CONTEXT-SPECIFIC SHORT-TERM ADAPTATION OF THE PHASE OF THE VESTIBULOOCULAR REFLEX

The phase of the angular vestibule-ocular reflex (VOR) is subject to a daptive control. We had previously found that adapting the phase of th e VOR also produced changes in drift on eccentric gaze-holding, implyi ng a change in the time constant of the velocity-to-position neural in tegrator. Here we attempted to dissociate changes in gaze-holding drif t from changes in the phase of the VOR. In normal human subjects, for 2 h, we alternated 5 min of VOR phase adaptation (sinusoids, 0.2 Hz) w ith 5 min of making saccades in the light with the head stationary. Af terwards, changes in VOR phase were the same (32% of requested) as tho se obtained with 1 h of phase adaptation alone, but changes in drift f ollowing saccades were much smaller than those found after phase adapt ation alone (0.8 degrees/s compared with 5 degrees/s). When measuring drift after VOR steps, however, the changes were closer to those found after phase adaptation alone (3.8 degrees/s). To test the relationshi p between gaze-holding drift after VOR steps and adaptive changes in V OR phase, we alternated sinusoidal VOR phase adaptation with normal VO R steps in the light. In this paradigm, the adaptive change in VOR pha se was about the same as with phase-adaptation alone (35%), but there was now little drift after saccades (1.9 degrees/s) or after VOR steps (0.7 degrees/s). We conclude that the state of the velocity-to-positi on neural integrator can be altered selectively and rapidly depending upon the task required. Such context-specific adaptation is advantageo us, because it allows adjustment of the phase of the VOR without degra ding the ability to hold eccentric fixation.