Biasing the semicircular canal cupula in excitatory direction decreases the gain of the vestibuloocular reflex for head impulses

Background: It has been shown that cold caloric irrigation decreases the vestibulo-ocular reflex (VOR) gain in the vertically positioned lateral canal as measured by head impulse testing. This effect is most probably caused by a sustained deflection of the cupula in the inhibitory direction. Objective: The aim of the present experiment was to answer the question if a sustained excitatory deflection increases the gain of the reflex in healthy volunteers. Methods: In order to deflect the cupula, cold caloric irrigation was applied in prone (forward head hanging) position. In this position cold thermal irrigation elicited an excitatory caloric nystagmus with an ipsilateral fast phase. Results: When head impulses were applied immediately after cold caloric irrigation, the gain of the VOR decreased in comparison to the values measured before irrigation. Conclusions: Together with the previous results cited above, these data show that biasing the cupula in either direction decreases its sensitivity with respect to high acceleration stimuli. This might occur because the deviation elicits a partial mechanical and electrophysiological saturation of the cupula