The human linear vestibulo-ocular reflex to transient accelerations: visual modulation of suppression and enhancement

Visual modulation of the linear vestibulo-ocular reflex (LVOR) was investigated in 4 normal subjects exposed to translational interaural transient accelerations of 0.08 g and 0.17 g. Binocular eye movements were recorded with the scleral search-coil technique. LVOR modulation with target proximity was studied using earth-fixed targets at distances of 30 and 60 cm (LVOR-E). LVOR suppression (LVOR-S) was provoked by similar targets which were head-fixed. For both conditions, linear acceleration evoked compensatory slow-phases whose velocities were, from onset, enhanced in proportion to acceleration and target proximity. At 80 ms after motion onset, i.e. before visually-guided eye movements could aid target fixation, gains (eye velocity/relative target velocity) during LVOR-E averaged 0.32 (S.D. 0.07) over all combinations of accelerations and target distances. At this time, eye velocities for LVOR-S were on average 33% lower than for LVOR-E (1.8 degrees/s vs. 2.7 degrees/s). During LVOR-S, a marked suppression of eye movements appeared at 102 ms (S.D. 10 ms). We conclude that mechanisms other than pursuit can be used to attenuate the LVOR at < 80 ms but their effect is weak. The marked suppression observed around 100 ms might be due to an early visual effect on vestibular pathways or by some independent voluntary control mechanism.