OTOLITH AND SEMICIRCULAR CANAL CONTRIBUTIONS TO THE HUMAN BINOCULAR RESPONSE TO ROLL OSCILLATION

Three normal human subjects were oscillated about their naso-occipital asis in a supine position at 0.4 Hz and 0.1 Hz. both in darkness and in the light with a structured fixation target. The same subjects were oscillated in roll about an upright position, at the same frequencies , in darkness; and also about axes directed 20 degrees and 40 degrees to the left and to the right of the midsagittal plane, itt 0.4 Hz, in darkness. Three-dimensional binocular eve movements were recorded usin g video-oculography. All stimuli induced a predominantly torsional nys tagmus with small disconjugate head-vertical (skew) and conjugate head -horizontal components. For roll oscillation, the torsional sieve phas e velocity gain ii-as higher in the light and generally increased with rile stimulation frequency. In darkness, only one subject had signifi cantly higher torsional gains in the upright compared to the supine po sition (12% difference), suggesting that the otolith contribution to t ile roll response is minimal at the frequencies tested. The slow phase velocity gain of the skew increased with fixation in two subjects, bu t decreased in the third subject: these changes were related to change s in eye vergence. In the upright position, with oblique axes of rotat ion. the head-vertical eye movements were asymmetrical, with the outer most eye executing a larger amplitude movement. The disconjugate head- vertical eye movements observed can be explained by the pattern of ver tical semicircular canal stimulation and their connections to the extr aocular muscles. In humans, skewing of tile eyes may compensate for th e eccentricity of the foveae which lie in the temporal retina and unde rgo vertical translations in opposite directions when the eyes tort.