INTERACTION OF LINEAR AND ANGULAR VESTIBULOOCULAR REFLEXES OF HUMAN-SUBJECTS IN RESPONSE TO TRANSIENT MOTION

The possibility of synergistic interaction between the canal and otoli th components of the horizontal vestibule-ocular reflex (VOR) was eval uated in human subjects by subtracting the response to pure angular ro tation (AVOR) from the response to combined angular and translational motion (ALVOR) and comparing this difference with the VOR to isolated linear motion (LVOR). Assessments were made with target fixation at 60 cm and in darkness. Linear stimuli were acceleration steps attaining 0.25 g in less than 80 ms. To elicit responses to combined translation al and angular head movements, the subjects were seated on a Barany ch air with the head displaced forwards 40 cm from the axis of rotation. The chair was accelerated at approximately 300 deg/s(2) to 127 deg/s p eak angular velocity, the tangential acceleration of the head being co mparable with that of isolated translation. Estimates of the contribut ion of smooth pursuit to responses in the light were made from compari sons of isolated pursuit of similar target trajectories, In the dark t he slow phase eye movements evoked by combined canal-otolith stimuli w ere higher in magnitude by approximately a third than the sum of those produced by translation and rotation alone. In the light, the relativ e target displacement during isolated linear motion was similar to the difference in relative target displacements during eccentric and cent red rotation. However, the gain of the translational component of comp ensatory eye movement during combined translational and angular motion was approximately unity, in contrast to the gain of the response to i solated linear motion, which was approximately a half. Pursuit perform ance was always poorer than target following during self-motion. The L VOR responses in the light were greater than the sum of the LVOR respo nses in the dark with pursuit eye movements. We conclude that, in resp onse to transient motion, there is a synergistic enhancement of the tr anslational VOR with concurrent canal stimulation and that the enhance ment of the LVOR in the light is not due solely to pursuit.