Feedforward versus feedback modulation of human vestibular-evoked balance responses by visual self-motion information


Corresponding author B. L. Day: MRC Human Movement Group, Sobell Department for Motor Neuroscience and Movement Disorders, Institute of Neurology, UCL, Queen Square, London WC1N 3BG, UK. Email: b.day@ion.ucl.ac.uk


Visual information modulates the balance response evoked by a pure vestibular perturbation (galvanic vestibular stimulation, GVS). Here we investigate two competing hypotheses underlying this visual–vestibular interaction. One hypothesis assumes vision acts in a feedforward manner by altering the weight of the vestibular channel of balance control. The other assumes vision acts in a feedback manner through shifts in the retinal image produced by the primary response. In the first experiment we demonstrate a phenomenon that is predicted by both hypotheses: the GVS-evoked balance response becomes progressively smaller as the amount of visual self-motion information is increased. In the second experiment we independently vary the pre-stimulus and post-stimulus visual environments. The rationale is that feedback effects would depend only upon the post-stimulus visual environment. Although the post-stimulus visual environment did affect later parts of the response (after ∼400 ms), the pre-stimulus visual environment had a strong influence on the size of the early part of the response. We conclude that both feedforward and feedback mechanisms act in concert to modulate the GVS-evoked response. We suggest this dual interaction that we observe between visual and vestibular channels is likely to apply to all sensory channels that contribute to balance control.