SHORT-TERM VESTIBULE-OCULAR REFLEX ADAPTATION IN HUMANS .1. EFFECT ONTHE OCULAR MOTOR VELOCITY-TO-POSITION NEURAL INTEGRATOR

We investigated the effect of short-term vestibule-ocular reflex (VOR) adaptation in normal human subjects on the dynamic properties of the velocity-to-position ocular motor integrator that holds positions of g aze. Subjects sat in a sinusoidally rotating chair surrounded by an op tokinetic nystagmus drum. The movement of the visual surround (drum) w as manipulated relative to the chair to produce an increase(x 1.7 view ing), decrease (x 0.5, x 0 viewing), or reversal (x(-2.5) viewing) of VOR gain. Before and after 1 h of training, VOR gain and gaze-holding after eccentric saccades in darkness were measured. Depending on the t raining paradigm, eccentric saccades could be followed by centrifugal drift (after x 0.5 viewing), implying an unstable integrator, or by ce ntripetal drift [after x 1.7 or x (-2.5) viewing], implying a leaky in tegrator. The changes in the neural integrator appear to be context sp ecific, so that when the VOR was tested in non-training head orientati ons, both the adaptive change in VOR gain and the changes in the neura l integrator were much smaller. The changes in VOR gain were on the or der of 10% and the induced drift velocities were several degrees per s ecend at 20 deg eccentric positions in the orbit. We propose that (1) the changes in the dynamic properties of the neural integrator reflect an attempt to modify the phase (timing) relationships of the VOR and (2) the relative directions of retinal slip and eye velocity during he ad rotation determine whether the integrator becomes unstable (and int roduces more phase lag) or leaky (and introduces less phase lag).