Proceedings of The Physiological Society
Cardiff University (2009) Proc Physiol Soc 17, C19
The effect of actual and expected light touch contact on the response to galvanic vestibular stimulation
C. J. Osler1, M. Lakie1, R. F. Reynolds1
1. School of Sport and Exercise Sciences, University of Birmingham, Birmingham, United Kingdom.
Information from the somatosensory system modulates the postural response to a vestibular-evoked perturbation (galvanic vestibular stimulation, GVS; for review see Fitzpatrick and Day, 2004). A previous study investigating the effect of lightly touching a fixed support reported a reduced muscular response to GVS compared to normal standing (Britton et al., 1993). Here, we further investigate this modulation. Firstly, we studied the effect of small quantified changes in the light touch contact force on the evoked whole body response. Secondly, we investigated if merely an expected change in light touch information could modulate the response to GVS. In the first experiment subjects (n=9) were instructed to stand in the required start position (with vision occluded and their back in contact with a foam pad) while GVS (1 mA for 3 seconds) was applied. Five conditions each studying different target contact forces were used (0N, 0.5N, 1N, 3N and 6N). Results showed the GVS-evoked lateral body displacement was strongly affected by condition (F4,32 = 30.48, p<0.001; repeated measures ANOVA); it was significantly attenuated in all light contact conditions compared to normal standing (all p<0.05; pairwise comparisons with Bonferroni adjustment), including the lowest contact force condition (mean ± S.D.; 0.52 ± 0.13N) where the mechanical advantage provided was minimal. This suggests the attenuated postural response to GVS with light touch contact is partly due to additional cutaneous sensory information and not merely mechanical stabilisation. In light contact conditions the response size was then scaled according to the measured contact force (r2 = 0.35, p<0.001; linear regression). In the second experiment the light touch information available to the subjects could be withdrawn by way of a servo motor. From the same start position the subjects (n=10) self-triggered GVS (2 mA for 0.6 seconds). Subjects expected a simultaneous removal of light touch information in some trials (expected no contact condition) and no such removal in the remainder (expected contact condition). However, in 20% of the expected no contact trials the removal of light touch information did not actually occur (unexpected contact condition). Results showed the postural response was again significantly affected by condition (F2,18 = 17.28, p<0.001; repeated measures ANOVA). However, in contact conditions results showed that GVS evoked a sway response that was not affected by whether the sensory information attained from light touch contact was expected or unexpected (p = 1.00; pairwise comparison with Bonferroni adjustment). This suggests that merely an expected change cannot modulate the evoked response in the same way as an actual change in light contact information.
Where applicable, experiments conform with Society ethical requirements