Brain, Vol. 125, No. 9, 2081-2088,
September 2002
© 2002 Guarantors of Brain
Vestibular-evoked postural responses in the absence of somatosensory information
1 MRC Human Movement Group, Sobell Department for Motor Neuroscience and Movement Disorders, Institute of Neurology, University College London, London and 2 Department of Clinical Neurosciences, University of Southampton and Poole Hospital, UKCorrespondence to: B. L. Day, MRC Human Movement Group, Sobell Department for Motor Neuroscience and Movement Disorders, Institute of Neurology, Queen Square, London WC1N 3BG, UKE-mail: b.day@ion.ucl.ac.uk
Received December 13, 2001. Revised March 25, 2002. Accepted March 27, 2002.
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In order to investigate the ways in which sensory channels interact to control balance, we measured the postural response evoked by galvanic vestibular stimulation (GVS) in a rare subject (I. W.) with a large-fibre sensory neuronopathy. I. W. has no sensations of cutaneous light touch and movement/position sense below the neck, and without vision he has no knowledge of where his limbs and body are in space. He was tested with and without vision while seated. With eyes closed, I. W.’s responses to pure vestibular stimuli were an order of magnitude larger than those of healthy controls. In other respects his responses were normal. Part of this phenomenon may have been due to lack of response modification by somatosensory feedback. However, the initial development of his ground reaction force, which is the earliest mechanical indicator of the response, differed from that of a control subject from its beginning. Similarly, opening his eyes resulted in a reduction (>50%) of the response from its beginning. We propose that these early changes reflect changes in initial response selection, possibly by alterations in the gain of vestibulopostural channels. We suggest that similar gain changes operate in healthy subjects and occur through a fast dynamic process. A model is put forward in which the weight of each sensory channel is adjusted continuously in a competitive manner according to the balance-relevant information content of the other sensory channels. As a secondary issue, the nature of I. W.’s head and trunk tilt response provides insight into the question of which vestibular afferents are recruited by GVS. I. W.’s responses consisted of an initial, relatively fast tilt followed by a slower, continuous tilt. When the stimulus was turned off, his body partially tilted back at an intermediate velocity. We modelled this behaviour as the algebraic sum of a position response and a constant velocity response. We suggest that these two components arise from stimulation of otolith and semicircular canal afferents, respectively.
Keywords: galvanic vestibular stimulation; balance; deafferentation; vestibular; proprioception
Abbreviations: GVS = galvanic vestibular stimulation
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The brain uses many types of sensory information to keep the body upright. Separate strands of research have shown how information from the vestibular, visual and somatosensory systems all feed into the controlling process. But it is not evident how information from these diverse sources is combined to produce unified motor behaviour. If one thinks about the body entering an unstable state, say the start of a fall from a standing position, then it is clear that all three sensory channels would provide their own brand of information signalling the fall. The vestibular system and visual system would signal changes in head and eye position with respect to the external world while the somatosensory system would signal motion of the joints as well as changes in muscle state and contact force between the feet and ground. Thus, the developing fall would be indicated by a unique pattern of sensory information involving all channels, and the particular pattern of sensory signals would determine the appropriate motor response. One might therefore expect that the balance control process would be organized to resolve multisensory patterns of input. This concept seems flawed, however, since artificial stimulation of single sensory channels can evoke postural responses. Thus, isolated movement of the visual environment (Lestienne et al., 1977
; Bronstein and Buckwell, 1997), excitation of muscle or cutaneous receptors (Eklund, 1972; Hiyashi et al., 1981; Kavounoudias et al., 1998; Kavounoudias et al., 2001) or changes in vestibular afferent input (Njiokiktjien and Folkerts, 1971; Coates, 1973; Nashner and Wolfson, 1974) are each able, on their own, to produce a postural response. Yet it is difficult to think of a physical situation in which loss of balance would disturb only one sensory channel.
An alternative model is that each sensory channel produces postural changes independently through private sensorimotor pathways. The net response then arises from the summation of outputs from each pathway. This idea is compatible with the finding that the postural responses evoked by separate stimulation of two sensory channels simply sum when the two channels are stimulated together (Hlavacka et al., 1995; Kavounoudias et al., 2001). However, such a model does not allow any interaction between the sensory channels at a premotor stage, and there is evidence that such interactions occur. For example, the direction of postural response evoked by a pure vestibular perturbation is dependent on the position of the head relative to the feet (Nashner and Wolfson, 1974; Lund and Broberg, 1983; Pastor et al., 1993), and the size of the response is modified by the availability of visual information (Njiokiktjien and Folkerts, 1971; Smetanin et al., 1990; Britton et al., 1993; Fitzpatrick et al., 1994). Hence there are circumstances in which the different sensory channels appear to interact at some level in the control process.
In the present study we investigated further this issue of sensory interaction for the control of balance. Our main question is: in what way does the motor response to an input from a single sensory channel depend upon the information available in the other channels? We attempt to answer this for the specific case of how the postural response evoked by a pure vestibular input is modified by the availability of somatosensory and visual information. While visual information can be manipulated simply by asking subjects to close or open their eyes, manipulation of somatosensory information is less straightforward. Our approach was to compare the behaviour of an intact subject with that of a rare subject (I. W.) with a large-fibre sensory neuronopathy that has left him without the sensations of cutaneous light touch and movement/position sense below the neck (Cole and Sedgwick, 1992). Without large myelinated sensory nerve fibre function, I. W. is unable to feel anything but temperature and pain and without vision has no knowledge of where his limbs and body are in space.
To achieve a pure vestibular test stimulus, we used the technique of galvanic vestibular stimulation (GVS). When applied to standing human subjects, the stimulus evokes a stereotyped whole-body postural response involving the neck, trunk and legs (Day et al., 1997). Because our subject is unable to stand securely with eyes closed, we chose to study his postural responses while he was seated. In seated healthy subjects, GVS evokes a very small but consistent response (Day et al., 1997). Animal work has shown that GVS acts by modulating the spontaneous firing frequency of vestibular afferents (Lowenstein, 1955; Goldberg et al., 1984; Courjon et al., 1987). In man, the mechanism is assumed to be the same, although it is not known what proportion or which types of vestibular afferents are recruited by the stimulus. The postural response evoked by GVS in I. W., as well as being relevant to theories of sensory interaction for balance control, also provides some insight into this question.
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The experimental procedure was performed with ethics committee approval and informed consent was obtained according to the Declaration of Helsinki.
Subject I. W. has a large-fibre sensory neuronopathy (Sterman et al., 1980). Though cutaneous sensation is absent below the collar line, he appears to have preserved neck proprioception, either because these afferents are all preserved or, perhaps more likely, because sufficient afferents remain to preserve function. Initially he was completely incapacitated by his loss of feedback for movement, but he taught himself to move again and subsequently to walk and live independently, using visual feedback and cognitive control of movement (Cole, 1995).
The subject was seated on a stool (50 cm high) without arm or back support. The stool and the subject’s feet were situated on a force-plate that registered the net ground reaction- force vector and its point of application on the plate’s surface (Kistler type 9287; Kistler Instrumente, Winterthur, Switzerland). His feet were placed wide apart on the floor and his hands were clasped together in front while his forearms rested against his thighs. Therefore his posture was inherently stable and he was able to close his eyes without fear of falling. For extra security, one of the authors stood by his side to provide support in the event of instability. For comparison, one author (J. C., age-matched to I. W.), who also had never previously experienced galvanic vestibular stimulation (GVS), underwent the same procedure while seated with a similar posture.
Bipolar constant-current GVS was applied via 2.5 cm diameter self-adhesive flexible electrodes (PALS; Nidd Valley, Knaresborough, UK) fixed to the mastoid processes. Before a trial began the subject closed his eyes. Data collection started 1–3 s later and lasted for 12 s. Stimuli were applied for 4 s starting 4 s after the start of each trial. The polarity of each stimulus was varied in a pseudorandom manner across trials. GVS current intensity was varied in blocked trials, each block consisting of 12 stimuli (six of each polarity). The order of the first three blocks was 0.5, 1.0 and 1.5 mA. In a fourth block the subject was tested with eyes open using a stimulus current of 1.5 mA.
Motion of the head and trunk was recorded in three dimensions with a sampling frequency of 200 Hz using an opto-electronic motion analysis system (Selspot II; Selcom, Partille, Sweden), which recorded the coordinates of four infrared-emitting diodes (IRED) attached to the body segments. Two IREDs, placed 16.7 cm apart, were attached to a vertically orientated bar fixed to a rigid headband. IREDs were fixed to the skin overlying the C7 and L2 spinous processes. These data were further processed by averaging over every four data points, which reduced signal noise and reduced the effective sampling frequency to 50 Hz. From these processed data, tilt of the head and the trunk in the frontal plane was calculated.
Responses were obtained from the changes in body segment tilt measured over the 4 s stimulation period. These data were analysed separately for I. W. and the control subject using a two-factor general linear model. The factors were Current (three levels) and Body segment (two levels). Comparisons between I. W. and the control subject and the influence of vision on the response were analysed using Student’s t-test.
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Sensations arising from GVS
When GVS is applied to seated healthy subjects they usually do not report spontaneously on the sensation other than maybe the effect of cutaneous stimulation under the electrodes. With eyes closed, some may report slight dizziness or apparent motion. In contrast, I. W. freely reported that he found the sensation surprising and interesting. With a stimulus intensity of 1 mA, he reported that he felt ‘woolly-headed’, which he likened to a floating sensation. At 1.5 mA the sensation was more disturbing in that he felt he had ‘lost contact with the world’. These apparently exaggerated sensations were reflected in his postural responses evoked by GVS.
Direction, size and distribution of response
Throughout every recording, I. W.’s head tended to drift by slowly tilting towards the left (Fig. 1). Vision reduced the drift but did not prevent it totally (Fig. 1). The drift presumably represents a motor bias that somatosensory feedback would ordinarily correct. GVS evoked reproducible lateral tilts (roll) of the head and trunk in I. W. (Fig. 1). The direction of tilt depended upon stimulus polarity and was such that the anodal ear was tilted down, as shown previously in healthy subjects (Day et al., 1997). After an initial, relatively fast tilt response, the body segments continued tilting at a slower rate while the stimulus remained on.
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For all intensities and for both body segments, I. W.’s tilt amplitudes (change in tilt from the beginning to the end of the 4 s stimulus in single trials) were an order of magnitude larger than those of the control subject (Fig. 1 and Table 1; P < 0.001 for all comparisons, t-test). Although these analyses rely on the behaviour of a single control subject, we also present some summary data on the response to GVS obtained previously from 10 healthy subjects whilst seated (Day et al., 1997
). These previous data on body segment tilt [eyes closed, mean (standard deviation) head tilt 0.44 (0.15)°; trunk tilt 0.31 (0.16)°], which were obtained using a stimulus intensity of 0.7 mA, are comparable to those of the present control subject.
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There was an effect of stimulus intensity on tilt amplitude for both I. W. [Current, F(2,33) = 44.59, P < 0.001] and the control subject [F(2,33) = 3.88, P < 0.05]. The sizes of the mean head tilt responses at 1.0 and 1.5 mA relative to that at 0.5 mA were similar for the two subjects (I. W., 194 and 253%, respectively; control, 200 and 237%).
The distribution of the response was comparable for the two subjects in that the head tilted significantly more than the trunk [Segment, I. W., F(1,33) = 202.19, P < 0.001; control, F(1,33) = 37.51, P < 0.001]. The similarity was apparent when the trunk tilt was expressed as a percentage of the head tilt [49% for I. W. and 43% for the control subject; t = 0.77, degrees of freedom (df) = 94, P > 0.05].
I. W. showed a significant interaction [Current x Segment; F(2,33) = 18.44, P < 0.001], which was due to the smaller increments of tilt with increasing stimulus intensity for the trunk compared with the head, but this effect did not reach significance for the control subject [F(2,33) = 1.805, P > 0.05].
Effect of vision
As shown in the averaged traces of Fig. 1 and in Table 1, opening the eyes during stimulation dramatically decreased I. W.’s tilt responses (head, t = 9.83, df = 22, P < 0.001; trunk, t = 9.04, df = 22, P < 0.001). However, vision did not significantly alter the tilt responses of the control subject (head, t = 0.56, df = 22, P > 0.05; trunk, t = 0.59, df = 22, P > 0.05).
Components of the response
As noted above, I. W.’s responses consisted of two components. Figure 2A shows the grand mean tilt response for the head and trunk. The traces depicted are averaged across all three stimulus intensities and both stimulus polarities. Responses obtained with the anode on the left mastoid were multiplied by –1 before averaging so that upward deflections represent tilt in the direction of the anode. Slow drifts in tilt (Fig. 1) were artificially removed from each 12 s trace prior to averaging by subtracting a straight line with an equation given by linear regression of the 4 s prestimulus period. The on response was characterized by a relatively fast rate of tilt for the first 1 s or so of stimulation, and this was followed by a slower tilt rate until the end of stimulation. When the stimulus was turned off, both segments tilted back at a constant rate for a brief period but without returning fully to their starting positions.
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This behaviour resembles the sum of two responses, as shown in Fig. 2B. One is a position response, which is characterized by a symmetrical ramp-hold-ramp profile, and the other is a constant-velocity response. The various component values of these two hypothetical responses were estimated from the slopes of best-fit lines determined by linear regression on selected segments of data. The regressions were performed on averaged data for each stimulus intensity separately. The model predicts that S1 (the initial slope, i.e. between 0.4 and 0.9 s after stimulus onset) should equal the sum of S2 (the slope of the constant velocity response between 1.6 and 4.0 s after stimulus onset) and –S3 (the ramp slope of the position response between 0.4 and 0.9 s after stimulus offset). Figure 2C and D shows that the predicted and actual values of the initial tilt response are in reasonable agreement for both the head and the trunk, particularly with stimulus intensities of 0.5 and 1.0 mA.
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The sensorimotor process controlling human balance is capable of considerable adaptation following a major loss of sensory input. A pure vestibular perturbation that barely affects the posture of a healthy seated subject evokes in our subject with severe somatosensory loss (I. W.) a substantial response that is an order of magnitude larger. This suggests that the sensory channels do indeed interact at some level in the balance control process. I. W.’s response is comparatively normal in other respects, such as the scaling with stimulus intensity and the relative contributions of head and trunk segments. One other study has looked at the impact of somatosensory loss on the response to GVS (Horak and Hlavacka, 2001
). The patients in that study suffered relatively mild loss compared with I. W., and their GVS-evoked responses, although somewhat larger than normal under certain circumstances, were of the same order of magnitude as those of healthy subjects. The extreme nature of I. W.’s response presumably results from his more severe deafferentation, which affects all parts of the body below the neck and not just the distal portions of the limbs. This is reasonable, given that somatosensory information from all parts of the postural chain is likely to be relevant for balance control and that the response to GVS is distributed to body segments from the neck to the feet (Day et al., 1997). The present results are relevant to two separate issues. First, they provide insight into which parts of the vestibular afferent system are recruited by GVS. Secondly, they provide unique information regarding the nature of sensory interactions for balance control. We discuss each of these in turn.
The vestibular afferent system recruited by GVS
Understanding the GVS-evoked afferent input is important because of current interest in the technique as a probe to study a number of different motor control processes. These include static balance (Iles and Pisini, 1992; Britton et al., 1993; Fitzpatrick et al., 1994; Day et al., 1997), voluntary movement (Smetanin et al., 1986; Cauquil and Day, 1998) and walking (Fitzpatrick et al., 1999; Bent et al., 2000; Jahn et al., 2000). GVS has even been put forward as the basis of a potential vestibular prosthetic to enhance balance performance (Scinicariello et al., 2001). Unfortunately there is little direct information available regarding which vestibular afferents are recruited by GVS. In man no direct recordings have been made from identified vestibular afferents. In the monkey GVS has been shown to affect afferents from semicircular canals and otoliths (Goldberg et al., 1984), but in this case the stimulating electrodes were placed directly in the perilymphatic space and so may not be equivalent to human transmastoid stimulation.
In human studies it has often been stated that the GVS-evoked postural response probably arises from stimulation of otolith afferents. One of the reasons for supposing this is that GVS evokes a relatively static body tilt that is compatible with utricular input. However, in healthy subjects this apparently simple tilt response is likely to be contaminated by the modifying influence of somatosensory feedback. Human GVS-evoked eye movements may provide clearer indirect information about which afferents are recruited, but even here a consensus has not yet been reached. Early work showed how GVS at relatively high intensities could produce nystagmic responses indicative of semicircular canal afferent stimulation (Hitzig, 1871; Pfaltz, 1970). More recent work has shown that the major eye movement response to GVS is ocular torsion (Zink et al., 1997, 1998; Severac Cauquil et al., 1998; Watson et al., 1998; Kleine et al., 1999), which is compatible with either otolith or semicircular canal afferent stimulation.
I. W.’s lack of feedback allowed an unfettered response to develop. Potentially, this provides a purer view of the vestibular input evoked by GVS. It should be pointed out that I. W.’s head-tilt response, which is the sum of body tilt and head-on-trunk tilt, should not strictly be considered free from feedback since there is some existing somatosensory information from the neck. Nevertheless, his head and trunk responded similarly and we consider feedback from his trunk to be essentially absent. I. W.’s response had two components, consisting of an initial relatively fast tilt followed by a slower tilt that continued for the duration of the stimulus. When the stimulus was turned off, his body partially tilted back at an intermediate velocity. A simple model that consists of the algebraic sum of a position response and a constant-velocity response has been shown to mimic this behaviour and to fit the data reasonably well. Accordingly, the actual response may consist of two such components that arise from stimulation of two classes of vestibular afferent. Good candidates for the position response and the velocity response would be otolith afferents (static tilt) and semicircular canal afferents (continuous tilt), respectively. With this interpretation, the afferents recruited by transmastoid GVS in man would have no particular otolithic or canalicular preponderance, as in the monkey.
Response modification through feedback versus selection
Somatosensory information has the potential to modify and shape the response to GVS through a reafferent feedback mechanism. As the response unfolds, the resulting motion of the joints, together with changes in muscle state and contact forces, is signalled by the somatosensory system. This feedback information may then be acted upon to attenuate or arrest the developing GVS-evoked response. I. W.’s lack of such a somatosensory feedback mechanism probably contributed towards his exaggerated response size. By the same token, visual information could have altered the response through a visual feedback mechanism, which may partly explain why his responses were considerably smaller with eyes open than shut.
In addition to any reafferent feedback mechanism that may have acted, the availability of sensory information also influenced the initial response selection. The evidence comes from changes in the development of the response at a very early stage, before feedback had had a reasonable time to act. This is illustrated in Fig. 3, which shows the first few hundred milliseconds of the response. Three responses are superimposed after averaging across polarity (responses obtained with the left anode were multiplied by –1 before averaging so that upward deflections are in the direction of the anode) at one stimulus intensity. They are from the 1.5 mA mean response of (i) I. W. with eyes closed, (ii) I. W. with eyes open, and (iii) the control subject with eyes open. Therefore, the traces represent the response to a vestibular perturbation when it is received in the presence of no other sensory information, visual information, or somatosensory plus visual information, respectively. The figure shows the initial mean response to GVS as the pulse of force exerted by the subject on the floor, which acted to accelerate the body sideways in the direction of the anode, together with the resultant changes in tilt of the trunk and head. The three traces can be seen to separate from the beginning of the force pulse, itself the earliest mechanical event. We propose that these early changes reflect changes in initial response selection, possibly by alterations in the gain of vestibulopostural channels according to the availability of other sensory information.
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In I. W. this re-weighting of the vestibular channel may be part of a slow plastic adaptation that requires a chronic sensory loss in order to be established. However, similar effects were apparent in I. W. when vision was manipulated, which implies that gain changes may be produced by a normal dynamic process that operates on a much shorter time-scale. Such a dynamic process may underlie the phenomena in healthy subjects who show changes in the GVS-evoked response according to the availability of visual (Smetanin et al., 1990
; Britton et al., 1993; Fitzpatrick et al., 1994) or somatosensory information (Britton et al., 1993), or when the subject’s posture is altered (Day et al., 1997). Where this dynamic process operates is not yet known, but it could involve a single site or multiples sites anywhere downstream from, and including, the vestibular nuclei.
We are unable to say whether there has been a similar increase in gain of I. W.’s visual channel as this was not tested formally. However, it would not be surprising if this were so, given I. W.’s extreme dependence on visual information for the successful performance of most motor tasks (Cole, 1995). Indeed, I. W. claims that his balance is poor on a plane, train or boat and that when standing normally he consciously uses visual information, for example from a spot on the wall opposite, to assess his stability. But unlike this type of visual dependence, the increase in vestibular gain has not been achieved through high-level cognitive retraining. It appears rather to have been an automatic gain change of which I. W. was unaware, and which may have underlain his reported experience of ‘disconnection’ during GVS.
Sensory interactions for balance control
With this interpretation, the picture is one of a control process in which information from the three sensory channels (vestibular, visual and somatosensory) is weighted dynamically in order to regulate posture. Each channel has direct access to the motor system, such that a perturbation delivered to any one channel in isolation will produce a response. However, the response is computed with reference to the current information available from the undisturbed channels. The evidence presented here suggests that the gain of a particular input–output relationship is constantly updated in a competitive manner as a function of the amount or quality of information available in the other channels. By analogy, such a system can be likened to a proportional representation voting system, in which every single vote has an impact on the outcome but the strength of that impact depends upon the total number of votes. Thus, without somatosensory or visual information these channels have no vote and so vestibular input determines the behaviour completely. The response to GVS is then maximal. When vision is also available, the impact of the vestibular vote is lessened and so the response size is reduced. Of course, if both sensory systems were perturbed congruently, such that they vote for the same outcome, then the response size would be restored. Inclusion of somatosensory information weakens the vestibular (and visual) vote considerably more. However, a sensory channel’s share of the vote need not be constant. For example, in order to explain posturally related gain changes in response to GVS (Day et al., 1997), the somatosensory vote may be less when a person is standing with a wide base than when sitting, and less still when standing with a narrow base. The question then becomes, for any sensory channel, how its information content that is relevant to the control of balance, and hence its voting power, is determined by the brain from moment to moment.
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We wish to thank I. W. for his kind co-operation, Mr R. Bedlington for technical support, Dr R. C. Fitzpatrick for helpful discussion and Professor J. C. Rothwell for enabling the experiments.
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  A. D. Goodworth and R. J. Peterka Contribution of Sensorimotor Integration to Spinal Stabilization in Humans J Neurophysiol, July 1, 2009; 102(1): 496 - 512. [Abstract] [Full Text] [PDF]
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 N. Vuillerme and R. Cuisinier Sensory Supplementation through Tongue Electrotactile Stimulation to Preserve Head Stabilization in Space in the Absence of Vision Invest. Ophthalmol. Vis. Sci., January 1, 2009; 50(1): 476 - 481. [Abstract] [Full Text] [PDF]
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 R. a. Klint, J. B. Nielsen, J. Cole, T. Sinkjaer, and M. J. Grey Within-step modulation of leg muscle activity by afferent feedback in human walking J. Physiol., October 1, 2008; 586(19): 4643 - 4648. [Abstract] [Full Text] [PDF]
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 Y. K. NAYLOR and M. K. MCBEATH Gender differences in spatial perception of body tilt Atten Percept Psychophys, February 1, 2008; 70(2): 199 - 207. [Abstract] [PDF]
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B. L. Day and M. Guerraz Feedforward versus feedback modulation of human vestibular-evoked balance responses by visual self-motion information J. Physiol., July 1, 2007; 582(1): 153 - 161. [Abstract] [Full Text] [PDF]
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M. S. Welgampola and B. L. Day Craniocentric body-sway responses to 500 Hz bone-conducted tones in man J. Physiol., November 15, 2006; 577(1): 81 - 95. [Abstract] [Full Text] [PDF]
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J.-C. Lepecq, C. De Waele, S. Mertz-Josse, C. Teyssedre, P. T. B. Huy, P.-M. Baudonniere, and P.-P. Vidal Galvanic Vestibular Stimulation Modifies Vection Paths in Healthy Subjects J Neurophysiol, May 1, 2006; 95(5): 3199 - 3207. [Abstract] [Full Text] [PDF]
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M. Cenciarini and R. J. Peterka Stimulus-Dependent Changes in the Vestibular Contribution to Human Postural Control J Neurophysiol, May 1, 2006; 95(5): 2733 - 2750. [Abstract] [Full Text] [PDF]
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J. B. Fallon, L. R. Bent, P. A. McNulty, and V. G. Macefield Evidence for Strong Synaptic Coupling Between Single Tactile Afferents From the Sole of the Foot and Motoneurons Supplying Leg Muscles J Neurophysiol, December 1, 2005; 94(6): 3795 - 3804. [Abstract] [Full Text] [PDF]
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E Jankowska, P Krutki, and K Matsuyama Relative contribution of Ia inhibitory interneurones to inhibition of feline contralateral motoneurones evoked via commissural interneurones J. Physiol., October 15, 2005; 568(2): 617 - 628. [Abstract] [Full Text] [PDF]
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B. L. Day and R. C. Fitzpatrick Virtual head rotation reveals a process of route reconstruction from human vestibular signals J. Physiol., September 1, 2005; 567(2): 591 - 597. [Abstract] [Full Text] [PDF]
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I. Cathers, B. L Day, and R. C Fitzpatrick Otolith and canal reflexes in human standing J. Physiol., February 15, 2005; 563(1): 229 - 234. [Abstract] [Full Text] [PDF]
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 R. C. Fitzpatrick and B. L. Day Probing the human vestibular system with galvanic stimulation J Appl Physiol, June 1, 2004; 96(6): 2301 - 2316. [Abstract] [Full Text] [PDF]
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D. L Wardman, J. L Taylor, and R. C Fitzpatrick Effects of galvanic vestibular stimulation on human posture and perception while standing J. Physiol., September 15, 2003; 551(3): 1033 - 1042. [Abstract] [Full Text] [PDF]
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D. L Wardman, B. L Day, and R. C Fitzpatrick Position and velocity responses to galvanic vestibular stimulation in human subjects during standing J. Physiol., February 15, 2003; 547(1): 293 - 299. [Abstract] [Full Text] [PDF]
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