Postural adjustments associated with different unloadings of the forearm: effects of proprioceptive and cutaneous afferent deprivation

Slow touch and ultrafast pain fibres: Revisiting peripheral nerve classification

One hundred years ago, Erlanger and Gasser demonstrated that conduction velocity is correlated with the diameter of a peripheral nerve axon. Later, they also demonstrated that the functional role of the axon is related to its diameter: touch is signalled by large-diameter axons, whereas pain and temperature are signalled by small-diameter axons. Certain discoveries in recent decades prompt a modification of this canonical classification. Here, we review the evidence for unmyelinated (C) fibres signalling touch at a slow conduction velocity and likely contributing to affective aspects of tactile information. We also review the evidence for large-diameter Aβ afferents signalling pain at ultrafast conduction velocity and likely contributing to the rapid nociceptive withdrawal reflex. These discoveries imply that conduction velocity is not as clear-cut an indication of the functional role of the axon as previously thought. We finally suggest that a future taxonomy of the peripheral afferent nervous system might be based on the combination of the axońs molecular expression and electrophysiological response properties.

The neural foundations of handedness: insights from a rare case of deafferentation

The role of proprioceptive feedback on motor lateralization remains unclear. We asked whether motor lateralization is dependent on proprioceptive feedback by examining a rare case of proprioceptive deafferentation (GL). Motor lateralization is thought to arise from asymmetries in neural organization, particularly at the cortical level. For example, we have previously provided evidence that the left hemisphere mediates optimal motor control that allows execution of smooth and efficient arm trajectories, while the right hemisphere mediates impedance control that can achieve stable and accurate final arm postures. The role of proprioception in both of these processes has previously been demonstrated empirically, bringing into question whether loss of proprioception will disrupt lateralization of motor performance. In this study, we assessed whether the loss of online sensory information produces deficits in integrating specific control contributions from each hemisphere by using a reaching task to examine upper limb kinematics in GL and five age-matched controls. Behavioral findings revealed differential deficits in the control of the left and right hands in GL and performance deficits in each of GL's hands compared with controls. Computational simulations can explain the behavioral results as a disruption in the integration of postural and trajectory control mechanisms when no somatosensory information is available. This rare case of proprioceptive deafferentation provides insights into developing a more accurate understanding of handedness that emphasizes the role of proprioception in both predictive and feedback control mechanisms.NEW & NOTEWORTHY The role of proprioceptive feedback on the lateralization of motor control mechanisms is unclear. We examined upper limb kinematics in a rare case of peripheral deafferentation to determine the role of sensory information in integrating motor control mechanisms from each hemisphere. Our empirical findings and computational simulations showed that the loss of somatosensory information results in an impaired integration of control mechanisms, thus providing support for a complementary dominance hypothesis of handedness.

Case Studies in Neuroscience: The central and somatosensory contributions to finger interdependence and coordination: lessons from a study of a "deafferented person"

We tested finger force interdependence and multifinger force-stabilizing synergies in a patient with large-fiber peripheral neuropathy ("deafferented person"). The subject performed a range of tasks involving accurate force production with one finger and with four fingers. In one-finger tasks, nontask fingers showed unintentional force production (enslaving) with an atypical pattern: very large indices for the lateral (index and little) fingers and relatively small indices for the central (middle and ring) fingers. Indices of multifinger synergies stabilizing total force and of anticipatory synergy adjustments in preparation to quick force pulses were similar to those in age-matched control females. During constant force production, removing visual feedback led to a slow force drift to lower values (by ~25% over 15 s). The results support the idea of a neural origin of enslaving and suggest that the patterns observed in the deafferented person were reorganized based on everyday manipulation tasks. The lack of significant changes in the synergy index shows that synergic control can be organized in the absence of somatosensory feedback. We discuss the control of the hand in deafferented persons within the α-model of the equilibrium-point hypothesis and suggest that force drift results from an unintentional drift of the control variables to muscles toward zero values. NEW & NOTEWORTHY We demonstrate atypical patterns of finger enslaving and unchanged force-stabilizing synergies in a person with large-fiber peripheral neuropathy. The results speak strongly in favor of central origin of enslaving and its reorganization based on everyday manipulation tasks. The data show that synergic control can be implemented in the absence of somatosensory feedback. We discuss the control of the hand in deafferented persons within the α-model of the equilibrium-point hypothesis.

Seeing Your Foot Move Changes Muscle Proprioceptive Feedback

Multisensory effects are found when the input from single senses combines, and this has been well researched in the brain. Presently, we examined in humans the potential impact of visuo-proprioceptive interactions at the peripheral level, using microneurography, and compared it with a similar behavioral task. We used a paradigm where participants had either proprioceptive information only (no vision) or combined visual and proprioceptive signals (vision). We moved the foot to measure changes in the sensitivity of single muscle afferents, which can be altered by the descending fusimotor drive. Visual information interacted with proprioceptive information, where we found that for the same passive movement, the response of muscle afferents increased when the proprioceptive channel was the only source of information, as compared with when visual cues were added, regardless of the attentional level. Behaviorally, when participants looked at their foot moving, they more accurately judged differences between movement amplitudes, than in the absence of visual cues. These results impact our understanding of multisensory interactions throughout the nervous system, where the information from different senses can modify the sensitivity of peripheral receptors. This has clinical implications, where future strategies may modulate such visual signals during sensorimotor rehabilitation.

Does Proprioception Influence Human Spatial Cognition? A Study on Individuals With Massive Deafferentation

When navigating in a spatial environment or when hearing its description, we can develop a mental model which may be represented in the central nervous system in different coordinate systems such as an egocentric or allocentric reference frame. The way in which sensory experience influences the preferred reference frame has been studied with a particular interest for the role of vision. The present study investigated the influence of proprioception on human spatial cognition. To do so, we compared the abilities to form spatial models of two rare participants chronically deprived of proprioception (GL and IW) and healthy control participants. Participants listened to verbal descriptions of a spatial environment, and their ability to form and use a mental model was assessed with a distance-comparison task and a free-recall task. Given that the loss of proprioception has been suggested to specifically impair the egocentric reference frame, the deafferented individuals were expected to perform worse than controls when the spatial environment was described in an egocentric reference frame. Results revealed that in both tasks, one deafferented individual (GL) made more errors than controls while the other (IW) made less errors. On average, both GL and IW were slower to respond than controls, and reaction time was more variable for IW. Additionally, we found that GL but not IW was impaired compared to controls in visuo-spatial imagery, which was assessed with the Minnesota Paper Form Board Test. Overall, the main finding of this study is that proprioception can influence the time necessary to use spatial representations while other factors such as visuo-spatial abilities can influence the capacity to form accurate spatial representations.

Proprioceptive loss and the perception, control and learning of arm movements in humans: evidence from sensory neuronopathy

It is uncertain how vision and proprioception contribute to adaptation of voluntary arm movements. In normal participants, adaptation to imposed forces is possible with or without vision, suggesting that proprioception is sufficient; in participants with proprioceptive loss (PL), adaptation is possible with visual feedback, suggesting that proprioception is unnecessary. In experiment 1 adaptation to, and retention of, perturbing forces were evaluated in three chronically deafferented participants. They made rapid reaching movements to move a cursor toward a visual target, and a planar robot arm applied orthogonal velocity-dependent forces. Trial-by-trial error correction was observed in all participants. Such adaptation has been characterized with a dual-rate model: a fast process that learns quickly, but retains poorly and a slow process that learns slowly and retains well. Experiment 2 showed that the PL participants had large individual differences in learning and retention rates compared to normal controls. Experiment 3 tested participants’ perception of applied forces. With visual feedback, the PL participants could report the perturbation’s direction as well as controls; without visual feedback, thresholds were elevated. Experiment 4 showed, in healthy participants, that force direction could be estimated from head motion, at levels close to the no-vision threshold for the PL participants. Our results show that proprioceptive loss influences perception, motor control and adaptation but that proprioception from the moving limb is not essential for adaptation to, or detection of, force fields. The differences in learning and retention seen between the three deafferented participants suggest that they achieve these tasks in idiosyncratic ways after proprioceptive loss, possibly integrating visual and vestibular information with individual cognitive strategies.

Schizotypal traits and forearm motor control against self-other produced action in a bimanual unloading task

The present study investigated the relation between schizotypy and motor control against self- or other-produced action. We used an unloading task to focus on the timing component of anticipatory motor control. In the task, a weight was removed from a participants' hand by the participants themselves or by an experimenter (voluntary versus imposed unloading). Postural disturbance at the removal timing was measured as an index of predictive function in motor control. We hypothesized that the postural disturbance in the voluntary unloading would be positively related to schizotypal traits; however, the results did not support this theory. The results showed almost zero correlation between the schizotypy scores and the postural disturbance in the voluntary unloading condition. In contrast, the schizotypy scores positively correlated with the postural disturbance in the imposed unloading condition. These findings were replicated across two participant groups and two schizotypy scales. Further analyses on subscales of the schizotypy questionnaire found moderate levels of positive correlation between each subscale for Cognitive-Perceptual and Disorganization factors and the disturbance. Accordingly, the present study did not support the idea that non-pathological individuals with high schizotypal traits have deficits in prediction of self-produced actions, at least for a temporal domain. Instead, the results suggested that individuals with high schizotypal traits, particularly for the positive and disorganization symptoms, are not good at responding to others-produced actions. The schizophrenic symptoms were discussed in terms of the failure in the processes executed after calculating prediction of sensory consequences and dysfunction in internal models for "other people".

Balance perturbations

Impairments of balance and gait leading to loss of mobility, falls, and disability are common occurrences in many neurologic conditions and with older age. Much of our current understanding about posture and balance control and its impairments has come from investigations of how healthy individuals and those with neurologic disorders respond to situations that perturb standing balance during instructed voluntary tasks or in reaction to externally imposed challenges to stability. Knowledge obtained from these investigations has come from documenting the physical and physiologic characteristics of the perturbations together with the body's electrophysiologic, structural, kinetic, kinematic, and behavioral responses. From these findings, basic mechanisms, diagnostic and pathologic criteria, and targets for clinical care have been identified while continued gaps in understanding have been exposed. In this chapter, we synthesize and discuss current concepts and understanding concerning the sensorimotor control of posture and balance while standing. We draw insights gained from perturbation studies investigating these functions in healthy adults, and those with neurologic pathologies.

Threshold position control of anticipation in humans: a possible role of corticospinal influences

KEY POINTS

Sudden unloading of preloaded wrist muscles elicits motion to a new wrist position. Such motion is prevented if subjects unload muscles using the contralateral arm (self-unloading). Corticospinal influences originated from the primary motor cortex maintain tonic influences on motoneurons of wrist muscles before sudden unloading but modify these influences prior to the onset and until the end of self-unloading. Results are interpreted based on the previous finding that intentional actions are caused by central, particularly corticospinal, shifts in the spatial thresholds at which wrist motoneurons are activated, thus predetermining the attractor point at which the neuromuscular periphery achieves mechanical balance with environment forces. By maintaining or shifting the thresholds, descending systems let body segments go to the equilibrium position in the respective unloading tasks without the pre-programming of kinematics or muscle activation patterns. The study advances the understanding of how motor actions in general, and anticipation in particular, are controlled.

ABSTRACT

The role of corticospinal (CS) pathways in anticipatory motor actions was evaluated using transcranial magnetic stimulation (TMS) of the primary motor cortex projecting to motoneurons (MNs) of wrist muscles. Preloaded wrist flexors were suddenly unloaded by the experimenter or by the subject using the other hand (self-unloading). After sudden unloading, the wrist joint involuntarily flexed to a new position. In contrast, during self-unloading the wrist remained almost motionless, implying that an anticipatory postural adjustment occurred. In the self-unloading task, anticipation was manifested by a decrease in descending facilitation of pre-activated flexor MNs starting ∼72 ms before changes in the background EMG activity. Descending facilitation of extensor MNs began to increase ∼61 ms later. Conversely, these influences remained unchanged before sudden unloading, implying the absence of anticipation. We also tested TMS responses during EMG silent periods produced by brief muscle shortening, transiently resulting in similar EMG levels before the onset and after the end of self-unloading. We found reduced descending facilitation of flexor MNs after self-unloading. To explain why the wrist excursion was minimized in self-unloading due to these changes in descending influences, we relied on previous demonstrations that descending systems pre-set the threshold positions of body segments at which muscles begin to be activated, thus predetermining the equilibrium point to which the system is attracted. Based on this notion, a more consistent explanation of the kinematic, EMG and descending patterns in the two types of unloading is proposed compared to the alternative notion of direct pre-programming of kinematic and/or EMG patterns.

Prediction in the Vestibular Control of Arm Movements

The contribution of vestibular signals to motor control has been evidenced in postural, locomotor, and oculomotor studies. Here, we review studies showing that vestibular information also contributes to the control of arm movements during whole-body motion. The data reviewed suggest that vestibular information is used by the arm motor system to maintain the initial hand position or the planned hand trajectory unaltered during body motion. This requires integration of vestibular and cervical inputs to determine the trunk motion dynamics. These studies further suggest that the vestibular control of arm movement relies on rapid and efficient vestibulomotor transformations that cannot be considered automatic. We also reviewed evidence suggesting that the vestibular afferents can be used by the brain to predict and counteract body-rotation-induced torques (e.g., Coriolis) acting on the arm when reaching for a target while turning the trunk.

Motor unit firing pattern, synchrony and coherence in a deafferented patient

The firing of spinal motoneurons (MNs) is controlled continuously by inputs from muscle, joint and skin receptors. Besides altering MN synaptic drive, the removal of these inputs is liable to alter the synaptic noise and, thus, the variability of their tonic activity. Sensory afferents, which are a major source of common and/or synchronized inputs shared by several MNs, may also contribute to the coupling in the time and frequency domains (synchrony and coherence, respectively) observed when cross-correlation and coherence analyses are applied to the discharges of MN pairs. Surprisingly, no consistent changes in firing frequency, nor in synchrony and coherence were reported to affect the activity of 3 pairs of motor units (MUs) tested in a case of sensory polyradiculoneuropathy (SPRNP), leading to an irreversible loss of large diameter sensory afferents (Farmer et al., 1993). Such a limited sample, however, precludes a definite conclusion about the actual impact that a chronic loss of muscle and cutaneous afferents may have on the firing properties of human MUs. To address this issue, the firing pattern of 92 MU pairs was analyzed at low contraction force in a case of SPRNP leading similarly to a permanent loss of proprioceptive inputs. Compared with 8 control subjects, MNs in this patient tended to discharge with slightly shorter inter-spike intervals but with greater variability. Synchronous firing tended to occur more frequently with a tighter coupling in the patient. There was no consistent change in coherence in the 15–30 Hz frequency range attributed to the MN corticospinal drive, but a greater coherence was observed below 5 Hz and between 30 and 60 Hz in the patient. The possible origins of the greater irregularity in MN tonic discharges, the tighter coupling of the synchronous firing and the changes in coherence observed in the absence of proprioceptive inputs are discussed.

Balance control interferes with the tracing performance of a pattern with mirror-reversed vision in older persons

When tracing a template with mirror-reversed vision (or distorted vision), the sensory information arising from the movement does not match the expected sensory consequences. In such situations, participants have to learn a new visuomotor mapping in order to trace the template with an accuracy and speed approaching that observed when tracing with direct vision. There are several suggestions that such visuomotor learning requires lowering the gain of the proprioceptive inputs. Generally, subjects learn this task in a seated condition offering a stable postural platform. Adapting to the new visuomotor relationship in a standing condition could add complexity and even hinder sensorimotor adaptation because balance control and processing of additional information typically interfere with each other. To examine this possibility, older individuals and young adults (on average, 70 and 22 years of age, respectively) were assigned to groups that trained to trace a shape with mirror-reversed vision in a seated or a standing condition for two sessions. For a third session, the seated groups (young and elderly) transferred to the standing condition while the standing groups continued to perform the tracing task while standing. This procedure allowed comparing the tracing performance of all groups (with the same amount of practice) in a standing condition. The standing groups also did a fourth session in a seated condition. Results show that older participants initially exposed to the standing condition were much slower to trace the template than all other groups (including the older group that performed the tracing task while seated). This slowness did not result from a baseline general slowness but from a genuine interference between balance control and the visuomotor conflict resulting from tracing the pattern with mirror-reversed vision. Besides, the Standing-Old participants that transferred to a seated condition in the fourth session immediately improved their tracing by reducing the total displacement covered by the pen to trace the template. Interestingly, the results did not support a transfer-appropriate practice hypothesis which suggests that training in a standing condition (at the third session) should have benefited the performance of those individuals who initially learned to trace the mirror pattern in a standing condition. This has important clinical implications: training at adapting to new sensory contexts or environmental conditions in conditions that do not challenge balance control could be necessary if one desires to attenuate the detrimental consequences on the postural or motor performances brought up by the interference between maintaining balance and the sensory reweighing processes.

The effects of sensorimotor training on anticipatory postural adjustment of the trunk in chronic low back pain patients

[Purpose] This study aimed to examine the effects of sensorimotor training on the anticipatory postural adjustment (APA) of chronic low back pain (CLBP) patients. [Subjects and Methods] Fourteen CLBP patients were randomly assigned to Group II (ordinary physical therapy, n=7) and Group III (sensorimotor training, n=7). In addition, a normal group (Group I) consisting of seven subjects was chosen as the control group. The two CLBP groups received their own treatment five times per week, for four weeks, for 40 minutes each time. Changes in pain and functional performance evaluation were examined by the visual analogue scale (VAS) and the Oswestry Disability Index (ODI). In order to look at the change in APA, muscle onset time was examined using electromyography (EMG). [Results] Group III showed significant changes in both VAS and ODI. According to comparison of the results for muscle onset time, there were significant decreases in Group III's transversus abdominis muscle (TrA) and external oblique muscle (EO) in the standing and sitting positions. There were significant differences between Group II and III in terms of the TrA in the sitting position. [Conclusion] Sensorimotor training makes patients capable of learning how to adjust muscles, thereby alleviating pain and improving muscle performance.

Anatomical and functional enhancements of the insula after loss of large primary somatosensory fibers

Brain changes associated with the loss of a sensory modality such as vision and audition have previously been reported. Here, we examined the effect of loss of discriminative touch and proprioception on cortical thickness and functional connectivity. We performed structural magnetic resonance imaging and resting-state functional magnetic resonance imaging scans on a 60-year-old female who at age 31 suffered a selective loss of large-diameter myelinated primary afferents and, therefore, relies mainly on her intact thin-fiber senses (temperature, pain, itch, and C-fiber touch) and vision to negotiate her environment. The patient showed widespread cortical thinning compared with 12 age-matched female controls. In contrast, her right anterior insula was significantly thick. Seed-based resting-state analysis revealed that her right anterior insula had increased connectivity to bilateral posterior insula. A separate independent component analysis revealed the increased connectivity between the insula and visual cortex in the patient. As the insula is an important processing area for temperature and C-fiber tactile information, the increased intrainsular and insular-visual functional connectivity could be related to the patient's use of C-fiber (gentle) touch and temperature information in conjunction with visual information to navigate her environment. We, thus, demonstrated plasticity in networks involving the insular cortex following denervation of large-diameter somatosensory afferents.

Prediction of the body rotation-induced torques on the arm during reaching movements: evidence from a proprioceptively deafferented subject

Reaching for a target while rotating the trunk generates substantial Coriolis and centrifugal torques that push the arm in the opposite direction of the rotations. These torques rarely perturb movement accuracy, suggesting that they are compensated for during the movement. Here we tested whether signals generated during body motion (e.g., vestibular) can be used to predict the torques induced by the body rotation and to modify the motor commands accordingly. We asked a deafferented subject to reach for a memorized visual target in darkness. At the onset of the reaching, the patient was rotated 25° or 40° in the clockwise or the counterclockwise directions. During the rotation, the patient's head remained either fixed in space (Head-Fixed condition) or fixed on the trunk (Head Rotation condition). At the rotation onset, the deafferented patient's hand largely deviated from the mid-sagittal plane in both conditions. The hand deviations were compensated for in the Head Rotation condition only. These results highlight the computational faculty of the brain and show that body rotation-related information can be processed for predicting the consequence of the rotation dynamics on the reaching arm movements.

Absence of equifinality of hand position in a double-step unloading task

Equifinality, during arm reaching movements, relates to the capacity of the neuromuscular system to attain the same final position in the presence or absence of transient perturbations. There have been several controversies regarding equifinality in the literature. A brief elastic perturbation, applied during a fast arm movement or just before its initiation, typically does not affect final arm position. On the other hand, several experiments have shown that velocity-dependent perturbations, such as Coriolis force or negative damping, while transient in nature, have a significant effect on final arm position when compared to unperturbed movements. In this study, an unloading paradigm was used to study the role of reflexes with respect to equifinality. The effects on final arm position of suddenly decreasing a static load maintained by fourteen subjects were analyzed. Subjects maintained an initial load produced by a double-joint manipulandum moving in the horizontal plane. The load was suddenly decreased, either in one or in two successive steps with different time intervals, resulting in a rapid reflex-mediated change in arm position. Unloading led to short-latency changes in the activity of shoulder and elbow muscles and significant variations in tonic activity. It was found that the final hand position was shorter for double- versus single-step unloading if the time between two successive changes in load was greater than 100 ms. With a shorter time interval, the final hand positions were the same. This difference in final hand positions was inversely proportional to the hand velocity at the time of the second change in load. Further, agonist/antagonist co-activation increased in double-step unloading. Thus, the change in both the load and the movement velocity may influence the magnitude of the unloading reflex. This may be indicative of a dependence of stretch reflexes on velocity. Perturbation may cause a reflex-mediated increase in joint stiffness, which could explain why equifinality is not preserved after some perturbations, such as velocity-dependant external forces.

Insights into the control of arm movement during body motion as revealed by EMG analyses

Recent studies have revealed that vestibulomotor transformations contribute to maintain the hand stationary in space during trunk rotation. Here we tested whether these vestibulomotor transformations have the same latencies and whether they are subject to similar cognitive control than the visuomotor transformations during manual tracking of a visual target. We recorded hand displacement and shoulder-muscle activity in two tasks: a stabilization task in which subjects stabilized their hand during passive 30 degrees body rotations, and a tracking task in which subjects tracked with their finger a visual target as it moved 30 degrees around them. The EMG response times recorded in the stabilization task (approximately 165 ms) were twice as short as those observed for the tracking task (approximately 350 ms). Tested with the same paradigm, a deafferented subject showed EMG response times that closely matched those recorded in healthy subjects, thus, suggesting a vestibular origin of the arm movements. Providing advance information about the direction of the required arm movement reduced the response times in the tracking task (by approximately 115 ms) but had no significant effect in the stabilization task. Generally, when providing false information about movement direction in the tracking task, an EMG burst first appeared in the muscle moving the arm in the direction opposite to the actual target motion (i.e., in accord with the precueing). This behavior was rarely observed in the stabilization task. These results show that the sensorimotor transformations that move the arm relative to the trunk have shorter latencies when they originate from vestibular inputs than from visual information and that vestibulomotor transformations are more resistant to cognitive processes than visuomotor transformations.

Somatotopic organization of gentle touch processing in the posterior insular cortex

A network of thin (C and A delta) afferents relays various signals related to the physiological condition of the body, including sensations of gentle touch, pain, and temperature changes. Such afferents project to the insular cortex, where a somatotopic organization of responses to noxious and cooling stimuli was recently observed. To explore the possibility of a corresponding body-map topography in relation to gentle touch mediated through C tactile (CT) fibers, we applied soft brush stimuli to the right forearm and thigh of a patient (GL) lacking A beta afferents, and six healthy subjects during functional magnetic resonance imaging (fMRI). For improved fMRI analysis, we used a highly sensitive multivariate voxel clustering approach. A somatotopic organization of the left (contralateral) posterior insular cortex was consistently demonstrated in all subjects, including GL, with forearm projecting anterior to thigh stimulation. Also, despite denying any sense of touch in daily life, GL correctly localized 97% of the stimuli to the forearm or thigh in a forced-choice paradigm. The consistency in activation patterns across GL and the healthy subjects suggests that the identified organization reflects the central projection of CT fibers. Moreover, substantial similarities of the presently observed insular activation with that described for noxious and cooling stimuli solidify the hypothesized sensory-affective role of the CT system in the maintenance of physical well-being as part of a thin-afferent homeostatic network.

Unmyelinated tactile afferents have opposite effects on insular and somatosensory cortical processing

A previous functional magnetic resonance imaging (fMRI) study of an A-beta deafferented subject (GL) showed that stimulation of tactile C afferents (CT) activates insular cortex whereas no activation was seen in somatosensory cortices. Psychophysical studies suggested that CT afferents contribute to affective but not to discriminative aspects of tactile stimulation. We have now examined cortical processing following CT stimulation in a second similarly deafferented subject (IW), as well as revisited the data from GL. The results in IW showed similar activation of posterior insular cortex following CT stimulation as in GL and so strengthen the view that CT afferents underpin emotional aspects of touch. In addition, CT stimulation evoked significant fMRI deactivation in somatosensory cortex in both subjects supporting the notion that CT is not a system for discriminative touch.

Functional role of unmyelinated tactile afferents in human hairy skin: sympathetic response and perceptual localization

In addition to A-beta fibres the human hairy skin has unmyelinated (C) fibres responsive to light touch. Previous functional magnetic resonance imaging (fMRI) studies in a subject with a neuronopathy who specifically lacks A-beta afferents indicated that tactile C afferents (CT) activate insular cortex, whereas no response was seen in somatosensory areas 1 and 2. Psychophysical tests suggested that CT afferents give rise to an inconsistent perception of weak and pleasant touch. By examining two neuronopathy subjects as well as control subjects we have now demonstrated that CT stimulation can elicit a sympathetic skin response. Further, the neuronopathy subjects' ability to localize stimuli which activate CT afferents was very poor but above chance level. The findings support the interpretation that the CT system is well suited to underpin affective rather than discriminative functions of tactile sensations.

Vestibular signal processing in a subject with somatosensory deafferentation: the case of sitting posture

Background

The vestibular system of the inner ear provides information about head translation/rotation in space and about the orientation of the head with respect to the gravitoinertial vector. It also largely contributes to the control of posture through vestibulospinal pathways. Testing an individual severely deprived of somatosensory information below the nose, we investigated if equilibrium can be maintained while seated on the sole basis of this information.

Results

Although she was unstable, the deafferented subject (DS) was able to remain seated with the eyes closed in the absence of feet, arm and back supports. However, with the head unconsciously rotated towards the left or right shoulder, the DS's instability markedly increased. Small electrical stimulations of the vestibular apparatus produced large body tilts in the DS contrary to control subjects who did not show clear postural responses to the stimulations.

Conclusion

The results of the present experiment show that in the lack of vision and somatosensory information, vestibular signal processing allows the maintenance of an active sitting posture (i.e. without back or side rests). When head orientation changes with respect to the trunk, in the absence of vision, the lack of cervical information prevents the transformation of the head-centered vestibular information into a trunk-centered frame of reference of body motion. For the normal subjects, this latter frame of reference enables proper postural adjustments through vestibular signal processing, irrespectively of the orientation of the head with respect to the trunk.

Vision without Proprioception Modulates Cortico-spinal Excitability during Hand Motor Imagery

Several studies have shown a cortico-spinal facilitation during motor imagery. This facilitation effect is weaker when the actual hand posture is incompatible with the imagined movement. To determine whether the source of this interference effect arises from online proprioceptive information, we examined transcranial magnetic stimulation (TMS)-induced motor-evoked potentials during motor imagery in the deafferented subject G.L. The patient and 7 control subjects were asked to close their eyes and imagine joining the tips of the thumb and the little finger while maintaining a hand posture compatible or incompatible with the imagined movement. Contrary to control subjects' performance, G.L.'s results show that the facilitation observed during motor imagery was independent of the hand posture. To examine how vision of the hand interacts with the imagery process, G.L. and control subjects performed the same task with the eyes open. Like control subjects, when G.L. looked at her hand, a greater facilitation was observed when her hand posture was compatible with the imagined movement than when it was incompatible. These results suggest that in the absence of proprioception, vision may facilitate or inhibit motor representations and support the idea that limb position in the brain is organized around multisensory representations.

Threshold position control of arm movement with anticipatory increase in grip force

The grip force holding an object between fingers usually increases before or simultaneously with arm movement thus preventing the object from sliding. We experimentally analyzed and simulated this anticipatory behavior based on the following notions. (1) To move the arm to a new position, the nervous system shifts the threshold position at which arm muscles begin to be recruited. Deviated from their activation thresholds, arm muscles generate activity and forces that tend to minimize this deviation by bringing the arm to a new position. (2) To produce a grip force, with or without arm motion, the nervous system changes the threshold configuration of the hand. This process defines a threshold (referent) aperture (R(a)) of appropriate fingers. The actual aperture (Q(a)) is constrained by the size of the object held between the fingers whereas, in referent position R(a), the fingers virtually penetrate the object. Deviated by the object from their thresholds of activation, hand muscles generate activity and grip forces in proportion to the gap between the Q(a) and R(a). Thus, grip force emerges since the object prevents the fingers from reaching the referent position. (3) From previous experiences, the system knows that objects tend to slide off the fingers when arm movements are made and, to prevent sliding, it starts narrowing the referent aperture simultaneously with or somewhat before the onset of changes in the referent arm position. (4) The interaction between the fingers and the object is accomplished via the elastic pads on the tips of fingers. The pads are compressed not only due to the grip force but also due to the tangential inertial force ("load") acting from the object on the pads along the arm trajectory. Compressed by the load force, the pads move back and forth in the gap between the finger bones and object, thus inevitably changing the normal component of the grip force, in synchrony with and in proportion to the load force. Based on these notions, we simulated experimental elbow movements and grip forces when subjects rapidly changed the elbow angle while holding an object between the index finger and the thumb. It is concluded that the anticipatory increase in the grip force with or without correlation with the tangential load during arm motion can be explained in neurophysiological and biomechanical terms without relying on programming of grip force based on an internal model.

The contribution of non-digital afferent signals to grip force adjustments evoked by brisk unloading of the arm or the held object

OBJECTIVE

Earlier studies suggest that grip force adjustments evoked by mechanical perturbations result more from cutaneous signals from the fingertips, than from afferent signals from the supporting limb. Generally an increase in tangential load at the fingertips induces an increase in grip force, whereas a decrease in load induces the opposite reaction. Some data suggest that prior knowledge and experience influences the magnitude of grip force adjustments.

METHODS

This study examines the relative contribution of digital and arm afferent signals in the context of brisk involuntary upward flexions obtained either by unloading the arm (ARM) or the held object (OBJECT). Following the perturbation, the tangential load at the fingertips increased in ARM, but decreased in OBJECT. A subsidiary goal was to compare the performance of naive subjects with the performance of trained and informed subjects.

RESULTS

When the perturbation was completely unexpected, grip force increased sharply after OBJECT and ARM unloading. By contrast, when subjects had prior knowledge and experience with the upcoming perturbation, grip responses were clearly differentiated; grip force increased after ARM, but decreased after OBJECT.

CONCLUSIONS

These results challenge the view that cutaneous signals of the fingertips are the driving signals of grip force responses. Instead, afferent signals from the flexed arm would account well for the lack of difference between grip force responses in ARM and OBJECT under unpredictable conditions. These data provide clear evidence that prior knowledge and experience influences reactive grip force control, since subjects became able to repress unnecessary grip force modulation in OBJECT.

SIGNIFICANCE

These data have implications for understanding the initiation and the modulation of grip force adjustments.

Threshold position control and the principle of minimal interaction in motor actions

The answer to the question of how the nervous system controls multiple muscles and body segments while solving the redundancy problem in choosing a unique action from the set of many possible actions is still a matter of controversy. In an attempt to clarify the answer, we review data showing that motor actions emerge from central resetting of the threshold position of appropriate body segments, i.e. the virtual position at which muscles are silent but deviations from it will elicit activity and resistive forces (threshold position control). The difference between the centrally-set threshold position and the sensory-signaled actual position is responsible for the activation of neuromuscular elements and interactions between them and the environment. These elements tend to diminish the evoked activity and interactions by minimizing the gap between the actual position and the threshold position (the principle of minimal interaction). Threshold control per se does not solve the redundancy problem: it only limits the set of possible actions. The principle of minimal interaction implies that the system relies on the natural capacity of neuromuscular elements to interact between themselves and with the environment to reduce this already restricted set to a unique action, thus solving the redundancy problem in motor control. This theoretical framework appears to be helpful in the explanation of the control and production of a variety of actions (reaching movements, specification of different hand configurations, grip force generation, and whole-body movements such as sit-to-stand or walking). Experimental tests of this theory are provided. The prediction that several types of neurons specify referent control variables for motor actions may be tested in future studies. The theory may also be advanced by applying the notion of threshold control to perception and cognition.

Unmyelinated tactile afferents underpin detection of low-force monofilaments

Human hairy but not glabrous skin has unmyelinated (C) tactile (CT) afferents that project to insular cortex. We studied two subjects with the rare sensory neuronopathy syndrome who lack A-beta fibers but have relatively preserved C-fiber function. Weak monofilaments were detected on hairy skin alone. Hence, the ability to detect light touch does not depend entirely on the A-beta somatosensory system; CT afferents may contribute to the detection of weak monofilaments.

Internally driven control of reaching movements: A study on a proprioceptively deafferented subject

We investigated the possibility of controlling reaching movements on the sole basis of central mechanisms, i.e., without peripheral feedback on hand and target positions. A deafferented subject (GL) and control subjects reached with the unseen hand for a straight-ahead target that could be displaced laterally at movement onset. The shifted target was continuously or briefly lit, or not visible. In this latter condition, a beep from either side of subjects' head single-handedly signaled the change in the movement goal, so that movements could only be controlled through an internal representation of the memorised target position. Compared to controls, GL showed quantitatively similar corrections (77% of the target displacement, on an average) and similar reaction times to the target shift (mean = 516 ms), regardless of target visual information. These results highlight a remarkable capacity for controlling reaching movements on the sole basis of internally driven processes. On the other hand, trajectories in double-step trials differed drastically between GL and controls. Controls' trajectories were composed of two segments, the second of which brought the hand directly toward the displaced target. The patient produced three-segment, stair-like trajectories. The first and third segments were mainly in the sagittal plane and the second segment was a vector-image of the lateral target shift. A control experiment showed that GL's trajectories were not the result of a voluntary strategy used to adjust movement trajectory in the absence of peripheral information on hand position. We suggest that GL's trajectories reflect a deficit in interjoint coordination in the absence of proprioception.

Independent control of voluntary movements and associated anticipatory postural responses in a bimanual task

OBJECTIVE

When one hand loads the other arm, EMG responses in the stationary arm anticipate the load. This study used transcranial magnetic stimulation over each hemisphere to clarify the relationship between a voluntary movement on one side and the anticipatory postural response on the other.

METHODS

Subjects (n = 7) performed elbow flexion movements of one arm as a reaction-time task. Because subjects' arms were linked, flexion about one elbow resulted in extension force about the other, and an anticipatory response occurred in those elbow flexor muscles. After the 'go' signal and before the predicted onset of EMG, transcranial magnetic stimuli were delivered over one or other motor cortex.

RESULTS

Stimulation contralateral to the reaction-time movement delayed the onset of voluntary EMG (46 ms in right biceps, 77 ms left) but did not alter the onset of EMG in the postural arm. Stimulation contralateral to the anticipatory postural response delayed only the postural EMG (left 96 ms, right 52 ms).

CONCLUSIONS

Thus, the associated voluntary and postural responses were delayed independently by stimuli over their respective contralateral motor cortex.

SIGNIFICANCE

This suggests that, although timing of responses may be linked by an initial signal, the response from each motor cortex develops independently.

Internal models for bi-manual tasks

Co-ordinated bi-manual actions form the basis for many everyday motor skills. In this review, the internal model approach to the problem of bi-manual co-ordination is presented. Bi-manual coordinative tasks are often regarded as a hallmark of complex action. They are often associated with object manipulation, whether the holding of a single object between the two hands or holding an object in each hand. However, the task of movement and control is deceptively difficult even when we execute an action with a single hand without holding an object. The simplest voluntary action requires the problems of co-ordination, timing and interaction between neural, muscular and skeletal structures to be overcome. When we are making a movement whilst holding an object, a further requirement is that an internal model is able to predict the dynamics of the object that is being held as well as the dynamics of the motor system. There has been extensive work examining the formation of internal models when acting in novel environments. The majority of studies examine uni-lateral learning of a task generally to the participant's dominant hand. However, many everyday motor tasks are bi-manual, and the existing findings regarding the learning of internal models in uni-manual tasks and their subsequent generalization highlights the complexities that must underlie the formation of bi-manual tasks. Our ability to perform bi-manual tasks raises interesting questions about how internal models are specified for co-ordinative actions, and also for how the motor system learns to represent the properties of objects.

Sensori-motor adaptation to knee osteoarthritis during stepping-down before and after total knee replacement

Background

Stepping-down is preceded by a shift of the center of mass towards the supporting side and forward. The ability to control both balance and lower limb movement was investigated in knee osteoarthritis patients before and after surgery. It was hypothesized that pain rather than knee joint mobility affects the coordination between balance and movement control.

Methods

The experiment was performed with 25 adult individuals. Eleven were osteoarthritic patients with damage restricted to one lower limb (8 right leg and 3 left leg). Subjects were recruited within two weeks before total knee replacement by the same orthopedic surgeon using the same prosthesis and technics of surgery. Osteoarthritic patients were tested before total knee replacement (pre-surgery session) and then, 9 of the 11 patients were tested one year after the surgery when re-educative training was completed (post-surgery session). 14 adult individuals (men: n = 7 and women: n = 7) were tested as the control group.

Results

The way in which the center of mass shift forward and toward the supporting side is initiated (timing and amplitude) did not vary within patients before and after surgery. In addition knee joint range of motion of the leading leg remained close to normal before and after surgery. However, the relative timing between both postural and movement phases was modified for the osteoarthritis supporting leg (unusual strategy for stepping-down) before surgery. The "coordinated" control of balance and movement turned to be a "sequential" mode of control; once the body weight transfer has been completed, the movement onset is triggered. This strategy could be aimed at shortening the duration-time supporting on the painful limb. However no such compensatory response was observed.

Conclusion

The change in the strategy used when supporting on the arthritis and painful limb could result from the action of nociceptors that lead to increased proprioceptor thresholds, thus gating the proprioceptive inputs that may be the critical afferents in controlling the timing of the coordination between balance and movement initiation control.

Perception of non-voluntary brief contractions in normal subjects and in a deafferented patient

The accuracy of force perception by human subjects in the absence of voluntary motor command was evaluated by exploring how they perceived isometric twitches of wrist extensor muscles produced by external stimulation. Twelve normal subjects and a well-known patient lacking large-diameter afferent fibres (GL) performed estimation, production and reproduction tasks. Magnetic stimulation of the radial nerve and, in normal subjects, transcranial magnetic stimulation (TMS) of the motor cortex were used to produce weak brief non-voluntary twitches. In estimation tasks, the subjects had to ascribe verbal marks on a 1-5 scale to the forces of stimulation-induced twitches. Loose covariations of marks and forces were observed, while directions of force variations between successive twitches were relatively well detected. GL did less well than normal subjects in detecting directions of force variations. In production tasks, subjects had to produce twitches matching verbal command marks in a 1-5 range, with or without visual feedback. Performances of normal subjects and GL resembled those of estimation tasks and were not improved by visual feedback. In reproduction tasks, subjects had to duplicate stimulation-induced test twitches: first without visual feedback, second with and third again without. Large errors were observed but all subjects did better with visual feedback. In the third step, improvement with respect to the first one was significantly more marked with TMS than with peripheral stimulation. GL improved her performance in the third step, possibly because she could use information provided by group III and group IV afferents still present in her nerves. Altogether, for normal subjects (1) the performances in estimation tasks are consistent with the known behaviour of Golgi tendon organs as observed in animal experiments, and (2) results observed in reproduction tasks suggest that cortical stimulation might elicit, in addition to corticospinal activation of motoneurones, collateral discharges that could be stored as a memory of motor command.

Children show decreased dynamic balance after mild traumatic brain injury

OBJECTIVE

To compare the balance skills of children after mild traumatic brain injury (TBI) with that of noninjured children matched for age, sex, and premorbid level of physical activity.

DESIGN

Cohort study.

SETTING

Pediatric trauma center.

PARTICIPANTS

Thirty-eight children aged 7 to 16 years (mean, 12.2+/-2.8 y) were recruited in each group. Children with mild TBI had a mean Glasgow Coma Scale score of 14.8 and were considered normal on a neurologic assessment at hospital discharge. Noninjured children were friends of those with mild TBI.

INTERVENTION

Assessments of balance were conducted at 1, 4, and 12 weeks after mild TBI and at corresponding time intervals for the controls.

MAIN OUTCOME MEASURES

The balance subtest of the Bruininks-Oseretsky Test of Motor Proficiency (BOTMP), the Pediatric Clinical Test of Sensory Interaction for Balance (P-CTSIB), and the Postural Stress Test (PST) were used.

RESULTS

Over the time interval of the study, analyses of variance revealed that children with mild TBI performed significantly worse than the noninjured group on the BOTMP balance subtest (P<.001) and on the PST (P=.031), as well as on the eyes-closed conditions in the P-CTSIB tandem position (P=.05).

CONCLUSIONS

Children with a mild TBI still showed balance deficits at 12 weeks postinjury. These deficits should be taken into consideration when planning a return to physical activities, particularly to those that require subtle balance skills.

Muscle modes during shifts of the center of pressure by standing persons: effect of instability and additional support

Muscle synergies in postural tasks have recently been studied using the framework of the uncontrolled manifold (UCM) hypothesis. A set of three hypothetical control variables, named M-modes, derived from the activity of 11 postural muscles, were identified. It was shown that postural synergies composed of these three M-modes preserve a certain shift of the center of pressure (COP) when subjects perform postural tasks while standing on a stable surface. In the present study we investigated the effects of support surface instability and availability of a light touch or grasp of a stable external support on the M-modes and their co-variation. The study was performed in two sessions. In the first session subjects released a load behind the body under four conditions: standing on a stable surface with no support (ST), standing on an unstable surface with no support (UN), standing on an unstable surface with a light touch (UN,T) and standing on an unstable surface with grasp of a stable object (UN,G). In the second session subjects performed two tasks: an arm movement backward and voluntary sway forward (towards the toes) under three conditions--ST, UN and UN,T. Principal component analysis was used to identify M-modes from data in the first session, and a UCM analysis was performed to study M-mode synergies in postural stabilization from data in the second session. A 'menu' of five M-modes was found, which were named either reciprocal M-modes or co-contraction M-modes based on the agonist-antagonist relationship of muscles comprising each mode. For a given task, subjects chose any three of these five M-modes in a subject- and task-specific manner. The reciprocal and co-contraction M-modes occurred equally frequently whether subjects stood on a stable or unstable support surface or whether a light touch was available or not. However, the co-contraction M-modes predominated when grasp of an object was available. In this condition, when the arm could be used for stabilization, there were M-modes uniting hip and shoulder muscles. However, the identified M-mode synergies were not found to lead to a consistent shift in the COP in any of the stability conditions. Possible reasons for this finding are discussed.

Recurrent inhibition of wrist extensor motoneurones: a single unit study on a deafferented patient

In order to document the effects of recurrent inhibition on the firing times of human alpha-motoneurones during natural motor behaviour, a case study was performed on a deafferented patient. The fact that this subject had completely lost the large-diameter sensory afferents provided us with a unique opportunity of selectively stimulating the motor axons in the nerves. The tonic activity of single motor units (n = 21) was recorded in the extensor carpi radialis muscles while applying randomly timed antidromic electrical stimuli to the radial nerve. The peristimulus time histogram analysis showed the presence of biphasic inhibitory effects, including an early, short-lasting component followed by a longer-lasting component occurring 20-40 ms later. The interspike interval (ISI) during which the stimulation occurred was generally lengthened as compared to the previous ISIs. The stimulation was most effective when delivered early (20-30 ms) after a spike. It was also effective, although less so, when delivered at the end of the ISI (70-100 ms after a spike). The lengthening effect sometimes extended over one or two of the subsequent ISIs. The lengthening effect of the motor axon stimulation was followed by an excitatory-like effect, which took the form of a shortening that affected up to five ISIs after the stimulation. The biphasic inhibitory effects and the subsequent facilitatory effects are discussed in terms of the dual nature of the synaptic processes involved in the recurrent inhibitory network, the postactivation facilitation/depression processes and the mutual inhibition occurring between Renshaw cells.

Unmyelinated tactile afferents signal touch and project to insular cortex

There is dual tactile innervation of the human hairy skin: in addition to fast-conducting myelinated afferent fibers, there is a system of slow-conducting unmyelinated (C) afferents that respond to light touch. In a unique patient lacking large myelinated afferents, we found that activation of C tactile (CT) afferents produced a faint sensation of pleasant touch. Functional magnetic resonance imaging (fMRI) analysis during CT stimulation showed activation of the insular region, but not of somatosensory areas S1 and S2. These findings identify CT as a system for limbic touch that may underlie emotional, hormonal and affiliative responses to caress-like, skin-to-skin contact between individuals.

Sequential control signals determine arm and trunk contributions to hand transport during reaching in humans

When reaching towards objects placed outside the arm workspace, the trunk assumes an active role in transport of the hand by contributing to the extent of movement while simultaneously maintaining the direction of reach. We investigated the spatial-temporal aspects of the integration of the trunk motion into reaching. Specifically, we tested the hypothesis that the efficiency ('gain') of the arm-trunk co-ordination determining the contribution of the trunk to the extent of hand movement may vary substantially with the phase of reaching. Sitting subjects made fast pointing movements towards ipsi- and a contralateral targets placed beyond the reach of the right arm so that a forward trunk motion was required to assist in transporting the hand to the target. Sight of the arm and target was blocked before the movement onset. In randomly selected trials, the trunk motion was unexpectedly prevented by an electromagnet. Subjects were instructed to make stereotypical movements whether or not the trunk was arrested. In non-perturbed trials, most subjects began to move the hand and trunk simultaneously. In trunk-blocked trials, it was impossible for the hand to cover the whole pointing distance but the hand trajectory and velocity profile initially matched those from the trials in which the trunk motion was free, approximately until the hand reached its peak velocity. The arm inter-joint co-ordination substantially changed in response to the trunk arrest at a minimal latency of 40 ms after the perturbation onset. The results suggest that when the trunk was free, the influence of the trunk motion on the hand trajectory and velocity profile was initially neutralized by appropriate changes in the arm joint angles. Only after the hand had reached its peak velocity did the trunk contribute to the extent of pointing. Previous studies suggested that the central commands underlying the transport component of arm movements are completed when the hand reaches peak velocity. These studies, together with the present finding that the trunk only begins to contribute to the hand displacement at peak hand velocity, imply that the central commands that determine the contributions of the arm and the trunk to the transport of the hand are generated sequentially, even though the arm and trunk move in parallel.

Influence of perturbation induced by an anticipated load on human motor regulatory systems

To investigate how human motor regulatory systems are modified by prior knowledge of a predictable external perturbation, six normal human subjects, each when sitting on a chair, were required to maintain a stable elbow flexion angle (90 degrees) while different weight perturbations were applied (0.5 kg or 2-kg loads). Loads were applied either by the experimenter Without Anticipation or With Anticipation by the subject's own contralateral hand. Acceleration of the forearm movement (elbow extension and flexion) by loads and electromyograms (EMGs) of the biceps brachii (BB) and the triceps brachii (TB) muscle were recorded. Under With Anticipation conditions, preceding EMG activities of BB and TB muscles prior to the onset time of perturbation were clearly observed. Furthermore, the amount of these preceding EMG activities was larger in the heavy load perturbation than in the light load perturbation. Under Without Anticipation conditions, however, these preceding EMG activities were not observed. In the preceding EMG activities, EMG bursts (latency 20 msec.) of a presumed stretch reflex induced by the perturbation were clearly observed. Thus, the function of anticipatory adjustment of mainitaining the elbow angle definitely appears to optimize limb stability in the case of the mechanical self-applied perturbation. Furthermore, the extent of the anticipatory adjustment of the elbow angle was dependent on the predicted magnitude of load.

Effects of upper limb unilateral isometric efforts on postural stabilization in subjects with hemiparesis

OBJECTIVE

To characterize postural stabilization during a progressive unilateral isometric abduction of the upper limb in a seated position in healthy subjects and subjects with hemiparesis.

DESIGN

Convenience sample.

SETTING

University secondary care rehabilitation center.

PATIENTS

Twelve patients with hemiparesis and 12 subjects without neurologic disorder.

INTERVENTIONS

Subjects were seated on a forceplate, with forearms fixed in cuffs mounted on a force transducer. Two trials per side of isometric abduction of arm were conducted. The orthogonal force and torque exerted was measured for each arm.

MAIN OUTCOME MEASURES

Forces at the upper limbs and at the seat, global motor performance, spasticity of upper limb, grip force, and dexterity.

RESULTS

Results of analyses of variance showed differences in the magnitude of the contralateral limb forces generated by subjects with hemiparesis and healthy subjects (p <.05). Normalized contralateral forces in the nonparetic upper limb associated with paretic isometric efforts were higher than those associated with nonparetic efforts and higher than those associated with efforts in healthy subjects.

CONCLUSION

These results suggest that postural stabilization during isometric efforts is impaired in subjects with hemiparesis.

Mechanically evoked cerebral potentials to sudden ankle dorsiflexion in human subjects during standing

Mechanically evoked cerebral potentials (MECP) were studied in humans standing on a movable platform with three different stance widths. A sudden platform tilt of 4 degrees produced ankle dorsiflexion and resulted in scalp potentials of five distinct components, the earliest being a positive deflection at 35/60 ms. Their latencies have shown fairly consistent values among the three stance widths, while the amplitudes underwent some significant changes under the wide stance condition as compared with tightly close feet. These findings were interpreted as purporting evidence of altered somaesthetic afferent input from lower limbs during standing with widely apart support surface.