Seeing the hand boosts feeling on the cheek

Tactile acuity improves when subjects look at the stimulated body part, even when vision does not provide any information relevant to touch. This “visual enhancement of touch” (VET), might involve modulation of primary somatosensory cortex (SI) processing by multimodal information related to the body. SI shows a characteristic somatotopic organization, with the face and hand represented laterally and adjacent to each other, and the foot represented more medially.The aim of this study was to investigate whether VET is limited to the viewed body part, spreads to all body parts, or generalizes only to body parts represented in SI closely to the viewed body part. Tactile acuity was assessed in healthy subjects on the hand, face and foot, while subjects viewed either their stimulated hand, their foot or were blindfolded: viewing the hand, compared to blindfold condition, enhanced tactile sensitivity on the hand and also on the face, but not on the foot. Conversely, viewing the foot, compared to blindfold condition, improved touch only on the foot. Two control experiments assured this effect was due to viewing the body and not to directing visuo-spatial attention toward the location of tactile stimulation. The present results show that VET acts accordingly to a somatotopic gradient based on SI organization, suggesting that this multisensory effect may occur within SI. This finding might have a possible application to facilitate the recovery of tactile deficits in patients with a lesion of somatosensory cortices.     

Early visual deprivation alters multisensory processing in peripersonal space

Multisensory peripersonal space develops in a maturational process that is thought to be influenced by early sensory experience. We investigated the role of vision in the effective development of audiotactile interactions in peripersonal space. Early blind (EB), late blind (LB) and sighted control (SC) participants were asked to lateralize auditory, tactile and audiotactile stimuli. The experiment was conducted with the hands uncrossed or crossed over the body midline in order to alter the relationship between personal and peripersonal spatial representations. First, we observed that the crossed posture results in a greater detrimental effect for tactile performance in sighted subjects but a greater deficit in auditory performance in early blind ones. This result is interpreted as evidence for a visually driven developmental process that automatically remaps tactile and proprioceptive spatial representation into an external framework. Second, we demonstrate that improved reaction times observed in the bimodal conditions in SC and LB exceeds that predicted by probability summation in both conditions of postures, indicating neural integration of different sensory information. In EB, nonlinear summation was obtained in the uncrossed but not in the crossed posture. We argue that the default use of an anatomically anchored reference system in EB prevents effective audiotactile interactions in the crossed posture due to the poorly aligned spatial coordinates of these two modalities in such conditions. Altogether, these results provide compelling evidence for the critical role of early vision in the development of multisensory perception and action control in peripersonal space.     

Bare hands and attention: evidence for a tactile representation of the human body

If brain lesions impair the allocation of attention to a representation of the body surface and the hand may serve as an attentional focus or “wand”, one might expect that somatosensory deficits caused by cerebral lesions would be ameliorated by contact with the ipsilesional hand.

To test this prediction, tactile detection tasks were administered to two subjects with right hemisphere lesions. Subject CB’s left tactile extinction was investigated in conditions in which the degree of contact between the right and left hands and the spatial relationship between his hands was systematically varied. His left tactile extinction was significantly reduced by touch of the right hand. Similarly, extinction at the left knee was ameliorated by touch of the knee by the right hand; touch of the right foot had no effect. Subject NC’s ability to detect a tactile stimulus delivered to the left side was systematically assessed in conditions in which the hands touched and the spatial relationship between the hands was varied. His ability to detect a touch on the left hand improved in conditions in which the left hand was touched by the right hand. This effect was not observed if direct contact between the two hands was prevented by inserting a thin cloth between the hands. For both subjects, placing the right hand in close proximity to the left hand or altering the spatial location of the hands relative to the body did not influence performance. These data demonstrate that the hand may serve as a conduit for attention and provide strong evidence for a distinct representation of the body surface that is at least in part independent of spatial representations.     

Activity patterns of human somatosensory cortex adapt dynamically to stimulus properties

Long-term synchronous tactile stimulation of two sites of the body results in integrated, overlapping cortical representations whereas asynchronous stimulation leads to segregated representations. To investigate the cortical capacity to adapt dynamically to stimulation properties 22 subjects were stimulated at digits 1, 3 and 5 of both hands in either random or fixed order. Changes in the functional organization of the somatosensory cortex were inferred by neuromagnetic source analysis based on somatosensory evoked magnetic fields. Compared to the stimulation in random sequence, the stimulation in fixed order revealed a reduction in distance between the cortical representation of D1 and D3. We conclude that the pattern of activation in the somatosensory cortex adapts dynamically to the spatio-temporal characteristics of the stimuli.     

From maps to form to space: Touch and the body schema

Evidence from patients has shown that primary somatosensory representations are plastic, dynamically changing in response to central or peripheral alterations, as well as experience. Furthermore, recent research has also demonstrated that altering body posture results in changes in the perceived sensation and localization of tactile stimuli. Using evidence from behavioral studies with brain-damaged and healthy subjects, as well as functional imaging, we propose that the traditional concept of the body schema should be divided into three components. First are primary somatosensory representations, which are representations of the skin surface that are typically somatotopically organized, and have been shown to change dynamically due to peripheral (usage, amputation, deafferentation) or central (lesion) modifications. Second, we argue for a mapping from a primary somatosensory representation to a secondary representation of body size and shape (body form representation). Finally, we review evidence for a third set of representations that encodes limb position and is used to represent the location of tactile stimuli relative to the subject using external, non-somatotopic reference frames (postural representations).     

When action is not enough: tool-use reveals tactile-dependent access to Body Schema

Proper motor control of our own body implies a reliable representation of body parts. This information is supposed to be stored in the Body Schema (BS), a body representation that appears separate from a more perceptual body representation, the Body Image (BI). The dissociation between BS for action and BI for perception, originally based on neuropsychological evidence, has recently become the focus of behavioural studies in physiological conditions. By inducing the rubber hand illusion in healthy participants, Kammers et al. (2009) showed perceptual changes attributable to the BI to which the BS, as indexed via motor tasks, was immune. To more definitively support the existence of dissociable body representations in physiological conditions, here we tested for the opposite dissociation, namely, whether a tool-use paradigm would induce a functional update of the BS (via a motor localization task) without affecting the BI (via a perceptual localization task). Healthy subjects were required to localize three anatomical landmarks on their right arm, before and after using the same arm to control a tool. In addition to this classical task-dependency approach, we assessed whether preferential access to the BS could also depend upon the way positional information about forearm targets is provided, to subsequently execute the same task. To this aim, participants performed either verbally or tactually driven versions of the motor and perceptual localization tasks. Results showed that both the motor and perceptual tasks were sensitive to the update of the forearm representation, but only when the localization task (perceptual or motor) was driven by a tactile input. This pattern reveals that the motor output is not sufficient per se, but has to be coupled with tactually mediated information to guarantee access to the BS. These findings shade a new light on the action-perception models of body representations and underlie how functional plasticity may be a useful tool to clarify their operational definition.     

Somatosensory cortical representation of the body size

The knowledge of the size of our own body parts is essential for accurately moving in space and efficiently interact with objects. A distorted perceptual representation of the body size often represents a core diagnostic criterion for some psychopathological conditions. The metric representation of the body was shown to depend on somatosensory afferences: local deafferentation indeed causes a perceptual distortion of the size of the anesthetized body part. A specular effect can be induced by altering the cortical map of body parts in the primary somatosensory cortex. Indeed, the present study demonstrates, in healthy adult participants, that repetitive Transcranial Magnetic Stimulation to the somatosensory cortical map of the hand in both hemispheres causes a perceptual distortion (i.e., an overestimation) of the size of the participants' own hand (Experiments 1–3), which does not involve other body parts (i.e., the foot, Experiment 2). Instead, the stimulation of the inferior parietal lobule of both hemispheres does not affect the perception of the own body size (Experiment 4). These results highlight the role of the primary somatosensory cortex in the building up and updating of the metric of body parts: somatosensory cortical activity not only shapes our somatosensation, it also affects how we perceive the dimension of our body.     

Visuotactile learning and body representation: an ERP study with rubber hands and rubber objects

We studied how the integration of seen and felt tactile stimulation modulates somatosensory processing, and investigated whether visuotactile integration depends on temporal contiguity of stimulation, and its coherence with a preexisting body representation. During training, participants viewed a rubber hand or a rubber object that was tapped either synchronously with stimulation of their own hand, or in an uncorrelated fashion. In a subsequent test phase, somatosensory event-related potentials (ERPs) were recorded to tactile stimulation of the left or right hand, to assess how tactile processing was affected by previous visuotactile experience during training. An enhanced somatosensory N140 component was elicited after synchronous, compared with uncorrelated, visuotactile training, irrespective of whether participants viewed a rubber hand or rubber object. This early effect of visuotactile integration on somatosensory processing is interpreted as a candidate electro-physiological correlate of the rubber hand illusion that is determined by temporal contiguity, but not by preexisting body representations. ERP modulations were observed beyond 200 msec poststimulus, suggesting an attentional bias induced by visuotactile training. These late modulations were absent when the stimulation of a rubber hand and the participant's own hand was uncorrelated during training, suggesting that preexisting body representations may affect later stages of tactile processing.     

Somatosensory saccades reveal the timing of tactile spatial remapping

Remapping tactile events from skin to external space is an essential process for human behaviour. It allows us to refer tactile sensations to their actual externally based location, by combining anatomically based somatosensory information with proprioceptive information about the current body posture. We examined the time course of tactile remapping by recording speeded saccadic responses to somatosensory stimuli delivered to the hands. We conducted two experiments in which arm posture varied (crossed or uncrossed), so that anatomical and external frames of reference were either put in spatial conflict or were aligned. The data showed that saccade onset latencies in the crossed hands conditions were slower than in the uncrossed hands condition, suggesting that, in the crossed hands condition, remapping had to be completed before a correct saccade could be executed. Saccades to tactile stimuli when the hands were crossed were sometimes initiated to the wrong direction and then corrected in-flight, resulting in a turn-around saccade. These turn-around saccades were more likely to occur in short-latency responses, compared to onset latencies of saccades that went straight to target. The latter suggests that participants were postponing their saccade until the time the tactile event was represented according to the current body posture. We propose that the difference between saccade onset latencies of crossed and uncrossed hand postures, and between the onset of a turn-around saccade and a straight saccade in the crossed hand posture, reveal the timing of tactile spatial remapping.     

The rubber hand illusion and its application to clinical neuroscience

The rubber hand illusion (RHI) is a perceptual experience which often occurs when an administered tactile stimulation of a person's real hand hidden from view, occurs synchronously with a corresponding visual stimulation of an observed rubber hand placed in full vision of the person in a position corresponding to where their real hand might normally be. The perceptual illusion is that the person feels a sense of "ownership" of the rubber hand which they are looking at. Most studies have focused on the underlying neural properties of the illusion and the experimental manipulations that lead to it. The illusion could also be used for exploring the sense of limb and prosthetic ownership for people after amputation. Cortical electrodes such as those used in sensorimotor stimulation surgery for pain may provide an opportunity to further understand the cortical representation of the illusion and possibly provide an opportunity to modulate the individual's sense of body ownership. Thus, the RHI might also be a critical tool for development of neurorehabilitative interventions that will be of great interest to the neurosurgical and rehabilitation communities.     

Tactile awareness and limb position in neglect: functional magnetic resonance imaging

We studied a patient with right parietal damage for whom tactile stimuli on the right/ipsilesional hand (projecting to the intact left hemisphere) were extinguished from awareness during double simultaneous stimulation, when his right hand was positioned in the left/contralesional space. This demonstrates the role of an egocentric spatial reference frame in attention that can determine awareness of stimuli despite intact sensory pathways. Using functional magnetic resonance imaging to elucidate the neural correlates of such perceptual extinction, we found that limb position modulated neural responses to tactile stimuli at early cortical stages (SI) in the intact hemisphere. Activity in bilateral middle frontal gyri also was modulated by limb position and may contribute to integrate sensory inputs into a supramodal, egocentric representation of space.     

Influence of concurrent tactile stimulation on somatosensory evoked potentials following posterior tibial nerve stimulation in man

A topographical study was made of SEPs following stimulation of the right posterior tibial nerve at the ankle, with and without concurrent tactile stimulation of the soles of either foot or the palm of the right hand. Effects of the interfering stimulus were best demonstrated by subtracting the wave forms to derive "difference' potentials. The majority of SEP components were significantly attenuated by tactile stimulation of the ipsilateral foot, and the difference wave form was of similar morphology to the control response. Components of opposite polarity peaking at 39 msec were consistent with the field of a cortical generator with dipolar properties, situated in the contralateral hemisphere just posterior to the vertex with the positive poles oriented towards the ipsilateral side. By analogy with median SEP findings, these potentials were believed to originate in the foot region of area 3b where neurones are mainly concerned with cutaneous sensory processing. When the tactile stimulus was applied to the contralateral foot, difference potentials maximally recorded just posterior to the vertex were of smaller amplitude but similar morphology to ipsilateral foot difference components. This suggested the possibility that input from the two lower extremities may converge at cortical or subcortical level, the effect being manifested in the response of certain neurones in area 3b. With both contralateral foot and ipsilateral hand stimulation, other difference potentials were present which suggested that there may be cortical regions responding to combinations of sensory stimuli applied to various parts of the body surface.     

Multiple tactile maps in the human cerebellum

Functional imaging studies of the cerebellum have mostly investigated motor performance or have been limited to the anterior lobe and therefore the somatosensory representations in the human cerebellum have not been fully demonstrated. We used fMRI of the entire cerebellum during tactile stimulation of the hand and foot in six normal subjects. Our results demonstrate that the tactile projections to the cerebellum in humans are represented in both the anterior and posterior lobes. in agreement with previous functional imaging studies, our results show a large-scale, between-limb somatotopy comparable to that shown in early animal studies.     

Superior tactile performance and learning in professional pianists: evidence for meta-plasticity in musicians

Musician's brains constitute an interesting model for neuroplasticity. Imaging studies demonstrated that sensorimotor cortical representations are altered in musicians, which was assumed to arise from the development of skilled performance. However, the perceptual consequences of the cortical changes remain elusive. Here we ask whether cortical reorganization induced by professional musical skill training is paralleled by the evolution of other, unrelated perceptual abilities. We therefore studied psychophysically tactile spatial acuity as an indirect marker of cortical changes in professional pianists and non-musician control subjects using a simultaneous two-point discrimination paradigm. We show that long-lasting piano practising resulted in lower spatial discrimination thresholds in comparison to non-musicians. In musicians, individual discrimination thresholds were linearly correlated with the daily training duration, indicating a direct link between tactile acuity and the degree of piano practising. To investigate whether the superior acuity in pianists is subject to further improvement, we used a Hebbian stimulation protocol of tactile coactivation known to improve spatial tactile acuity. Three hours of coactivation further reduced their discrimination thresholds. The coactivation-induced gain in pianists was significantly larger in comparison to control subjects and correlated with the years of heavy daily practising (>3 h/day), but not with the total years including casual playing. Our results suggest that despite already high-level performance in pianists, Hebbian learning was more effective in musicians than in controls. This implies stronger capacities for plastic reorganization and points to enhanced learning abilities implicating a form of meta-plasticity in professional pianists.     

Increased excitability of somatosensory cortex in aged humans is associated with impaired tactile acuity

Aging affects all levels of neural processing, including changes of intracortical inhibition and cortical excitability. Paired-pulse stimulation, the application of two stimuli in close succession, is a useful tool to investigate cortical excitability in humans. The paired-pulse behavior is characterized by the second response being significantly suppressed at short stimulus onset asynchronies. While in rat somatosensory cortex, intracortical inhibition has been demonstrated to decline with increasing age, data from human motor cortex of elderly subjects are controversial and there are no data for the human somatosensory cortex (SI). Moreover, behavioral implications of age-related changes of cortical excitability remain elusive. We therefore assessed SI excitability by combining paired-pulse median nerve stimulation with recording somatosensory evoked potentials in 138 healthy subjects aged 17-86 years. We found that paired-pulse suppression was characterized by substantial interindividual variability, but declined significantly with age, confirming reduced intracortical inhibition in elderly subjects. To link the age-related increase of cortical excitability to perceptual changes, we measured tactile two-point discrimination in a subsample of 26 aged participants who showed either low or high paired-pulse suppression. We found that tactile performance was particularly impaired in subjects showing markedly enhanced cortical excitability. Our data demonstrate that paired-pulse suppression of human SI is significantly reduced in older adults, and that age-related enhancement of cortical excitability correlates with degradation of tactile perception. These findings indicate that cortical excitability constitutes an important mechanism that links age-related neurophysiological changes to behavioral alterations in humans.     

Facilitating effect of 15-Hz repetitive transcranial magnetic stimulation on tactile perceptual learning

Recent neuroimaging studies have revealed that tactile perceptual learning can lead to substantial reorganizational changes of the brain. We report here for the first time that combining high-frequency (15 Hz) repetitive transcranial magnetic stimulation (rTMS) over the primary somatosensory cortex (SI) with tactile discrimination training is capable of facilitating operant perceptual learning. Most notably, increasing the excitability of SI by 15-Hz rTMS improved perceptual learning in spatial, but not in temporal, discrimination tasks. These findings give causal support to recent correlative data obtained by functional magnetic resonance imaging studies indicating a differential role of SI in spatial and temporal discrimination learning. The introduced combination of rTMS and tactile discrimination training may provide new therapeutical potentials in facilitating neuropsychological rehabilitation of functional deficits after lesions of the somatosensory cortex.     

Tactile coactivation resets age-related decline of human tactile discrimination

OBJECTIVE: For young subjects, it is well-documented that training and practice improve sensorimotor performance. However, little is known about how the typically observed age-related decline of sensorimotor abilities can be ameliorated by sensory stimulation. METHODS: As an alternative approach to training, we have introduced a tactile coactivation protocol involving Hebbian synaptic plasticity to improve tactile performance on a short timescale of a few hours. RESULTS: By applying coactivation on the index finger to drive perceptual learning, we demonstrate that in the elderly, aged 65 to 89 years, the age-related impairment of tactile two-point discrimination can be mitigated substantially. In elderly adults, tactile-acuity thresholds increased to 3.5mm compared with 1.5mm found in young adults, whereas 50-year-old subjects showed intermediate performance. As a result of coactivation, discrimination thresholds of the 80-year-old adults came to match those typically found at an age of 50, demonstrating that age-related decline in tactile performance is not irreversible, but rather subject to considerable restoration by specific stimulation protocols. INTERPRETATION: Because the preservation of sufficient tactile acuity into advanced age is an important prerequisite for the maintenance of autonomous living, we believe that the concept of coactivation might turn out to be beneficial in preserving everyday sensorimotor competence in the elderly through new forms of therapeutic interventions.     

Viewing the body prepares the brain for touch: effects of TMS over somatosensory cortex

Viewing the body can improve tactile perception. We investigated whether this could be due to a remodeling of somatosensory cortical areas during vision of the body. Single-pulse transcranial magnetic stimulation (TMS) was delivered over the primary and secondary somatosensory areas of subjects who showed clear visual-tactile enhancement while they performed a tactile grating discrimination task. Before the tactile stimulus, subjects viewed either their right index finger through a semisilvered mirror or an object reflected by the mirror and positioned to appear in the same location as the finger. In a first experiment we observed that TMS over primary somatosensory cortex significantly reduced subjects' accuracy whilst viewing the hand. No such reduction was found when subjects viewed a neutral object. In a second experiment, we disrupted the activity of primary and secondary somatosensory areas in different sessions. When stimulating the primary somatosensory cortex, a reduction in accuracy was again found while viewing the hand, but not a neutral object. TMS over secondary somatosensory cortex had no effect in any condition. Our results show that vision of the body may act at an early stage in stimulus elaboration and perception, allowing an anticipatory tuning of the neural circuits in primary somatosensory cortex that underlie tactile acuity.     

NK1 receptor activation by geniohyoid primary afferents modulates parasympathetic postganglionic neuronal excitability in the rat

Brain activation patterns derived from neurofunctional methods are often implicitly regarded as being directly related to subjective perceptual experience in an iso- or at least homomorph manner, neglecting the operational differences between these two dimensions. This paper (a) introduces a method for assessing 'perceptual maps' of stimulation patterns presented to the body surface, providing a means to parametrically relate neural representation and subjective percept, and (b) applies this method to demonstrate the existence of 'somatotopic maps' of hot and painful stimulus patterns independent from mechanoceptive co-activation. Brief (90 ms) CO2 laser pulses were presented in an array of multiple stimulation sites on the dorsal forearms (N. radialis area, C7 dermatome) of healthy subjects. Perceived locations were indicated with a 3D tracker without touching the skin, and (mis-)localizations in distal-proximal direction were analyzed. Stimuli were localized with overall mean errors of 22 mm (SD: 16 mm) toward the wrist and 24 mm (SD: 18 mm) toward the elbow. Somatotopic representation of thermal-nociceptive stimuli could be demonstrated in all subjects, independent from mechanoceptive co-activation. The perceptual maps revealed striking individual (mis-)localization patterns, many subjects exhibiting 'stretched', some 'condensed' somatotopic representations. In estimating the mapping parameters from physical to perceptual space linear regressions generally provided a good fit (adj. R2>0.80 in 10 out of 12 subjects). Nonlinear models were advantageous in some subjects only. Our method can be useful in assessing inter-individual differences or experimentally induced shifts in somatotopic processing.     

Circadian fluctuations in three types of sensory modules in healthy subjects

This study was designed to observe and compare the circadian fluctuations in tactile sense, joint reposition sense and two-point discrimination in healthy subjects. Twenty-one healthy adult subjects received perceptual ability tests through these three different sensory modules at approximately 9：00, 13：00 and 18：00 in a day. The distribution of ranking for perceptual ability was significantly different among the three different time points in each individual, with highest perceptual ability in the evening compared with noon and morning, in terms of tactile sense and two-point discrimination. These findings suggest that the perceptual ability of healthy subjects fluctuates according to the time points in a day.