Parallel processing of somatosensory information: a theory

Neural encoding of actual and imagined touch within human posterior parietal cortex

In the human posterior parietal cortex (PPC), single units encode high-dimensional information with partially mixed representations that enable small populations of neurons to encode many variables relevant to movement planning, execution, cognition, and perception. Here, we test whether a PPC neuronal population previously demonstrated to encode visual and motor information is similarly engaged in the somatosensory domain. We recorded neurons within the PPC of a human clinical trial participant during actual touch presentation and during a tactile imagery task. Neurons encoded actual touch at short latency with bilateral receptive fields, organized by body part, and covered all tested regions. The tactile imagery task evoked body part-specific responses that shared a neural substrate with actual touch. Our results are the first neuron-level evidence of touch encoding in human PPC and its cognitive engagement during a tactile imagery task, which may reflect semantic processing, attention, sensory anticipation, or imagined touch.

Neural Basis of Touch and Proprioception in Primate Cortex

The sense of proprioception allows us to keep track of our limb posture and movements and the sense of touch provides us with information about objects with which we come into contact. In both senses, mechanoreceptors convert the deformation of tissues-skin, muscles, tendons, ligaments, or joints-into neural signals. Tactile and proprioceptive signals are then relayed by the peripheral nerves to the central nervous system, where they are processed to give rise to percepts of objects and of the state of our body. In this review, we first examine briefly the receptors that mediate touch and proprioception, their associated nerve fibers, and pathways they follow to the cerebral cortex. We then provide an overview of the different cortical areas that process tactile and proprioceptive information. Next, we discuss how various features of objects-their shape, motion, and texture, for example-are encoded in the various cortical fields, and the susceptibility of these neural codes to attention and other forms of higher-order modulation. Finally, we summarize recent efforts to restore the senses of touch and proprioception by electrically stimulating somatosensory cortex. © 2018 American Physiological Society. Compr Physiol 8:1575-1602, 2018.

Passive exercise to improve quality of life, activities of daily living, care burden and cognitive functioning in institutionalized older adults with dementia - a randomized controlled trial study protocol

Background

Dementia affects cognitive functioning, physical functioning, activities of daily living (ADLs), and quality of life (QOL). Pharmacological treatments to manage, cure or prevent dementia remain controversial. Therefore development of non-pharmacological approaches to prevent, or at least delay the onset and progression of dementia is urgently needed. Passive exercise is proposed to be such a non-pharmacological alternative. This study primarily aims to investigate the effects of three different forms of passive exercise on QOL and ADLs of institutionalized patients with dementia. The secondary aims are to assess the effects of three different forms of passive exercise on cognitive functioning and physical functioning of institutionalized patients with dementia as well as on care burden of both the primary formal and primary informal caregivers of these patients.

Methods

This is a multicenter randomized controlled trial. Three forms of passive exercise are distinguished; motion simulation (MSim), whole body vibration (WBV), and a combination of both MSim + WBV. Intervention effects are compared to a control group receiving regular care. Institutionalized patients with dementia follow a six-week intervention program consisting of four 4–12 min sessions a week.

The primary outcome measures QOL and ADLs and secondary outcome measure care burden are assessed with questionnaires filled in by the primary formal and informal caregivers of the patient. The other secondary outcome measures cognitive and physical functioning are assessed by individual testing. The four groups are compared at baseline, after 6 weeks of intervention, and 2 weeks after the intervention has ended.

Discussion

This study will provide insight in the effects of different forms of passive exercise on QOL, ADLs, cognitive and physical functioning and care burden of institutionalized patients with dementia and their primary formal and informal caregivers. The results of this study might support the idea that passive exercise can be an efficient alternative for physical activity for patients not able to be or stay involved in active physical exercise.

Trial registration

The Netherlands National Trial Register (NTR6290). Retrospectively registered 29 March 2017.

Tactile texture signals in primate primary somatosensory cortex and their relation to subjective roughness intensity

This study investigated the hypothesis that a simple intensive code, based on mean firing rate, could explain the cortical representation of subjective roughness intensity and its invariance with scanning speed. We examined the sensitivity of neurons in the cutaneous, finger representation of primary somatosensory cortex (S1) to a wide range of textures [1 mm high, raised-dot surfaces; spatial periods (SPs), 1.5-8.5 mm], scanned under the digit tips at different speeds (40-115 mm/s). Since subjective roughness estimates show a monotonic increase over this range and are independent of speed, we predicted that the mean firing rate of a subgroup of S1 neurons would share these properties. Single-unit recordings were made in four alert macaques (areas 3b, 1 and 2). Cells whose discharge rate showed a monotonic increase with SP, independent of speed, were particularly concentrated in area 3b. Area 2 was characterized by a high proportion of cells sensitive to speed, with or without texture sensitivity. Area 1 had intermediate properties. We suggest that area 3b and most likely area 1 play a key role in signaling roughness intensity, and that a mean rate code, signaled by both slowly and rapidly adapting neurons, is present at the level of area 3b. Finally, the substantial proportion of neurons that showed a monotonic change in discharge limited to a small range of SPs (often independent of response saturation) could play a role in discriminating smaller changes in SP.

Whole body vibration improves cognition in healthy young adults

This study investigated the acute effects of passive whole body vibration (WBV) on executive functions in healthy young adults. Participants (112 females, 21 males; age: 20.5±2.2 years) underwent six passive WBV sessions (frequency 30 Hz, amplitude approximately 0.5 mm) and six non-vibration control sessions of two minutes each while sitting on a chair mounted on a vibrating platform. A passive WBV session was alternated with a control session. Directly after each session, performance on the Stroop Color-Block Test (CBT), Stroop Color-Word Interference Test (CWIT), Stroop Difference Score (SDS) and Digit Span Backward task (DSBT) was measured. In half of the passive WBV and control sessions the test order was CBT-CWIT-DSBT, and DSBT-CBT-CWIT in the other half. Passive WBV improved CWIT (p = 0.009; effect size r = 0.20) and SDS (p = 0.034; r = 0.16) performance, but only when the CBT and CWIT preceded the DSBT. CBT and DSBT performance did not change. This study shows that two minutes passive WBV has positive acute effects on attention and inhibition in young adults, notwithstanding their high cognitive functioning which could have hampered improvement. This finding indicates the potential of passive WBV as a cognition-enhancing therapy worth further evaluation, especially in persons unable to perform active forms of exercise.

Modality-based organization of ascending somatosensory axons in the direct dorsal column pathway

The long-standing doctrine regarding the functional organization of the direct dorsal column (DDC) pathway is the "somatotopic map" model, which suggests that somatosensory afferents are primarily organized by receptive field instead of modality. Using modality-specific genetic tracing, here we show that ascending mechanosensory and proprioceptive axons, two main types of the DDC afferents, are largely segregated into a medial-lateral pattern in the mouse dorsal column and medulla. In addition, we found that this modality-based organization is likely to be conserved in other mammalian species, including human. Furthermore, we identified key morphological differences between these two types of afferents, which explains how modality segregation is formed and why a rough "somatotopic map" was previously detected. Collectively, our results establish a new functional organization model for the mammalian direct dorsal column pathway and provide insight into how somatotopic and modality-based organization coexist in the central somatosensory pathway.

The functional architecture of S1 during touch observation described with 7 T fMRI

Recent studies indicate that the primary somatosensory cortex (S1) is active not only when touch is physically perceived but also when it is merely observed to be experienced by another person. This social responsivity of S1 has important implications for our understanding of S1 functioning. However, S1 activity during touch observation has not been characterized in great detail to date. We focused on two features of the S1 functional architecture during touch observation, namely the topographical arrangement of index and middle finger receptive fields (RFs), and their dynamic shrinkage during concurrent activation. Both features have important implications for human behavior. We conducted two fMRI studies at 7 T, one where touch was physically perceived, and one where touch was observed. In the two experiments, participants either had their index finger and/or middle finger stimulated using paintbrushes, or just observed similar touch events on video. Our data show that observing and physically experiencing touch elicits overlapping activity changes in S1. In addition, observing touch to the index finger or the middle finger alone evoked topographically arranged activation foci in S1. Importantly, when co-activated, the index and middle finger RFs not only shrank during physical touch perception, but also during touch observation. Our data, therefore, indicate a similarity between the functional architecture of S1 during touch observation and physical touch perception with respect to single-digit topography and RF shrinkage. These results may allow the tentative conclusion that even primary somatosensory experiences, such as physical touch perception, can be shared amongst individuals.

Anisotropic distribution of thalamocortical boutons in barrels

A characteristic feature of the somatosensory cortex in rodents is the presence of discrete cellular aggregates in layer 4 (barrels) that process input from the mystacial vibrissae. Just like thalamic cells that relay vibrissal information to the barrels, barrel cells display directional preference to whisker motion. The present study examined whether the projection of single thalamic cells into a barrel is consistent with the existence of an orderly map of direction preference. The direction preference of single thalamic cells was assessed, and axonal projections were visualized after juxtacellular labeling with biotinylated dextran. Results show that the terminal field of individual thalamic neurons in a barrel is markedly anisotropic and that the location of boutons with respect to the somatotopic map is either positively or negatively correlated with the angular tuning of the thalamic neuron. These results indicate that angular tuning is not represented across a systematic map with fixed anteroposterior/mediolateral coordinates in a barrel. The actual significance of the direction-dependent segregation of thalamocortical terminals in barrels may only come to light in the context of active sensing.

Molecular identification of rapidly adapting mechanoreceptors and their developmental dependence on ret signaling

In mammals, the first step in the perception of form and texture is the activation of trigeminal or dorsal root ganglion (DRG) mechanosensory neurons, which are classified as either rapidly (RA) or slowly adapting (SA) according to their rates of adaptation to sustained stimuli. The molecular identities and mechanisms of development of RA and SA mechanoreceptors are largely unknown. We found that the "early Ret(+)" DRG neurons are RA mechanoreceptors, which form Meissner corpuscles, Pacinian corpuscles, and longitudinal lanceolate endings. The central projections of these RA mechanoreceptors innervate layers III through V of the spinal cord and terminate within discrete subdomains of the dorsal column nuclei. Moreover, mice lacking Ret signaling components are devoid of Pacinian corpuscles and exhibit a dramatic disruption of RA mechanoreceptor projections to both the spinal cord and medulla. Thus, the early Ret(+) neurons are RA mechanoreceptors and Ret signaling is required for the assembly of neural circuits underlying touch perception.

Correlation of spatial event plots with simulated population responses of mechanoreceptive fibers

The experimental setup for generating spatial event plots (SEPs) from single mechanoreceptive fibers of the skin was computationally simulated. The generic fibers used in the simulations were similar to the rapidly adapting fibers (RAs), and had variable refractoriness and receptive-field size. The speed, lateral shift, and the contact width of the drum scanned across the receptive field of the fiber are adjustable parameters. The stimulus patterns used on the drum mimicked stimuli used by several other investigators. These were dot patterns, grating patterns, and the letter "E". First, the effects of simulation parameters on the SEPs were studied. The simulation output confirms the results of physiological experiments that SEPs contain information on the spatiotemporal resolution of the fiber. The next series of simulations involved generating SEPs of fibers obtained from the same or varying spatial distributions of receptive fields. Three hypothetical distributions were used: homogeneous rectangular, uniformly random, and Gaussian. The momentary population response at each case was found using the technique by Johansson and Vallbo (Brain Res 184: 353-366, 1980). The population responses were not isomorphic images of the stimulus patterns due to the variations in field sizes and locations. However, every fiber, no matter which distribution it came from, generated almost identical SEPs given similar response properties. Furthermore, the SEPs looked like the outline of the stimulus. These observations show that SEPs do not contain information about the population response. Therefore, reconstructing the population response using SEPs can result in misleading conclusions on central-nervous-system processing and should be viewed cautiously when formulating psychophysical/physiological linking hypotheses.

Organization of cortical processing for facial movements during licking in cats

We proposed that cortical organization for the execution of adequate licking in cats was processed under the control of two kinds of affiliated groups for face and jaw & tongue movements (Hiraba H, Sato T. 2005A. Cerebral control of face, jaw, and tongue movements in awake cats: Changes in regional cerebral blood flow during lateral feeding Somatosens Mot Res 22:307-317). We assumed the cortical organization for face movements from changes in MRN (mastication-related neuron) activities recorded at area M (motor cortex) and orofacial behaviors after the lesion in the facial SI (facial region in the primary somatosensory cortex). Although we showed the relationship between facial SI (area 3b) and area M (area 4delta), the property of area C (area 3a) was not fully described. The aim of this present study is to investigate the functional role of area C (the anterior part of the coronal sulcus) that transfers somatosensory information in facial SI to area M, as shown in a previous paper (Hiraba H. 2004. The function of sensory information from the first somatosensory cortex for facial movements during ingestion in cats Somatosens Mot Res 21:87-97). We examined the properties of MRNs in area C and changes in orofacial behaviors after the area C or area M lesion. MRNs in area C had in common RFs in the lingual, perioral, and mandibular parts, and activity patterns of MRNs showed both post- and pre-movement types. Furthermore, cats with the area C lesion showed similar disorders to cats with the area M lesion, such as the dropping of food from the contralateral mouth, prolongation of the period of ingestion and mastication, and so on. From these results, we believe firmly the organization of unilateral cortical processing in facial SI, area C, and area M for face movements during licking.

Laterodorsal nucleus of the thalamus: A processor of somatosensory inputs

The laterodorsal (LD) nucleus of the thalamus has been considered a "higher order" nucleus that provides inputs to limbic cortical areas. Although its functions are largely unknown, it is often considered to be involved in spatial learning and memory. Here we provide evidence that LD is part of a hitherto unknown pathway for processing somatosensory information. Juxtacellular and extracellular recordings from LD neurons reveal that they respond to vibrissa stimulation with short latency (median = 7 ms) and large magnitude responses (median = 1.2 spikes/stimulus). Most neurons (62%) had large receptive fields, responding to six and more individual vibrissae. Electrical stimulation of the trigeminal nucleus interpolaris (SpVi) evoked short latency responses (median = 3.8 ms) in vibrissa-responsive LD neurons. Labeling produced by anterograde and retrograde neuroanatomical tracers confirmed that LD neurons receive direct inputs from SpVi. Electrophysiological and neuroanatomical analyses revealed also that LD projects upon the cingulate and retrosplenial cortex, but has only sparse projections to the barrel cortex. These findings suggest that LD is part of a novel processing stream involved in spatial orientation and learning related to somatosensory cues.

Encoding of stimulus frequency and sensor motion in the posterior medial thalamic nucleus

In all sensory systems, information is processed along several parallel streams. In the vibrissa-to-barrel cortex system, these include the lemniscal system and the lesser-known paralemniscal system. The posterior medial nucleus (POm) is the thalamic structure associated with the latter pathway. Previous studies suggested that POm response latencies are positively correlated with stimulation frequency and negatively correlated with response duration, providing a basis for a phase locked loop-temporal decoding of stimulus frequency. We tested this hypothesis by analyzing response latencies of POm neurons, in both awake and anesthetized rats, to vibrissae deflections at frequencies between 0.3 and 11 Hz. We found no significant, systematic correlation between stimulation frequency and the latency or duration of POm responses. We obtained similar findings from recording in awake rats, in rats under different anesthetics, and in anesthetized rats in which the reticular activating system was stimulated. These findings suggest that stimulus frequency is not reliably reflected in response latency of POm neurons. We also tested the hypothesis that POm neurons respond preferentially to sensor motion, that is, they respond to whisking in air, without contacts. We recorded from awake, head-restrained rats while monitoring vibrissae movements. All POm neurons responded to passive whisker deflections, but none responded to noncontact whisking. Thus like their counterparts in the trigeminal ganglion, POm neurons may not reliably encode whisking kinematics. These observations suggest that POm neurons might not faithfully encode vibrissae inputs to provide reliable information on vibrissae movements or contacts.

Auditory cortex mapmaking: principles, projections, and plasticity

Maps of sensory receptor epithelia and computed features of the sensory environment are common elements of auditory, visual, and somatic sensory representations from the periphery to the cerebral cortex. Maps enhance the understanding of normal neural organization and its modification by pathology and experience. They underlie the derivation of the computational principles that govern sensory processing and the generation of perception. Despite their intuitive explanatory power, the functions of and rules for organizing maps and their plasticity are not well understood. Some puzzles of auditory cortical map organization are that few complete receptor maps are available and that even fewer computational maps are known beyond primary cortical areas. Neuroanatomical evidence suggests equally organized connectional patterns throughout the cortical hierarchy that might underlie map stability. Here, we consider the implications of auditory cortical map organization and its plasticity and evaluate the complementary role of maps in representation and computation from an auditory perspective.

Deviation from Weber's Law in the Non-Pacinian I Tactile Channel: A Psychophysical and Simulation Study of Intensity Discrimination

This study involves psychophysical experiments and computer simulations to investigate intensity discrimination in the non-Pacinian I (NP I) tactile channel. The simulations were based on an established population model for rapidly adapting mechanoreceptive fibers (Güuçlü & Bolanowski, 2004a). Several intensity codes were tested as decision criteria: number of active neurons, total spike count, maximal spike count, distribution of spike counts among the afferent population, and synchronization of spike times. Simulations that used the number of active fibers as the intensity code gave the most accurate results. However, the Weber fractions obtained from simulations are smaller than psychophysical Weber fractions, which suggests that only a subset of the afferent population is recruited for intensity discrimination during psychophysical experiments. Simulations could also capture the deviation from Weber's law, that is, the decrease of the Weber fraction as a function of the stimulus level, which was present in the psychophysical data. Since the psychophysical task selectively activated the NP I channel, the deviation effect is probably not due to the contribution of another tactile channel but rather is explicitly produced by the NP I channel. Moreover, because simulations with all tested intensity codes resulted in the same effect, the activity of the afferent population is sufficient to explain the deviation, without the need for a higher-order network. Depending on the intensity code used, the mechanical spread of the stimulus, rate-intensity functions of the tactile fibers, and the decreasing spike-phase jitter contribute to the deviation from Weber's law.

Decoding the auditory corticofugal systems

The status of the organization of the auditory corticofugal systems is summarized. These are among the largest pathways in the brain, with descending connections to auditory and non-auditory thalamic, midbrain, and medullary regions. Auditory corticofugal influence thus reaches sites immediately presynaptic to the cortex, sites remote from the cortex, as in periolivary regions that may have a centrifugal role, and to the cochlear nucleus, which could influence early central events in hearing. Other targets include the striatum (possible premotor functions), the amygdala and central gray (prospective limbic and motivational roles), and the pontine nuclei (for precerebellar control). The size, specificity, laminar origins, and morphologic diversity of auditory corticofugal axons is consonant with an interpretation of multiple roles in parallel descending systems.

Decoding the auditory corticofugal systems

The status of the organization of the auditory corticofugal systems is summarized. These are among the largest pathways in the brain, with descending connections to auditory and non-auditory thalamic, midbrain, and medullary regions. Auditory corticofugal influence thus reaches sites immediately presynaptic to the cortex, sites remote from the cortex, as in perolivary regions that may have a centrifugal role, and to the cochlear nucleus, which could influence early central events in hearing. Other targets include the striatum (possible premotor functions), the amygdala and central gray (prospective limbic and motivational roles), and the pontine nuclei (for precerebellar control). The size, specificity, laminar origins, and morphologic diversity of auditory corticofugal axons is consonant with an interpretation of multiple roles in parallel descending systems.

Degree of adaptability of the somatosensory cortex to change: Prospects for integration of bone-mounted dental prostheses

1. The topographic representation of the body surface in the somatosensory cortex provides an important model system for the in vivo study of neuronal plasticity, induced changes in somatotopy providing a direct measure of plasticity not available in most parts of the central nervous system. 2. Over the past two decades, animal experimentation in a number of laboratories has shown a remarkable degree of adaptability of the cortical representation following peripheral lesions and has had a widespread influence by challenging the once-accepted dogma that the brain is a structurally fixed organ. 3. Although some aspects of original stimulation will be missing, it is likely that receptors stimulated through bone conduction and compression by bone-mounted dental prostheses preserve some of the geometric and temporal relationships of original stimulation. By analogy with data obtained from the forearm representations, it would be expected that many features of the original cortical representations will be recreated. 4. There are also examples in the literature of perceptual learning without gross changes to the cortical representation (some being within a class of adaptability known as gain control) and it is likely that perceptual integration of many dental prostheses occurs within the limits of these neural adaptation mechanisms.

Response properties of whisker-related neurons in rat second somatosensory cortex

In addition to a primary somatosensory cortex (SI), the cerebral cortex of all mammals contains a second somatosensory area (SII); however, the functions of SII are largely unknown. Our aim was to explore the functions of SII by comparing response properties of whisker-related neurons in this area with their counterparts in the SI. We obtained extracellular unit recordings from narcotized rats, in response to whisker deflections evoked by a piezoelectric device, and compared response properties of SI barrel (layer IV) neurons with those of SII (layers II to VI) neurons. Neurons in both cortical areas have similar response latencies and spontaneous activity levels. However, SI and SII neurons differ in several significant properties. The receptive fields of SII neurons are at least five times as large as those of barrel neurons, and they respond equally strongly to several principal whiskers. The response magnitude of SII neurons is significantly smaller than that of neurons in SI, and SII neurons are more selective for the angle of whisker deflection. Furthermore, whereas in SI fast-spiking (inhibitory) and regular-spiking (excitatory) units have different spontaneous and evoked activity levels and differ in their responses to stimulus onset and offset, SII neurons do not show significant differences in these properties. The response properties of SII neurons suggest that they are driven by thalamic inputs that are part of the paralemniscal system. Thus whisker-related inputs are processed in parallel by a lemniscal system involving SI and a paralemniscal system that processes complimentary aspects of somatosensation.

Tonotopic and heterotopic projection systems in physiologically defined auditory cortex

Combined physiological and connectional studies show significant non-topographic extrinsic projections to frequency-specific domains in the cat auditory cortex. These frequency-mismatched loci in the thalamus, ipsilateral cortex, and commissural system complement the predicted topographic and tonotopic projections. Two tonotopic areas, the primary auditory cortex (AI) and the anterior auditory field (AAF), were electrophysiologically characterized by their frequency organization. Next, either cholera toxin beta subunit or cholera toxin beta subunit gold conjugate was injected into frequency-matched locations in each area to reveal the projection pattern from the thalamus and cortex. Most retrograde labeling was found at tonotopically appropriate locations within a 1 mm-wide strip in the thalamus and a 2-3 mm-wide expanse of cortex (approximately 85%). However, approximately 13-30% of the neurons originated from frequency-mismatched locations far from their predicted positions in thalamic nuclei and cortical areas, respectively. We propose that these heterotopic projections satisfy at least three criteria that may be necessary to support the magnitude and character of plastic changes in physiological studies. First, they are found in the thalamus, ipsilateral and commissural cortex; since this reorganization could arise from any of these routes and may involve each, such projections ought to occur in them. Second, they originate from nuclei and areas with or without tonotopy; it is likely that plasticity is not exclusively shaped by spectral influences and not limited to cochleotopic regions. Finally, the projections are appropriate in magnitude and sign to plausibly support such rearrangements; given the rapidity of some aspects of plastic changes, they should be mediated by substantial existing connections. Alternative roles for these heterotopic projections are also considered.

Motor and parietal cortical areas both underlie kinaesthesia

Tendon vibration has long been known to evoke perception of illusory movements through activation of muscle spindle primary endings. Few studies, however, have dealt with the cortical processes resulting in these kinaesthetic illusions. We conceived an fMRI experiment to investigate the cortical structures taking part in these illusory perceptions. Since muscle spindle afferents project onto different cortical areas involved in motor control it was necessary to discriminate between activation related to sensory processes and activation related to perceptual processes. To this end, we designed and compared different conditions. In two illusion conditions, covibration at different frequencies of the tendons of the right wrist flexor and extensor muscle groups evoked perception of slow or fast illusory movements. In a no illusion condition, covibration at the same frequency of the tendons of these antagonist muscle groups did not evoke a sensation of movement. Results showed activation of most cortical areas involved in sensorimotor control in both illusion conditions. However, in most areas, activation tended to be larger when the movement perceived was faster. In the no illusion condition, motor and premotor areas were little or not activated. Specific contrasts showed that perception of an illusory movement was specifically related to activation in the left premotor, sensorimotor, and parietal cortices as well as in bilateral supplementary motor and cingulate motor areas. We conclude that activation in motor as well as in parietal areas is necessary for a kinaesthetic sensation to arise.

Dynamic representational plasticity in sensory cortex

Studies of the effects of peripheral and central lesions, perceptual learning and neurochemical modification on the sensory representations in cortex have had a dramatic effect in alerting neuroscientists and therapists to the reorganizational capacity of the adult brain. An intriguing aspect of some of these investigations, such as partial peripheral denervation, is the short-term expression of these changes. Indeed, in visual cortex, auditory cortex and somatosensory cortex loss of input from a region of the peripheral receptor epithelium (retinal, basilar and cutaneous, respectively) induces rapid expression of ectopic, or expanded, receptive fields of affected neurons and reorganization of topographic maps to fill in the representation of the denervated area. The extent of these changes can, in some cases, match the maximal extents demonstrated with chronic manipulations. The rapidity, and reversibility, of the effects rules out many possible explanations which involve synaptic plasticity and points to a capacity for representational plasticity being inherent in the circuitry of a topographic pathway. Consequently, topographic representations must be considered as manifestations of physiological interaction rather than as anatomical constructs. Interference with this interaction can produce an unmasking of previously inhibited responsiveness. Consideration of the nature of masking inhibition which is consistent with the precision and order of a topographic representation and which has a capacity for rapid plasticity requires, in addition to stimulus-driven inhibition, a source of tonic input from the periphery. Such input, acting locally to provide tonic inhibition, has been directly demonstrated in the somatosensory system and is consistent with results obtained in auditory and visual systems.

Modeling population responses of rapidly-adapting mechanoreceptive fibers

The population response of rapidly-adapting (RA) fibers is one component of the physiological substrate of the sense of touch. Herein, we describe a computational scheme based on the population-response model by K.O. Johnson (J. Neurophysiol. 37: 48-72, 1974) which we extended by permitting the capability to include the spatial distributions of receptors in the glabrous skin linked to RA fibers. The hypothetical cases simulated were rectangular, uniformly random and proximo-distally Gaussian distributions. Each spatial organization produced qualitatively distinct population-response profiles that also varied due to stimulus parameters. The effects of stimulus amplitude, average innervation density and contactor-probe location were studied by considering various response measures: number of active fibers, summated firing rate and the average firing rate of a subset of the modeled population. The outcome of the measures were statistically compared among simulated anatomical distributions. The response is the same for rectangular and uniformly random distributions, both of which have a homogeneous innervation density. However, the Gaussian distribution produced statistically different responses when the measure was not averaged over the subset population which represented the receptive field of a higher-order neuron. These results indicate that, as well as stimulus parameters, the anatomical organization is a significant determinant of the population response. Therefore, reconstructing population activity for testing psychophysical hypotheses must presently be done with care until the organization of the receptors within the skin has been clarified.

Fitting pieces in the peripheral nerve puzzle

Findings from comparative microneurography are reviewed, i.e., data obtained by exploring human nerves with tungsten electrodes or concentric needle electrodes under similar conditions. It has emerged that activity in single myelinated fibers originates near nodes of Ranvier. Other data have shown that Ranvier nodes tend to cluster in certain regions of a fascicle and belong to fibers of the same modality which innervate the same skin area. This segregation involves all four main classes of myelinated low-threshold skin afferents. Fiber populations of the same modality may act as peripheral projection modules involved in somatosensory processing of tactile stimuli to cognitive levels. The fiber bundle arrangement of the nerves may be important for conserving functional gnosis in conditions where peripheral nerve fibers are lost. This organization may also be critical as a substrate to promote reinnervation after nerve cut followed by peripheral nerve suture. It is therefore less critical for an outgrowing fiber to find its exact distal counterpart. Even if misguided outgrowth occurs into the endoneurial tube of a neighboring distal fiber of the same modality with an adjacent receptive field, function can be reestablished. A precise nerve topography might also be of significance for obtaining a functionally satisfactory recovery after avulsion injuries treated by nerve root implantation into the spinal cord. Thus, there is in man an ordered nerve fiber organization, both in the periphery and in the CNS, which may have profound functional significance both under normal conditions and in disease.

Patterns in the brain. Neuronal population coding in the somatosensory system

The aim of this article is to review some basic principles of neural coding, with an emphasis on mechanisms of stimulus representation in ensembles of neurons. The theory of "across-neuron response patterns" (ANRPs), first suggested by Thomas Young (1802) and fully developed by Robert Erickson (1963-2000), is summarized and applied to the problem of coding in primary afferent fibers and cortical neurons of the somatosensory system. The basic premise of the theory is that precise information about stimulus features cannot be encoded by single neurons, but is encoded by patterns of activity across populations of neurons. Different stimuli produce uniquely different patterns of ensemble activity (ANRPs)-discrimination between two stimuli is based on the absolute difference in total amount of activity (neural mass difference) of the ANRPs for those stimuli. Review of the literature shows that ANRPs and related population codes can accurately represent and differentiate among various stimulus parameters that cannot be distinguished by single neurons alone. Finally, the behavior of neuronal ensembles can be used to account for the sensory-perceptual changes associated with plasticity of thalamocortical circuits following selective sensorimotor deprivation or experience.

Distribution of neurons immunoreactive for calcium-binding proteins varies across areas of cat primary somatosensory cortex

The primary somatosensory (SI) cortex in the cat contains four cytoarchitectonic areas that appear to contain separate body representations and have different functions. We tested whether functional differences among these areas are reflected in the densities of neurons containing each of three calcium-binding proteins: parvalbumin (PV), calbindin (CB), and calretinin (CR). Colocalization experiments revealed that CR was localized in a population of neurons distinct from those containing PV or CB. The general laminar distributions of the three calcium-binding proteins were similar to those described in other species and cortical areas, but there were significant density differences in layers II and III across SI. The density of PV-immunoreactive neurons was higher in areas 3b and 1 than in areas 3a and 2. CB-immunoreactive neurons were found in higher densities in anterior SI than in posterior SI, and the pattern of CR-immunoreactive neurons was reciprocal to that of CB, with significantly higher densities in posterior regions of SI. Since the firing characteristics of nonpyramidal neurons appear to be related to their calcium-binding protein content, differences in regional distributions of these neurons in layers II and III may contribute to functional differences between the cytoarchitectonic areas of SI cortex.

Spatial attention modulates the cortical somatosensory representation of the digits in humans

The topographic organization of the primary somatosensory cortex adapts to alterations of afferent input. Here, electric source imaging was used to show that spatial attention modifies cortical somatosensory representations in humans. The cortical representation of the electrically stimulated digit 2 (resp. digits 2 and 3) of the right hand was more medial along the somatosensory area 3b in subjects who focused attention on digit 4 of the right hand, while it was more lateral when subjects attended digit 4 of the contralateral hand. This effect was very fast since the direction of attention was changed every 6 min. The results indicate that cortical somatosensory representations not only depend on afferent input but vary when spatial attention is directed towards different parts of the body.

Illusory arm movements activate cortical motor areas: a positron emission tomography study

Vibration at approximately 70 Hz on the biceps tendon elicits a vivid illusory arm extension. Nobody has examined which areas in the brain are activated when subjects perceive this kinesthetic illusion. The illusion was hypothesized to originate from activations of somatosensory areas normally engaged in kinesthesia. The locations of the microstructurally defined cytoarchitectonic areas of the primary motor (4a and 4p) and primary somatosensory cortex (3a, 3b, and 1) were obtained from population maps of these areas in standard anatomical format. The regional cerebral blood flow (rCBF) was measured with (15)O-butanol and positron emission tomography in nine subjects. The left biceps tendon was vibrated at 10 Hz (LOW), at 70 or 80 Hz (ILLUSION), or at 220 or 240 Hz (HIGH). A REST condition with eyes closed was included in addition. Only the 70 and 80 Hz vibrations elicited strong illusory arm extensions in all subjects without any electromyographic activity in the arm muscles. When the rCBF of the ILLUSION condition was contrasted to the LOW and HIGH conditions, we found two clusters of activations, one in the supplementary motor area (SMA) extending into the caudal cingulate motor area (CMAc) and the other in area 4a extending into the dorsal premotor cortex (PMd) and area 4p. When LOW, HIGH, and ILLUSION were contrasted to REST, giving the main effect of vibration, areas 4p, 3b, and 1, the frontal and parietal operculum, and the insular cortex were activated. Thus, with the exception of area 4p, the effects of vibration and illusion were associated with disparate cortical areas. This indicates that the SMA, CMAc, PMd, and area 4a were activated associated with the kinesthetic illusion. Thus, against our expectations, motor areas rather than somatosensory areas seem to convey the illusion of limb movement.

Origins of medial geniculate body projections to physiologically defined zones of rat primary auditory cortex

Medial geniculate body neurons projecting to physiologically identified subregions of rat primary auditory cortex (area 41, Te1) were labeled with horseradish peroxidase in adult rats. The goals were to determine the type(s) of projection neuron and the spatial arrangement of these cells with respect to thalamic subdivisions. Maps of best frequency were made with single neuron or unit cluster extracellular recording at depths of 500-800 microm, which correspond to layers III-IV in Nissl preparations. Tracer injections were made in different cortical isofrequency regions (2, 11, 22, or 38 kHz, respectively). Labeled neurons were plotted on representative sections upon which the architectonic subdivisions were drawn independently. Most of the cells of origin lay in the ventral division in every experiment. Injections at low frequencies labeled bands of neurons laterally in the ventral division; progressively more rostral deposits at higher frequencies labeled bands or clusters more medially in the ventral division, and through most of its caudo-rostral extent. Medial division labeling was variable. Labeled cells were always in the lateral half of the nucleus and were often scattered. There were few labeled cells in the dorsal division. Seven types of thalamocortical neuron were identified: ventral division cells had a tufted branching pattern, while medial division neurons have heterogeneous shapes and sizes and were larger. Dorsal division neurons had a radiate branching pattern. The size range of labeled neurons spanned that of Nissl stained neuronal somata. Area 41 may receive two types of thalamic projection: ventral division input is strongly convergent, highly topographic, spatially focal, and restricted to one type of neuron only, while the medial division projection is more divergent, coarsely topographical, involves multiple cortical areas, and has several varieties of projection neuron. Despite species differences in local circuitry, many facets of thalamocortical organization are conserved in phylogeny.

Sustained attention modulates the immediate effect of de-afferentation on the cortical representation of the digits: source localization of somatosensory evoked potentials in humans

Long-term cortical reorganization of the somatotopic arrangement of the digits after alterations of the peripheral input is well established. Studies on the immediate effects of manipulating peripheral input have shown conflicting results indicating that additional factors might modulate cortical reorganization. We present a source localization study using somatosensory evoked potentials (SEP) following electric stimulation of digits one and five before and during anaesthesia of digits two, three and four in 10 normal volunteers. When attention was directed to a stimulus at the dorsal hand, the 3D-distance between digits one and five decreased during as compared to before anaesthesia. In contrast, this distance enlarged when subjects were not attending a particular stimulus. In this condition most subjects focused their attention on the clear sensation of the de-afferented hand region. These results indicate that attention modulates the effect of immediate cortical reorganization of the hand area during partial deafferentation. As an hypothesis: it may be speculated that the sensation of the de-afferentation results in increased synchronized activity of the de-afferented somatosensory cortex and, thus, to its enlarged representation. Conversely, if attention is directed to a different hand region, the representations of the neighboring digits may expand into the de-afferented cortex.

Distribution of nociceptive neurons in the ventrobasal complex of macaque thalamus

In urethane-chloralose anesthetized Japanese macaques, the distribution of nociceptive neurons within the thalamic ventrobasal (VB) complex was studied. Nociceptive specific (NS) and wide dynamic range (WDR) neurons were found in the periphery of the contralateral integument compartment of the VB complex. Thus, they formed a shell at the perimeter of this compartment with a somatotopic organization. The compartment consisted of large parts of nucleus ventralis posteromedialis (VPM) and nucleus ventralis posterolateralis, pars caudalis (VPLc). NS neurons were located more caudally than WDR neurons. In the NS zone of VPM, the forehead was represented caudally, and oral structures rostrally. In the ventral NS zone of VPM, there was a sequential representation of the tongue, gum and mandibular skin from the medial to the lateral edge. The hand was represented medially in the NS zone of VPLc, and its representation dominated in the rostral NS zone. There was a sequential representation of the cervical, thoracic, lumbar, sacral and caudal segments mediolaterally along the dorsal VPLc. In the medial half of ventral NS zone of VPLc, the upper body half was represented, and in the lateral half, the lower body half. The foot was represented at or near the medial edge of lateral half. In the rostral WDR zone, the trunk and peripheral face were represented.

Temporal-code to rate-code conversion by neuronal phase-locked loops

Peripheral sensory activity follows the temporal structure of input signals. Central sensory processing uses also rate coding, and motor outputs appear to be primarily encoded by rate. I propose here a simple, efficient structure, converting temporal coding to rate coding by neuronal phase-locked loops (PLL). The simplest form of a PLL includes a phase detector (that is, a neuronal-plausible version of an ideal coincidence detector) and a controllable local oscillator that are connected in a negative feedback loop. The phase detector compares the firing times of the local oscillator and the input and provides an output whose firing rate is monotonically related to the time difference. The output rate is fed back to the local oscillator and forces it to phase-lock to the input. Every temporal interval at the input is associated with a specific pair of output rate and time difference values; the higher the output rate, the further the local oscillator is driven from its intrinsic frequency. Sequences of input intervals, which by definition encode input information, are thus represented by sequences of firing rates at the PLL's output. The most plausible implementation of PLL circuits is by thalamocortical loops in which populations of thalamic "relay" neurons function as phase detectors that compare the timings of cortical oscillators and sensory signals. The output in this case is encoded by the thalamic population rate. This article presents and analyzes the algorithmic and the implementation levels of the proposed PLL model and describes the implementation of the PLL model to the primate tactile system.

Plasticity of primary somatosensory cortex paralleling sensorimotor skill recovery from stroke in adult monkeys

Adult owl and squirrel monkeys were trained to master a small-object retrieval sensorimotor skill. Behavioral observations along with positive changes in the cortical area 3b representations of specific skin surfaces implicated specific glabrous finger inputs as important contributors to skill acquisition. The area 3b zones over which behaviorally important surfaces were represented were destroyed by microlesions, which resulted in a degradation of movements that had been developed in the earlier skill acquisition. Monkeys were then retrained at the same behavioral task. They could initially perform it reasonably well using the stereotyped movements that they had learned in prelesion training, although they acted as if key finger surfaces were insensate. However, monkeys soon initiated alternative strategies for small object retrieval that resulted in a performance drop. Over several- to many-week-long period, monkeys again used the fingers for object retrieval that had been used successfully before the lesion, and reacquired the sensorimotor skill. Detailed maps of the representations of the hands in SI somatosensory cortical fields 3b, 3a, and 1 were derived after postlesion functional recovery. Control maps were derived in the same hemispheres before lesions, and in opposite hemispheres. Among other findings, these studies revealed the following 1) there was a postlesion reemergence of the representation of the fingertips engaged in the behavior in novel locations in area 3b in two of five monkeys and a less substantial change in the representation of the hand in the intact parts of area 3b in three of five monkeys. 2) There was a striking emergence of a new representation of the cutaneous fingertips in area 3a in four of five monkeys, predominantly within zones that had formerly been excited only by proprioceptive inputs. This new cutaneous fingertip representation disproportionately represented behaviorally crucial fingertips. 3) There was an approximately two times enlargement of the representation of the fingers recorded in cortical area 1 in postlesion monkeys. The specific finger surfaces employed in small-object retrieval were differentially enlarged in representation. 4) Multiple-digit receptive fields were recorded at a majority of emergent, cutaneous area 3a sites in all monkeys and at a substantial number of area 1 sites in three of five postlesion monkeys. Such fields were uncommon in area 1 in control maps. 5) Single receptive fields and the component fields of multiple-digit fields in postlesion representations were within normal receptive field size ranges. 6) No significant changes were recorded in the SI hand representations in the opposite (untrained, intact) control hemisphere. These findings are consistent with "substitution" and "vicariation" (adaptive plasticity) models of recovery from brain damage and stroke.

[Proprioceptive and cutaneous evoked somatosensory potentials in the baboon: cycles of GABAergic excitability and inhibition]

Double stimulations induce deep and long-lasting inhibition (0-300 ms) of the P16-N30 components of somatosensory potentials (SEP) evoked by sciatic or sural nerve stimulation. This inhibition is evidenced on both S1 and M1 cortical areas, demonstrating similar course and duration, whatever the source (right or left limb) and/or the modality (extero- or proprioceptive) of conditioning and testing afferences. The depth of this inhibition depends on the relative amplitude of the conditioning to testing SEP. After muscle injection of a subconvulsive dose of bicuculline, tSEPs are facilitated when individually elicited. When double stimulations are used, the inhibition of the SEP test is sharply reduced (with a 30-ms interstimulus delay). However, disinhibition of the conditioned SEP does not depend on separate individual SEP facilitations. Cortical GABAergic type a circuits are likely to be involved in inhibition of the conditioned SEP. This inhibition would be a non-invasive image of inhibitions that preserve the specificities of sensory messages in primary areas.

Peripheral afferents with common function cluster in the median nerve and somatotopically innervate the human palm

Concentric needle electrodes with a central core diameter of 20-30 microm were used to explore median nerve fascicles in man. Such electrodes can simultaneously monitor subtle electrophysiological and topographical features even within parts of a fascicle. Single-unit recordings from myelinated fibres were more easily obtained at some intrafascicular sites than others. Typically, groups of identified myelinated fibres in these regions, possibly corresponding to a cluster of Ranvier nodes, tended to be fibres responding to stimuli of the same modality. These afferents innervated the glabrous skin of the human hand and fingers in a somatotopic manner. In particular, the somatotopy even seemed to be present at the receptor level in the skin. This novel aspect of peripheral nerve organisation is probably of fundamental importance for the interplay between peripheral and central processes involved in somatosensation both under normal conditions and in disease. Some clinical implications of the findings are discussed.

Application of focused ultrasound for research on pain

Short pulses of focused ultrasound can stimulate the superficial and deep-seated receptor structures of human tissues and induce different somatosensory sensations including, in particular, pain sensations. Focused ultrasound as a new artificial stimulus for inducing pain has a number of advantages related with its non-invasiveness, the possibility of the precise control of stimulus parameters and the location of its action. The experimental procedures and the results of the application of focused ultrasound as a painful stimulus in physiological research and in clinical practice are discussed. Data concerning various kinds of pain sensations, values of ultrasound thresholds of pain in different parts of the hand, as well as the discussion of the main effective factors of focused ultrasound responsible for the induction of pain, are presented in this review.

Trends in the anatomical organization and functional significance of the mammalian thalamus

The last decade has witnessed major changes in the experimental approach to the study of the thalamus and to the analysis of the anatomical and functional interrelations between thalamic nuclei and cortical areas. The present review focuses on the novel anatomical approaches to thalamo-cortical connections and thalamic functions in the historical framework of the classical studies on the thalamus. In the light of the most recent data it is here discussed that: a) the thalamus can subserve different functions according to functional changes in the cortical and subcortical afferent systems; b) the multifarious thalamic cellular entities play a crucial role in the different functional states.

Consequences of covert orienting to non-informative stimuli of different modalities: a unitary mechanism?

Reaction time (RT) to visual targets is lengthened following non-informative cues presented in the same location, or in different locations but in the same hemifield as the targets. RT lengthening is best accounted for by the voluntary suppression of an overt orienting toward the location of the cue: this veto produces an inhibition of the overall motor reactivity towards stimuli presented in the entire hemifield of the cue. This paper shows that ipsilateral inhibition is not unique to the visual system, since the same directional constraints in motor readiness are induced with somatosensory stimulation. RT is slower when a somatic target delivered on a shoulder is preceded by an ipsilateral somatic cue compared to a contralateral one. The neural control of these orienting tendencies may involve the superior colliculus, which contains overlapping maps of the visual, somatosensory and auditory peripheries. This suggestion is reinforced by the presence of cross-modal inhibitory effects in paradigms involving visual cues and somatic targets or somatic cues and visual targets. While the time course of ipsilateral inhibition is similar in the visual and the somatic modalities, cross-modal inhibitory effects are different and somehow complementary when visual cues precede somatic targets (early short-lasting inhibition) or, respectively, somatic cues precede visual targets (late, long-lasting inhibition). An additional finding is that crossed-uncrossed RT differences (CUDs), presumably due to the anatomical relations between stimulus and response, are present in both modalities.

[Somatosensory evoked potentials: morphology and interareal S1-M1 relationships in the baboon (Papio papio)]

Primary somatosensory potentials (SEPs) were elicited by electrical stimulation of the medial sciatic (proprioceptive) or sural (cutaneous) nerves. They were detected by contacts on SI and MI dura on both cortical sides against a cephalic reference. SEPs were averaged (n = 20). Primary SI SEP consisted of positive, then negative, PI6/N30 waves. N30 was absent from MI records. Local electrocoagulation of the SI cortex on one side has entailed some reduction, but not suppression of together the homolateral MI and contralateral SI and MI SEP. The residual SEPs have increased in latencies by a few milliseconds. Additional coagulation of the MI area on the same side has resulted in loss of the SI and MI SEP on the opposite hemisphere when evoked by a stimulation ipsilateral to this intact cortex. Normal SEPs were elicited from the intact cortex by any of the used stimulation. No evoked signal could be evidence from the lesioned areas. It was concluded that negligible passive electrical diffusion from any SEP area was present onto any of the other SEP reception sites. From close comparison between the different records, we came to the following propositions: each of the SI and MI areas harbours a neural mass generator for SEPs elicited by contralateral nerve stimulation. SI and MI SEPs cannot be directly elicited by ipsilateral stimulus. SI and MI SEP ipsilateral to the nerve stimulation are due to some cortico-cortical trans-sagittal excitatory message arising from the contralateral SI/MI areas. Data stand for exteroceptive or proprioceptive stimulation as well. The absence of ipsilateral direct spino-cortical projection for SEP evidenced under barbiturate does also exist in the baboon after total recovery of surgery. A scheme is given which summarizes these active relationships between somesthetic areas.

[Somatosensory evoked potentials: interference and perceptual masking of cutaneous afferents in man]

Somatosensory evoked potentials (SEP) are attenuated following double electrical stimulation of the fingers (II + III). This effect is observed at cervical (N13), parietal (N20-P27) and frontal (P22-N30) levels. We simultaneously observed in the same subjects that cutaneous perception of the test-shock is completely suppressed with interstimulus intervals (ISI) within a 0-10 msec range. With 25-30 msec ISI, the perceptive function totally recovers, but SEP inhibition remains at 50 % of the control. The SEP reduction does not result in a perception deficit as long as the cortical-test response exceeds 50% of control. These results suggest that: SEP inhibition could be a local but durable phenomenon occurring at both cervical and cortical levels. Cutaneous perception does not necessitate a maximal SEP development. The perceptive process involves other associative areas (5,7...) and is activated when the primary cortical activation exceeds a certain threshold which was found at 50% of the unconditioned response voltage.

Covert orienting to non-informative cues: reaction time studies

Lateralized, non-informative visual cues lengthen reaction time (RT) to successive targets flashed in the same hemified. Early ipsilateral RT facilitation is limited to the co-occurrence of cues and targets. Inhibition from visual cues has sensory components which do not depend on orienting, as well as attentional components which are limited to one side of the vertical meridian. An inhibition of RT to targets ipsilateral to the cues has been found with somatic or auditory cues and targets, and also when somatic targets follow visual cues or visual targets follow somatic cues. The results reviewed in this paper (1) are best accounted for by directional constraints in motor readiness which are induced by the voluntary suppression of an overt orienting toward the location of the cue; (2) indicate that similar mechanisms of covert orienting operate in the whole peripersonal and near extrapersonal space; and (3) point to a common neural substrate mediating both intramodal and cross-modal effects.

Visual areas in the dorsal and medial extrastriate cortices of the marmoset

To define the number and limits of the visual areas in the primate extrastriate cortex, the visuotopy of the dorsal convexity and medial wall was studied by electrophysiological recordings in five marmosets anaesthetised with sufentanil and nitrous oxide and paralysed with pancuronium bromide. We identified five visuotopic representations in and around the densely myelinated zone between visual area 2 (V2) and the posterior parietal cortex. Most of the densely myelinated zone is formed by the homologue of the owl monkey's dorsomedial area (DM); thus, we also termed this area DM in the marmoset. Within DM, the lower quadrant representation is continuous, with central vision represented laterally, peripheral vision medially, the horizontal meridian caudally, and the vertical meridian rostrally. In contrast, the upper quadrant representation is split, with the central portion represented at the lateral edge of DM on the dorsal surface, and the periphery along the midline. Two other visual field representations, corresponding to the dorsointermediate area (DI) and to a new subdivision termed the dorsoanterior area (DA), are also densely myelinated but can be distinguished from DM based on the separation of the bands of Baillarger and visual topography. In addition, a homologue of the medial visual area (M) was identified. Our results reveal a highly complex visuotopy in primate cortex, with local discontinuities in representation and borders between areas that are often not coincident with either the horizontal or the vertical meridian. The topography of the dorsal extrastriate cortex in the marmoset strongly suggests that both visual area 3 (V3) and the parietooccipital area (PO) of other primates are portions of a single visuotopic representation, DM, and calls into question the existence of visual areas with partial or quadrantic representations of the visual field.