Vestibulo-ocular responses evoked via bilateral electrical stimulation of the lateral semicircular canals

We investigated the vestibulo-ocular responses (VORs) evoked by bilateral electrical stimulation of the nerves innervating horizontal semicircular canals in squirrel monkeys and compared these responses to those evoked by unilateral stimulation. In response to sinusoidal modulation of the electrical pulse rate, the VOR for bilateral stimulation roughly equals the addition of the responses evoked by unilateral right ear and unilateral left ear stimulation; the VOR time constants were about the same for bilateral and unilateral stimulation and both were much shorter than for normal animals. In response to individual pulse stimulation, the VOR evoked by bilateral stimulation closely matches the point-by-point addition of responses evoked by unilateral right ear and unilateral left ear stimulation. We conclude that, to first order, the VOR responses evoked by bilateral stimulation are the summation of the responses evoked by unilateral stimulation. These findings suggest that--from a physiologic viewpoint--unilateral and bilateral vestibular prostheses are about equally viable. Given these findings, one possible advantage of a bilateral prosthesis is higher gain. However, at least for short-term stimulation such as that studied herein, no inherent advantage in terms of the response time constant ("velocity storage") was found.