Primate translational vestibuloocular reflexes. II. Version and vergence responses to fore-aft motion

Primate translational vestibuloocular reflexes. II. Version and vergence re sponses to fore-aft motion. J. Neurophysiol. 83: 1648-1661, 2000. To mainta in binocular fixation on near targets during fore-aft translational disturb ances, largely disjunctive eye movements are elicited the amplitude and dir ection of which should be tuned to the horizontal and vertical eccentriciti es of the target. The eye movements generated during this task have been in vestigated here as trained rhesus monkeys fixated isovergence targets at di fferent horizontal and vertical eccentricities during 10 Hz fore-aft oscill ations. The elicited eye movements complied with the geometric requirements for binocular fixation, although not ideally. First, the corresponding ver gence angle for which the movement of each eye would be compensatory was co nsistently less than that dictated by the actual fixation parameters. Secon d, the eye position with zero sensitivity to translation was not straight a head, as geometrically required, but rather exhibited a systematic dependen ce on viewing distance and vergence angle. Third, responses were asymmetric , with gains being larger for abducting and downward compared with adductin g and upward gaze directions, respectively. As frequency was varied between 4 and 12 Hz, responses exhibited high-pass filter properties with signific ant differences between abduction and adduction responses. As a result of t hese differences, vergence sensitivity increased as a function of frequency with a steeper slope than that of version. Despite largely undercompensato ry version responses, vergence sensitivity was closer to ideal. Moreover, t he observed dependence of vergence sensitivity on vergence angle, which was varied between 2.5 and 10 MA, was largely linear rather than quadratic (as geometrically predicted). We conclude that the spatial tuning of eye veloc ity sensitivity as a function of gaze and viewing distance follows the gene ral geometric dependencies required for the maintenance of foveal visual ac uity. However, systematic deviations from ideal behavior exist that might r eflect asymmetric processing of abduction/adduction responses perhaps becau se of different functional dependencies of version and vergence eye movemen t components during translation.