THE HUMAN VERTICAL VESTIBULOOCULAR REFLEX DURING COMBINED LINEAR AND ANGULAR-ACCELERATION WITH NEAR-TARGET FIXATION

The purpose of this study was to examine the effect of fixation target distance on the human vestibuloocular reflex (VOR) during eccentric r otation in pitch. Such rotation induces both angular and linear accele ration. Eight normal subjects viewed earth-fixed targets that were eit her remote or near to the eyes during whole-body rotation about an ear th-horizontal axis that was ei ther oculocentric or 15 cm posterior (e ccentric) to the eyes. Eye and head movements were recorded using magn etic search coils. Using a servomotor-driven chair, passive whole-body rotations were delivered as trains of single-frequency sinusoids at f requencies from 0.8 to 2.0 Hz and as pseudorandom impulses of accelera tion. In the light, the visually enhanced VOR (VVOR) was recorded whil e subjects were asked to fixate targets at one of several distances. I n darkness, subjects were asked to remember targets that had been view ed immediately prior to the rotation. In order to eliminate slip of th e retinal image of a near target when the axis of rotation of the head is posterior to the eyes, the ideal gain (compensatory eye velocity d ivided by head velocity) of the VVOR and VOR must exceed 1.0. Both the VOR and VVOR were found to have significantly enhanced gains during s inusoidal and pseudorandom impulses of rotation (P<0.05). Enhancement of VVOR gain was greatest at low frequencies of head rotation and decr eased with in creasing frequency. However, enhanced VOR gain only slig htly exceeded 1.0, and VVOR gain enhancement was significantly lower t han the expected ideal values for the stimulus conditions employed (P< 0.05). During oculocentric rotations with near targets, both the VOR a nd VVOR tended to exhibit small phase leads that increased with rotati onal frequency. In contrast, during eccentric rotations with near targ ets, there were small phase lags that increased with frequency. Visual tracking contributes during ocular compensatory responses to sustaine d head rotation, although the latency of visual tracking reflexes exce eds 100 ms. In order to study initial vestibular responses prior to mo dification by visual tracking, we presented impulses of head accelerat ion in pseudorandom sequence of initial positions and directions, and evaluated the ocular response in the epoch from 25 to 80 ms after move ment onset. As with sinusoidal rotations, pseudorandom eccentric head rotation in the presence of a near, earth-fixed target was associated with enhancement of VVOR and VOR gains in the interval from 25 to 80 m s from movement onset. Despite the inability of visual tracking to con tribute to these responses, VVOR gain significantly exceeded VOR gain for pseudorandom accelerations. This gain enhancement indicates that t arget distance and linear motion of the head are considered by the hum an ocular motor system in adjustment of performance of the early VOR, prior to a contribution by visual following reflexes. Vergence was app ropriate to target distance during all VVOR rotations, but varied duri ng VOR rotations with remembered targets. For the 3-m target distance, vergence during the VOR was stable over each entire trial but slightl y exceeded the ideal value. For the 0. 1-m near target, instantaneous vergence during the VOR typically declined gradually in a manner not c orresponding to the time course of instantaneous VOR gain change; mean vergence over entire trials ranged from 60 to 90% of ideal, correspon ding to target distances for which ideal gain would be much higher tha n actually observed. These findings suggest a dissociation between ver gence and VOR gain during eccentric rotation with near targets in the frequency range from 0.8 to 2.0 Hz.