual scissors, commonly used by custom tailors for bespoke garment production. We assessed whether scissor design impacts physical demands (muscle activity, perceived discomfort, and wrist posture) and task performance (quality and perceived efficiency). Twenty-four novice volunteers each completed simulated cutting tasks in 24 conditions involving the factorial combinations of four scissor designs (SD), three workstation heights, and two fabric types. SD significantly affected all outcome measures, and differences between SDs were consistent across workstation heights and fabric types. Two wide-handles scissors appeared superior overall, which may be related to the distinct grip type employed with this type of design. These results suggest that careful scissor selection has the potential to both reduce injury risk and enhance performance during fabric cutting tasks, though future testing is needed under more realistic conditions.