Touch improvement at the hand transfers to the face

SummaryThe hand–face border is one of the most prominent features of the primate somatosensory cortex. A reduction of somatosensory input, following amputation or anesthesia, induces perceptual changes across this border that are explained by plastic competitive mechanisms [1–4]. Whether cross-border plasticity can be induced by learning processes relying on increased somatosensory input has been unclear. Here we report that training-independent learning [5] improves tactile perception, not only at the stimulated index finger, but also at the unstimulated face. These findings demonstrate that learning-induced tactile improvement can cross the hand–face border, suggesting that facilitation-based plasticity may operate in the healthy human brain

Role of perceptual improvement induced by repetitive somatosensory stimulation on motor behaviour

Somatosensory signals are essential to the motor system control and tactile impairments can result in significant reduction on the quality of motor abilities. Repetitive somatosensory stimulation (RSS) at a finger is known to improve performance in a two-points discrimination task (2PDT), supposedly thanks to the increase of the cortical representation of the stimulated body part in the somatosensory cortices. Yet, whether the RSS induced improvement on touch perception affects the activity of the motor cortex remains unknown. Here we assessed the effects of RSS on the motor system. Tactile performance and motor skills were evaluated before and after 3 hours training phase by using a 2PDT and a pegboard task. The training phase consisted of a real RSS, and a sham RSS. Results showed that the tactile performance increased for the trained finger (right index) only after the real RSS. Results on motor performance showed that reaction times were shorter for the right hand only in the group trained with sham RSS, whereas number of drops for the pegboard task were reduced for the left hand only in the real RSS trained group. We suggest that RSS can have an effect on the participants motor performance possibly through sensorimotor interactions

Breaking the boundaries of somatosensory plasticity: improved touch at the fingers transfers to the lips

Selected as Hot TopicInternational audienceIt is now well established that cortical plasticity occurs in the adult brain and that plastic phenomena are involved in many brain functions. Understanding the rules governing plasticity and its functional consequences is extremely challenging. A cutaneous coactivation (CA) protocol has been successfully used to study somatosensory plasticity (see Godde et al, 2003). CA relies on the Hebbian postulate and consists of synchronous and passive coactivation of several non-overlapping receptive fields which produces transient somatosensory changes. The aim of this study was to evaluate whether pure somatosensory plasticity, experimentally induced at a specific cutaneous location (i.e., right index fingertip) can cross somatotopically defined boundaries and functionally affect physically distant, but cortically adjacent body-part regions (i.e., the perioral lip region). A two-point discrimination (2PD) task was used to determine the spatial discrimination thresholds at the right and left index finger (RD2 & LD2), and at the right and left sides of the upper lip area. This test was performed during 4 sessions on 2 consecutive days. Between the third and fourth session, a vibrotactile CA was applied for three hours at the RD2 fingertip. Thirty participants were randomly attributed to either of two groups: a test group (n=15, mean age = 20.53 ± 2.26 years), in which subjects received a true CA at the RD2 fingertip; and a control group (n=15, mean age = 23.5 ± 3.25 years), in which the CA was replaced by a sham CA (stimulator fixed at the fingertip but turned off). Results showed that in the test group three hours of CA led to a significant decrease of the 2PD threshold at RD2 fingertip, from 1.77 ± 0.28 mm to 1.52 ± 0.43 mm (mean decrease of 15.26%), thus replicating previous results. Critically, a significant 2PD threshold decrease was also found at both sides of the upper lip area (from 6.02 ± 0.88 mm to 5.44 ± 1.24 mm, mean decrease of 10.29%). No effects were found at either the LD2 fingertip or at any body-part region of the control group. These results demonstrate that experimentally-induced plasticity following RD2 coactivation improves participants’ tactile discrimination performance not only at the coactivated body site, but also at cortically adjacent body sites. Thus, CA-induced somatosensory plasticity appears to cross somatotopically defined boundaries between cortical body-parts representations, spreading its functional consequences to cortically adjacent, but peripherally unstimulated body parts

Neuromagnetic correlates of adaptive plasticity across the hand-face border in human primary somatosensory cortex

International audienceIt is well established that permanent or transient reduction of somatosensory inputs, following hand deafferentation or anesthesia, induces plastic changes across the hand-face border, supposedly responsible for some altered perceptual phenomena such as tactile sensations being referred from the face to the phantom hand. It is also known that transient increase of hand somatosensory inputs, via repetitive somatosensory stimulation (RSS) at a fingertip, induces local somatosensory discriminative improvement accompanied by cortical representational changes in the primary somatosensory cortex (SI). We recently demonstrated that RSS at the tip of the right index finger induces similar training-independent perceptual learning across the hand-face border, improving somatosensory perception at the lips (Muret D, Dinse HR, Macchione S, Urquizar C, Farnè A, Reilly KT. Curr Biol 24: R736–R737, 2014). Whether neural plastic changes across the hand-face border accompany such remote and adaptive perceptual plasticity remains unknown. Here we used magnetoencephalography to investigate the electrophysiological correlates underlying RSS-induced behavioral changes across the hand-face border. The results highlight significant changes in dipole location after RSS both for the stimulated finger and for the lips. These findings reveal plastic changes that cross the hand-face border after an increase, instead of a decrease, in somatosensory inputs

A neural surveyor to map touch on the body

Perhaps the most recognizable sensory map in all of neuroscience is the somatosensory homunculus. Although it seems straightforward, this simple representation belies the complex link between an activation in a somatotopic map and the associated touch location on the body. Any isolated activation is spatially ambiguous without a neural decoder that can read its position within the entire map, but how this is computed by neural networks is unknown. We propose that the somatosensory system implements multilateration, a common computation used by surveying and global positioning systems to localize objects. Specifically, to decode touch location on the body, multilateration estimates the relative distance between the afferent input and the boundaries of a body part (e.g., the joints of a limb). We show that a simple feedforward neural network, which captures several fundamental receptive field properties of cortical somatosensory neurons, can implement a Bayes-optimal multilateral computation. Simulations demonstrated that this decoder produced a pattern of localization variability between two boundaries that was unique to multilateration. Finally, we identify this computational signature of multilateration in actual psychophysical experiments, suggesting that it is a candidate computational mechanism underlying tactile localization

Complex pattern of facial remapping in somatosensory cortex following congenital but not acquired hand loss.

Cortical remapping after hand loss in the primary somatosensory cortex (S1) is thought to be predominantly dictated by cortical proximity, with adjacent body parts remapping into the deprived area. Traditionally, this remapping has been characterised by changes in the lip representation, which is assumed to be the immediate neighbour of the hand based on electrophysiological research in non-human primates. However, the orientation of facial somatotopy in humans is debated, with contrasting work reporting both an inverted and upright topography. We aimed to fill this gap in the S1 homunculus by investigating the topographic organisation of the face. Using both univariate and multivariate approaches we examined the extent of face-to-hand remapping in individuals with a congenital and acquired missing hand (hereafter one-handers and amputees, respectively), relative to two-handed controls. Participants were asked to move different facial parts (forehead, nose, lips, tongue) during functional MRI (fMRI) scanning. We first confirmed an upright face organisation in all three groups, with the upper-face and not the lips bordering the hand area. We further found little evidence for remapping of both forehead and lips in amputees, with no significant relationship to the chronicity of their phantom limb pain (PLP). In contrast, we found converging evidence for a complex pattern of face remapping in congenital one-handers across multiple facial parts, where relative to controls, the location of the cortical neighbour - the forehead - is shown to shift away from the deprived hand area, which is subsequently more activated by the lips and the tongue. Together, our findings demonstrate that the face representation in humans is highly plastic, but that this plasticity is restricted by the developmental stage of input deprivation, rather than cortical proximity