Showing papers on "Somatosensory system published in 1997"

Pain affect encoded in human anterior cingulate but not somatosensory cortex.

Abstract: Recent evidence demonstrating multiple regions of human cerebral cortex activated by pain has prompted speculation about their individual contributions to this complex experience. To differentiate cortical areas involved in pain affect, hypnotic suggestions were used to alter selectively the unpleasantness of noxious stimuli, without changing the perceived intensity. Positron emission tomography revealed significant changes in pain-evoked activity within anterior cingulate cortex, consistent with the encoding of perceived unpleasantness, whereas primary somatosensory cortex activation was unaltered. These findings provide direct experimental evidence in humans linking frontal-lobe limbic activity with pain affect, as originally suggested by early clinical lesion studies.

Functional relevance of cross-modal plasticity in blind humans

Abstract: Functional imaging studies of people who were blind from an early age have revealed that their primary visual cortex can be activated by Braille reading and other tactile discrimination tasks1. Other studies have also shown that visual cortical areas can be activated by somatosensory input in blind subjects but not those with sight2,3,4,5,6,7. The significance of this cross-modal plasticity is unclear, however, as it is not known whether the visual cortex can process somatosensory information in a functionally relevant way. To address this issue, we used transcranial magnetic stimulation to disrupt the function of different cortical areas in people who were blind from an early age as they identified Braille or embossed Roman letters. Transient stimulation of the occipital (visual) cortex induced errors in both tasks and distorted the tactile perceptions of blind subjects. In contrast, occipital stimulation had no effect on tactile performance in normal-sighted subjects, whereas similar stimulation is known to disrupt their visual performance. We conclude that blindness from an early age can cause the visual cortex to be recruited to a role in somatosensory processing. We propose that this cross-modal plasticity may account in part for the superior tactile perceptual abilities of blind subjects.

Top-down modulation of early sensory cortex.

Abstract: Data from nine previous studies of human visual information processing using positron emission tomography were reanalyzed to contrast blood flow responses during passive viewing and active discriminations of the same stimulus array. The analysis examined whether active visual processing (i) increases blood flow in medial visual regions early in the visual hierarchy and (ii) decreases blood flow in auditory and somatosensory cortex. Significant modulation of medial visual regions was observed in six of nine studies, indicating that top-down processes can affect early visual cortex. Modulations showed several task dependencies, suggesting that in some cases the underlying mechanism was selective (e.g. analysis-or feature-specific) rather than non-selective. Replicable decreases at or near auditory Brodmann area (BA) left 41/42 were observed in two of five studies, but in different locations. Analyses that combined data across studies yielded modest but significant decreases. Replicable decreases were not found in primary somatosensory cortex but were observed in an insular region that may be a somatosensory association area. Decreases were also noted in the parietal operculum (perhaps SII) and BA 40. These results are inconsistent with a model in which the precortical input to task-irrelevant sensory cortical areas is broadly suppressed.

Essential role of POU–domain factor Brn-3c in auditory and vestibular hair cell development

Abstract: The Brn-3 subfamily of POU–domain transcription factor genes consists of three highly homologous members—Brn-3a, Brn-3b, and Brn-3c—that are expressed in sensory neurons and in a small number of brainstem nuclei. This paper describes the role of Brn-3c in auditory and vestibular system development. In the inner ear, the Brn-3c protein is found only in auditory and vestibular hair cells, and the Brn-3a and Brn-3b proteins are found only in subsets of spiral and vestibular ganglion neurons. Mice carrying a targeted deletion of the Brn-3c gene are deaf and have impaired balance. These defects reflect a complete loss of auditory and vestibular hair cells during the late embryonic and early postnatal period and a secondary loss of spiral and vestibular ganglion neurons. Together with earlier work demonstrating a loss of trigeminal ganglion neurons and retinal ganglion cells in mice carrying targeted disruptions in the Brn-3a and Brn-3b genes, respectively, the Brn-3c phenotype reported here demonstrates that each of the Brn-3 genes plays distinctive roles in the somatosensory, visual, and auditory/vestibular systems.

Essential role of POU- domain factor Brn-3c in auditory and vestibular hair cell development (inner ear development)

Abstract: The Brn-3 subfamily of POU-domain tran- scription factor genes consists of three highly homologous members—Brn-3a, Brn-3b, and Brn-3c—that are expressed in sensory neurons and in a small number of brainstem nuclei. This paper describes the role of Brn-3c in auditory and vestibular system development. In the inner ear, the Brn-3c protein is found only in auditory and vestibular hair cells, and the Brn-3a and Brn-3b proteins are found only in subsets of spiral and vestibular ganglion neurons. Mice carrying a targeted deletion of the Brn-3c gene are deaf and have impaired balance. These defects ref lect a complete loss of auditory and vestibular hair cells during the late embryonic and early postnatal period and a secondary loss of spiral and vestibular ganglion neurons. Together with earlier work dem- onstrating a loss of trigeminal ganglion neurons and retinal ganglion cells in mice carrying targeted disruptions in the Brn-3a and Brn-3b genes, respectively, the Brn-3c phenotype reported here demonstrates that each of the Brn-3 genes plays distinctive roles in the somatosensory, visual, and auditoryy vestibular systems.

Central Versus Peripheral Determinants of Patterned Spike Activity in Rat Vibrissa Cortex During Whisking

Abstract: We report on the relationship between single-unit activity in primary somatosensory vibrissa cortex of rat and the rhythmic movement of vibrissae. Animals were trained to whisk freely in air in search of food. Electromyographic (EMG) recordings from the mystatial pads served as a reference for the position of the vibrissae. A fast, oscillatory component in single-unit spike trains is correlated with vibrissa position within the whisk cycle. The phase of the correlation for different units is broadly distributed. A second, slowly varying component of spike activity correlates with the amplitude of the whisk cycle. For some units, the phase and amplitude correlations were of sufficient strength to allow the position of the whiskers to be accurately predicted from a single spike train. To determine whether the observed patterned spike activity was driven by motion of the vibrissae, as opposed to central pathways, we reversibly blocked the contralateral facial motor nerve during the behavioral task so that the rat whisked only on the ipsilateral side. The ipsilateral EMG served as a reliable reference signal. The fast, oscillatory component of the spike-EMG correlation disappears when the facial motor nerve is blocked. This implies that the position of vibrissae within a cycle is encoded through direct sensory activation. The slowly varying component of the spike-EMG correlation is unaffected by the block. This implies that the amplitude of whisking is likely to be mediated by corollary discharge. Our results suggest that motor cortex does not relay a reference signal to sensory cortex for positional information of the vibrissae during whisking.

Deactivation and reactivation of somatosensory cortex after dorsal spinal cord injury

Abstract: Sensory stimuli to the body are conveyed by the spinal cord to the primary somatosensory cortex. It has long been thought that dorsal column afferents of the spinal cord represent the main pathway for these signals, but the physiological and behavioural consequences of cutting the dorsal column have been reported to range from mild and transitory to marked. We have re-examined this issue by sectioning the dorsal columns in the cervical region and recording the responses to hand stimulation in the contralateral primary somatosensory cortex (area 3b). Following a complete section of the dorsal columns, neurons in area 3b become immediately and perhaps permanently unresponsive to hand stimulation. Following a partial section, the remaining dorsal column afferents continue to activate neurons within their normal cortical target territories, but after five or more weeks the area of activation is greatly expanded. After prolonged recovery periods of six months or more, the deprived hand territory becomes responsive to inputs from the face (which are unaffected by spinal cord section). Thus, area 3b of somatosensory cortex is highly dependent on dorsal spinal column inputs, and other spinal pathways do not substitute for the dorsal columns even after injury.

Immediate and simultaneous sensory reorganization at cortical and subcortical levels of the somatosensory system

Abstract: The occurrence of cortical plasticity during adulthood has been demonstrated using many experimental paradigms. Whether this phenomenon is generated exclusively by changes in intrinsic cortical circuitry, or whether it involves concomitant cortical and subcortical reorganization, remains controversial. Here, we addressed this issue by simultaneously recording the extracellular activity of up to 135 neurons in the primary somatosensory cortex, ventral posterior medial nucleus of the thalamus, and trigeminal brainstem complex of adult rats, before and after a reversible sensory deactivation was produced by subcutaneous injections of lidocaine. Following the onset of the deactivation, immediate and simultaneous sensory reorganization was observed at all levels of the somatosensory system. No statistical difference was observed when the overall spatial extent of the cortical (9.1 ± 1.2 whiskers, mean ± SE) and the thalamic (6.1 ± 1.6 whiskers) reorganization was compared. Likewise, no significant difference was found in the percentage of cortical (71.1 ± 5.2%) and thalamic (66.4 ± 10.7%) neurons exhibiting unmasked sensory responses. Although unmasked cortical responses occurred at significantly higher latencies (19.6 ± 0.3 ms, mean ± SE) than thalamic responses (13.1 ± 0.6 ms), variations in neuronal latency induced by the sensory deafferentation occurred as often in the thalamus as in the cortex. These data clearly demonstrate that peripheral sensory deafferentation triggers a system-wide reorganization, and strongly suggest that the spatiotemporal attributes of cortical plasticity are paralleled by subcortical reorganization.

Arm movement-related neurons in the visual area V6A of the macaque superior parietal lobule

Abstract: Area V6A is a cortical visual area located in the posterior face of the superior parietal lobule in the macaque monkey. It contains visual neurons as well as neurons not activated by any kind of visual stimulation. The aim of this study was to look for possible features able to activate these latter neurons. We tested 70 non-visual V6A neurons. Forty-three of them showed an arm movement-related neural discharge due to somatosensory stimulation and/or skeletomotor activity of the upper limbs of the animal. The arm movement-related neural discharge started before the onset of arm movement, often before the earliest electromyographic activity. Thus, although the discharge is probably supported by proprioceptive and tactile inputs it is not fully dependent on them. Arm movement-related neurons of area V6A seem to be well equipped for integrating motor signals related to arm movements with somatosensory signals evoked by those movements. Taking into account also the visual characteristics of V6A neurons, it seems likely that area V6A as a whole is involved in the visual guiding of reaching.

Corticovestibular interactions: anatomy, electrophysiology, and functional considerations.

Abstract: This review summarizes anatomical and electrophysiological observations related to corticovestibular interactions as a step toward understanding their possible functions. Vestibular information is represented in at least three distinct regions of the cerebral cortex in cats and monkeys: the parietal and somatosensory cortex and the parietoinsular vestibular cortex. In addition, vestibular-related signals are found in more extensive regions, including the motor and premotor regions and frontal eye fields. Most of these regions also project directly to the vestibular nuclei. In monkeys, at least six cortical regions have been identified, including the motor, somatosensory, parietal and temporal areas. Most of these regions receive vestibular projections via the thalamus. Most neurons in those cortical areas respond to head velocity and receive converging vestibular, visual and somatosensory input. Electrical stimulation of some of these cortical areas in anesthetized cats influences the activity of many vestibular nuclear neurons including those projecting to the spinal cord. Lesions of the parietal vestibular regions impair the vestibulo-ocular reflex (VOR) and visual suppression of the VOR as well as vestibular-related cognitive functions such as spatial perception and memory in human subjects. Diffuse cortical damage also results in similar impairment of the VOR and suppression of the VOR and possibly the vestibulo-collic reflex. Such impairments after cortical lesions may well be due in part to interruption of cortico-vestibular connections. Future studies in alert animals should focus on the role of different cortical regions projecting to the vestibular nuclei, specifically on how each affects the processing of vestibular signals that mediate vestibulo-motor reflexes and that are used for vestibular related cognitive processes.

Lingual deficits in BDNF and NT3 mutant mice leading to gustatory and somatosensory disturbances, respectively

Abstract: A combination of anatomical, histological and physiological data from wild-type and null-mutated mice have established crucial roles for BDNF and NT3 in gustatory and somatosensory innervation of the tongue, and indeed for proper development of the papillary surface of the tongue. BDNF is expressed in taste buds, NT3 in many surrounding epithelial structures. Absence of BDNF in mice leads to severely malformed taste bud-bearing papillae and severe reduction of taste buds, a loss of proper innervation of remaining taste buds and a loss of taste discrimination although not of the suckling reflex per se. In contrast, absence of NT3 leads to a massive loss of somatosensory innervation of lingual structures. These findings demonstrate distinct roles for BDNF and NT3 in the establishment of the complex innervation apparatus of the tongue with non-overlapping roles for the lingual gustatory and somatosensory systems. The distinction between different sensory modalities, being dependent on either BDNF or NT3 may also have clinical implications.

Functional localization of pain perception in the human brain studied by PET.

Abstract: To elucidate the functional localization and somatotopic organization of pain perception in the human cerebral cortex, we studied the regional cerebral blood flow using positron emission tomography during selective painful stimulation in six normal subjects. Response to a painful stimulus was elicited using a special CO 2 laser, which selectively activates nociceptive receptors, to the hand and foot. Multiple brain areas, including bilateral secondary somatosensory cortices (SII) and insula, and the frontal lobe and thalamus contralateral to the stimulus side, were found to be involved in the response to painful stimulation. While our data indicate that the bilateral SII play an important role in pain perception, they also indicate that there is no pain-related somatotopic organization in the human SII or insula.

Input-increase and input-decrease types of cortical reorganization after upper extremity amputation in humans.

Abstract: A plastic remodeling of regions in somatosensory cortex has previously been observed to occur in separate experimental paradigms in response to loss of somatosensory input and to increase in input. In this study, both types of cortical reorganization have been observed to occur concurrently in the same adult human nervous system as a result of a single intervention. Following upper extremity amputation, magnetic source imaging revealed that tactile stimulation of the lip evoked responses not only in the area of the somatosensory cortex corresponding to the face, but also within the cortical region that would normally correspond to the now absent hand. This “invasion” of the cortical amputation zone was accompanied by a significant increase in the size of the representation of the digits of the intact hand, presumably as a result of an increased importance of sensory stimulation consequent to increased dependence on that hand imposed by the loss of the contralateral extremity.

Lateralization and functional organization of the locus coeruleus projection to the trigeminal somatosensory pathway in rat.

Abstract: The primary goals of this study were to (1) examine the distribution of locus coeruleus (LC) neurons, which project to cortical and subcortical sites along the trigeminal somatosensory pathway in rats, and (2) determine the extent to which different regions within this ascending sensory system receive collateral projections from the same LC neuron. Long-Evans hooded rats received unilateral pressure injections of different combinations of retrograde fluorescent tracers into whisker-related regions of primary (SI) and secondary (SII) somatosensory cortices, the ventrobasal (VB) and posterior group (POm) nuclei of the thalamus, and the principalis nucleus of the trigeminal complex (PrV). Coronal sections (40–100 μm) through the LC were examined by fluorescence microscopy, and the distribution of retrogradely labeled cells was recorded. The major finding was that whisker-related regions of the cortex receive efferent projections from neurons concentrated in the caudal portion of the ipsilateral LC, whereas subcortical trigeminal somatosensory structures receive bilateral input from both LC nuclei. Despite the bilateral nature of the LC projection to subcortical sites, the majority of LC efferents to VB and POm thalamus originate in the ipsilateral LC nucleus, whereas projections to PrV originate primarily from the contralateral LC. An additional finding was that a relatively large proportion of LC cells, which project to a single somatosensory structure, also send axon collaterals to other relay sites along the same ascending somatosensory pathway. Taken together, these results suggest that the LC–noradrenergic system maintains a more selective relationship with functionally related efferent targets than has been previously appreciated. J. Comp. Neurol. 385:135–147, 1997. © 1997 Wiley-Liss, Inc.

Somatosensory fovea in the star-nosed mole: Behavioral use of the star in relation to innervation patterns and cortical representation

Abstract: Eleven fleshy appendages, or rays, surround each of the nostrils of the star-nosed mole Each ray is covered with tactile sensory organs, and each ray is represented in the cortex by a stripe of tissue visible in brain sections processed for cytochrome oxidase Here we report that the 11th, ventral ray is the behavioral focus of the nose This ray is preferentially used to explore prey items by touch, in a behavior pattern similar to the use of a fovea in the visual system After prey is first contacted with any nasal ray, subsequent touches are centered on the 11th ray Although the 11th ray is small and has relatively few sensory organs on its surface, it has the largest cortical representation, greatest area of cortex per sensory organ, and the highest innervation density per sensory organ In addition, the average area of cortex per primary afferent is highest for this ray We refer to the differential magnification of first-order afferents in the cortical representation as “afferent magnification” The patterns of both cortical magnification (cortical area per sensory organ) and afferent magnification (cortical area per afferent) of the rays correlated highly with the distribution of touches across the nose scored from videotaped behavior A simple model of star-nosed mole behavior predicts the distribution of touches across the rays and also correlates highly with both the actual pattern of behavior and the patterns of cortical magnification observed J Comp Neurol 387:215–233, 1997 © 1997 Wiley-Liss, Inc

Spatial frames of reference and somatosensory processing: a neuropsychological perspective

Abstract: In patients with lesions in the right hemisphere, frequently involving the posterior parietal regions, left-sided somatosensory (and visual and motor) deficits not only reflect a disorder of primary sensory processes, but also have a higher-order component related to a defective spatial representation of the body. This additional factor, related to right brain damage, is clinically relevant: contralesional hemianaesthesia (and hemianopia and hemiplegia) is more frequent in right brain-damaged patients than in patients with damage to the left side of the brain. Three main lines of investigation suggest the existence of this higher-order pathological factor. (i) Right brain-damaged patients with left hemineglect may show physiological evidence of preserved processing of somatosensory stimuli, of which they are not aware. Similar results have been obtained in the visual domain. (ii) Direction-specific vestibular, visual optokinetic and somatosensory or proprioceptive stimulations may displace spatial frames of reference in right brain-damaged patients with left hemineglect, reducing or increasing the extent of the patients' ipsilesional rightward directional error, and bring about similar directional effects in normal subjects. These stimulations, which may improve or worsen a number of manifestations of the neglect syndrome (such as extrapersonal and personal hemineglect), have similar effects on the severity of left somatosensory deficits (defective detection of tactile stimuli, position sense disorders). However, visuospatial hemineglect and the somatosensory deficits improved by these stimulations are independent, albeit related, disorders. (iii) The severity of left somatosensory deficits is affected by the spatial position of body segments, with reference to the midsagittal plane of the trunk. A general implication of these observations is that spatial (non-somatotopic) levels of representation contribute to corporeal awareness. The neural basis of these spatial frames includes the posterior parietal and the premotor frontal regions. These spatial representations could provide perceptual-premotor interfaces for the organization of movements (e.g. pointing, locomotion) directed towards targets in personal and extrapersonal space. In line with this view, there is evidence that the sensory stimulations that modulate left somatosensory deficits affect left motor disorders in a similar, direction-specific, fashion.

Differential control of cortical activity by the basal forebrain in rats: a role for both cholinergic and inhibitory influences.

Abstract: Using microdialysis and high-performance liquid chromatography, we measured acetylcholine (ACh) release simultaneously from two cortical sites in anesthetized rats. One site was always in the somatosensory cortex, and the other was in either the visual or the motor cortex. After baseline measurements were obtained, selected sites in the basal forebrain (BF) were stimulated to increase ACh release. Some BF sites provoked more release in one microdialysis probe than in the other, suggesting some degree of corticotropic organization of the cholinergic projections from the BF. BF sites optimal for release from the visual cortex were separated from optimal sites for release from the somatosensory cortex by greater distances than were the best sites for release from the somatosensory and the motor cortex. Stimulation of a single BF site often provoked similar release from the latter two cortical areas. Electrical stimulation of the BF also modified cortical neuronal activity. Activation of some BF sites provoked an intense discharge of many neurons in the vicinity of the cortical recording electrode, and the same stimulus site in the BF provoked release of large amounts of ACh in the cortex. Stimulation of other BF sites produced strong inhibition of ongoing cortical activity and no increase in cortical ACh release. When other sites were stimulated, they had no effect or they generated stereotyped bursting patterns in the cortex without any observable effect on ACh release. BF sites that generated inhibition of cortical neural activity were generally located near the sites that activated the cortex and provoked release of ACh. These data suggest an elaborate control of the sensory cortex by a mechanism involving both gamma-aminobutyric acid-containing and cholinergic neurons of the BF. J. Comp. Neurol. 381:53-67, 1997. © 1997 Wiley-Liss, Inc.

Chemical activators of sensory neurons

Abstract: Chemical activation of sensory neurons plays an important role in the somatosensory system The actions of both endogenous mediators such as excitatory amino acids, acetylcholine, bradykinin, and ATP, as well as selective exogenous activators of nociceptive sensory neurons are reviewed The physiological significance of these mediators in both nociception and other types of sensation are discussed

Neuronal activity of somatosensory cortex in a cross-modal (visuo-haptic) memory task

Abstract: Studies have shown that in the monkey′s associative cerebral cortex, cells undergo sustained activation of discharge while the animal retains information for a subsequent action. Recent work has revealed the presence of such ″memory cells″ in the anterior parietal cortex (Brodmann′s areas 3a, 3b, 1, and 2) – the early stage of the cortical somatosensory system. Here we inferred that, in a cross-modal visuo-haptic short-term memory task, somatosensory cells would react to visual stimuli associated with tactile features. Single-unit discharge was recorded from the anterior parietal cortex – including areas of hand representation – of monkeys performing a visuo-haptic delayed matching-to-sample task. Units changed firing frequency during the presentation of a visual cue that the animal had to remember for making a correct tactile choice between two objects at the end of a delay (retention period). Some units showed sustained activation during the delay. In some of them that activation differed depending on the cue. These findings suggest that units in somatosensory cortex react to visual stimuli behaviorally associated with tactile information. Further, the results suggest that some of these neurons are involved in short-term active memory and may, therefore, be part of cross-modal memory networks.

Granule Cell Activation of Complex-Spiking Neurons in Dorsal Cochlear Nucleus

Abstract: Dorsal cochlear nucleus (DCN) principal cells receive, in addition to their well known auditory inputs, various nonauditory inputs via a cerebellar-like granule cell circuit located in the superficial layers of the DCN. Activation of this circuit (granule cell axons make excitatory synapses on the principal cells but also contact inhibitory interneurons that project to the principal cells) produces strong inhibition of the principal cells. Here we investigate the role of cartwheel cells, homologs of cerebellar Purkinje cells, in producing this inhibition. The responses of type IV units (one type of principal cells) and of cartwheel cells were recorded to ortho- and antidromic activation of the granule cells (i.e., by stimulation of their inputs from the somatosensory cuneate and spinal trigeminal nuclei and by direct stimulation of their parallel fiber axons). Cartwheel cells were identified on the basis of recording depth and complex action potential shape. A four-pulse facilitation paradigm (four pulses at 50 msec intervals) was used; this stimulus allows separation of the apparently simple inhibitory somatosensory response of type IV units into a three-component (inhibition–excitation–inhibition) response. As expected, cartwheel cells are excited by granule cell activation; the latencies and four-pulse amplitudes of these responses correspond to the properties of the second, long-latency inhibitory component of type IV responses. The source of the first, short-latency inhibitory response is still unknown. Nevertheless, these results show that cartwheel cells convey inhibitory polysensory information to DCN principal cells.

The effects of hyperventilation on postural control mechanisms

Abstract: The effect of hyperventilation on postural balance was investigated. Voluntary hyperventilation increased body sway in normal subjects, particularly in the sagittal plane. The possibility that this hyperventilation-induced unsteadiness is due to interference with lower limb somatosensory input, vestibular reflexes or cerebellar function was assessed. (i) The effect of hyperventilation on peripheral compound sensory action potentials (SAPs) and somatosensory evoked potentials (SEPs) (recorded centrally, from the scalp) elicited by electrical stimulation of the sural nerve was measured in six normal adults. A reduction in the scalp SEP amplitude and an increase in the peripheral SAP amplitude were observed during hyperventilation, which reversed during the recovery period. These changes indicate increased peripheral neural excitability which could lead to a higher level of ectopic activity; the latter would interfere with central reception of peripheral input. (ii) The click-evoked vestibulo-collic reflex was recorded to study the effect of hyperventilation on vestibulo-spinal activity. EMG recordings from both sternocleidomastoid muscles of six healthy subjects were made in response to loud clicks presented to either ear. Neither the amplitude nor the latency of the response were altered significantly by hyperventilation. (iii) Eye-movement recordings were obtained in the six normal subjects to assess the effect of hyperventilation on the vestibulo-ocular reflex and its visual suppression, the latter being a function largely mediated by the cerebellum; no changes were detected. (iv) Three-dimensional eye-movement recordings and body-sway measurements were obtained in six patients with longstanding unilateral vestibular loss in order to evaluate if hyperventilation disrupts vestibular compensation. In all patients, a horizontal nystagmus either appeared or was significantly enhanced for > or = 60 s after voluntary hyperventilation. Sway was also enhanced by hyperventilation in these patients, particularly in the frontal plane. This study suggests that hyperventilation disrupts mechanisms mediating vestibular compensation. The increase in sway may be, at least partly, mediated by deranged peripheral and central somatosensory signals from the lower limbs. Hyperventilation seems to spare vestibular reflex activity and cerebellar-mediated eye movements.