Shrinkage of Somatosensory Hand Area in Subjects With Upper Extremity Dysmelia Revealed by Magnetoencephalography

Orofacial sensorimotor behaviour in unilateral chewing: A comparative analysis in asymptomatic population

The main aim of the present study was to evaluate the presence of differences in motor, somatosensory and laterality recognise variables between individuals with unilateral chewing (UC) versus bilateral chewing (BC). A cross-sectional study with a nonprobabilistic sample was conducted. Seventy asymptomatic individuals were grouped as UC or BC to assess differences in motor, somatosensory, and laterality recognise variables. The recorded variables were range of motion (ROM), electromyographic (EMG) activity, lip grip force, 2-point discrimination (2-PD), pain pressure threshold (PPT), and laterality recognition. Significant differences in EMG activity of the masseter and temporal muscles were found in group*side (p<.005). Significant changes in group*side (p<.005) were found in lip strength and 2-PD in the maxillary and mandibular branch. For laterality, significant between-group differences in accuracy were found (p=.037). Individuals with UC showed unilateral sensorimotor modifications compared with those with BC. Although the relationship between type of mastication and temporomandibular disorder (TMD) cannot be established, neurophysiological changes in UC could affect biomechanics and temporomandibular joint function and could predispose individuals to the onset, development, and maintenance of TMD because patients with TMD usually present an impairment in orofacial motor and sensory functions, with recruitment abnormalities of the masseter and temporal muscles during chewing.

Plasticity based on compensatory effector use in the association but not primary sensorimotor cortex of people born without hands

What forces direct brain organization and its plasticity? When brain regions are deprived of their input, which regions reorganize based on compensation for the disability and experience, and which regions show topographically constrained plasticity? People born without hands activate their primary sensorimotor hand region while moving body parts used to compensate for this disability (e.g., their feet). This was taken to suggest a neural organization based on functions, such as performing manual-like dexterous actions, rather than on body parts, in primary sensorimotor cortex. We tested the selectivity for the compensatory body parts in the primary and association sensorimotor cortex of people born without hands (dysplasic individuals). Despite clear compensatory foot use, the primary sensorimotor hand area in the dysplasic subjects showed preference for adjacent body parts that are not compensatorily used as effectors. This suggests that function-based organization, proposed for congenital blindness and deafness, does not apply to the primary sensorimotor cortex deprivation in dysplasia. These findings stress the roles of neuroanatomical constraints like topographical proximity and connectivity in determining the functional development of primary cortex even in extreme, congenital deprivation. In contrast, increased and selective foot movement preference was found in dysplasics' association cortex in the inferior parietal lobule. This suggests that the typical motor selectivity of this region for manual actions may correspond to high-level action representations that are effector-invariant. These findings reveal limitations to compensatory plasticity and experience in modifying brain organization of early topographical cortex compared with association cortices driven by function-based organization.

Typical action perception and interpretation without motor simulation

Every day, we interact with people synchronously, immediately understand what they are doing, and easily infer their mental state and the likely outcome of their actions from their kinematics. According to various motor simulation theories of perception, such efficient perceptual processing of others' actions cannot be achieved by visual analysis of the movements alone but requires a process of motor simulation--an unconscious, covert imitation of the observed movements. According to this hypothesis, individuals incapable of simulating observed movements in their motor system should have difficulty perceiving and interpreting observed actions. Contrary to this prediction, we found across eight sensitive experiments that individuals born with absent or severely shortened upper limbs (upper limb dysplasia), despite some variability, could perceive, anticipate, predict, comprehend, and memorize upper limb actions, which they cannot simulate, as efficiently as typically developed participants. We also found that, like the typically developed participants, the dysplasic participants systematically perceived the position of moving upper limbs slightly ahead of their real position but only when the anticipated position was not biomechanically awkward. Such anticipatory bias and its modulation by implicit knowledge of the body biomechanical constraints were previously considered as indexes of the crucial role of motor simulation in action perception. Our findings undermine this assumption and the theories that place the locus of action perception and comprehension in the motor system and invite a shift in the focus of future research to the question of how the visuo-perceptual system represents and processes observed body movements and actions.

Congenital foot deformation alters the topographic organization in the primate somatosensory system

Limbs may fail to grow properly during fetal development, but the extent to which such growth alters the nervous system has not been extensively explored. Here we describe the organization of the somatosensory system in a 6-year-old monkey (Macaca radiata) born with a deformed left foot in comparison to the results from a normal monkey (Macaca fascicularis). Toes 1, 3, and 5 were missing, but the proximal parts of toes 2 and 4 were present. We used anatomical tracers to characterize the patterns of peripheral input to the spinal cord and brainstem, as well as between thalamus and cortex. We also determined the somatotopic organization of primary somatosensory area 3b of both hemispheres using multiunit electrophysiological recording. Tracers were subcutaneously injected into matching locations of each foot to reveal their representations within the lumbar spinal cord, and the gracile nucleus (GrN) of the brainstem. Tracers injected into the representations of the toes and plantar pads of cortical area 3b labeled neurons in the ventroposterior lateral nucleus (VPL) of the thalamus. Contrary to the orderly arrangement of the foot representation throughout the lemniscal pathway in the normal monkey, the plantar representation of the deformed foot was significantly expanded and intruded into the expected representations of toes in the spinal cord, GrN, VPL, and area 3b. We also observed abnormal representation of the intact foot in the ipsilateral spinal cord and contralateral area 3b. Thus, congenital malformation influences the somatotopic representation of the deformed as well as the intact foot.

Differential grey matter changes in sensorimotor cortex related to exceptional fine motor skills

Functional changes in sensorimotor representation occur in response to use and lesion throughout life. Emerging evidence suggests that functional changes are paralleled by respective macroscopic structural changes. In the present study we used voxel-based morphometry to investigate sensorimotor cortex in subjects with congenitally malformed upper extremities. We expected increased or decreased grey matter to parallel the enlarged or reduced functional representations we reported previously. More specifically, we expected decreased grey matter values in lateral sensorimotor cortex related to compromised hand function and increased grey matter values in medial sensorimotor cortex due to compensatory foot use. We found a medial cluster of grey matter increase in subjects with frequent, hand-like compensatory foot use. This increase was predominantly seen for lateral premotor, supplementary motor, and motor areas and only marginally involved somatosensory cortex. Contrary to our expectation, subjects with a reduced number of fingers, who had shown shrinkage of the functional hand representation previously, did not show decreased grey matter values within lateral sensorimotor cortex. Our data suggest that functional plastic changes in sensorimotor cortex can be associated with increases in grey matter but may also occur in otherwise macroscopically normal appearing grey matter volumes. Furthermore, macroscopic structural changes in motor and premotor areas may be observed without respective changes in somatosensory cortex.

Precise mapping of the somatotopic hand area using neuromagnetic steady-state responses

The body surface is represented in somatotopically organized maps in the primary somatosensory cortex. Estimating the size of the hand area with neuromagnetic source analysis has been used as a metric for monitoring neuroplastic changes related to training, learning, and brain injury. Commonly, results were significant as group statistics only because source localization accuracy was limited by factors such as residual noise and head motion. In this study we aimed to develop a robust method for obtaining the somatotopic map of the hand area in individuals using the bootstrap framework. Furthermore, a comprehensive analysis of the different factors affecting the accuracy of the obtained maps was provided. We applied vibrotactile touch stimuli to the tip of the index finger or the ring finger of the right hand and recorded the 22-Hz steady-state response using MEG. Single equivalent dipole sources were localized in contralateral left somatosensory cortex. Bootstrap resampling revealed the confidence intervals for the source coordinates using a single block of 5 min MEG recording. Residual noise in the averaged evoked response predominantly affected source localization, and the related confidence interval was reciprocally related to the signal-to-noise ratio. Apparently, head movements within a block of MEG recording contributed less to the variability of source localization in cooperative volunteers. The results of the current study indicate that significant separations of index finger and ring finger representations along the somatotopic map can be revealed in an individual using bootstrap framework.

Allogreffe de main chez le nouveau-né agénésique: étude de faisabilité

Would a newborn with a single hand benefit from hand allograft? Transantebrachial aplasia is the chosen clinical form of agenesia in our interrogation. The feasibility study presents several aspects: 1) ethical and psychological aspects. Is this a desired surgery for agenesic population? Which are the functional, psychological and social situations of agenesic patient? Is the hand transplantation in newborn ethically acceptable? What is the parents' attitude toward agenesia? Can we envisage organ donation in neonatal period? 2) immunological aspects. The non-vital character of this condition and its' good functional tolerance cannot make accepting the risk of adverse effects of hand allotransplantation. Hence, one may consider this surgery only without immunosuppression. Can the peculiarities of the neonate "immature" immune system represent an opportunity of easier tolerance obtaining, avoiding immunosuppression? 3) anatomical and technical aspects. The proximal tissues at the level of amputation are all hypoplastic in agenesic patients. Can we efficaciously suture those structures with donor eutrophic tissues? 4) cognitive aspects. Is a neonate born with only one hand is able to use two? A feasibility study on such a subject needs to take into account all these aspects. This research is useful because, even if hand allograft in agenesic newborn will never be done, the provided information will allow to progress in the vaster domain of composite tissue allotransplantation in perinatology.

Studying the human somatosensory hand area: A new way to compare fMRI and MEG

Valid localization is a prerequisite to study plasticity of the somatosensory cortex in humans. We compared the localizations of left and right thumb and little finger in the primary somatosensory cortex obtained with fMRI and MEG. Representations were investigated in 11 healthy right-handed subjects using echoplanar fMRI and 122-channel MEG together with electric finger stimulation. Activation observed with fMRI was based on an increase in the BOLD signal. Most of the activation clusters (71.1%) were located on the lateral surface of the postcentral gyrus. Representations of thumb and little finger were 17mm apart on average and consistently showed a somatotopic arrangement with the thumb representation inferior, lateral, and anterior to the representation of the little finger. Activation observed with MEG was modelled by equivalent current dipoles. Dipole localization was compatible with an assumed origin of activation within the posterior wall of the central sulcus. The Euclidian distance between corresponding dipoles was 11.5mm on average with deviations from the expected spatial arrangement of 35, 30, and 20% in the x-, y- und z-direction, respectively. Our study demonstrates how relative localization of somatosensory activations can serve as an indicator for localization validity when comparing different methods or studying somatosensory plasticity.