Context-specific short-term adaptation of the phase of the vestibulo-ocular reflex

The phase of the angular vestibulo-ocular reflex (VOR) is subject to adaptive control. We had previously found that adapting the phase of the VOR also produced changes in drift on eccentric gaze-holding, implying a change in the time constant of the velocity-to-position neural integrator. Here we attempted to dissociate changes in gaze-holding drift 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) with 5 min of making saccades in the light with the head stationary. Afterwards, changes in VOR phase were the same (32% of requested) as those obtained with 1 h of phase adaptation alone, but changes in drift following saccades were much smaller than those found after phase adaptation 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 relationship between gaze-holding drift after VOR steps and adaptive changes in VOR phase, we alternated sinusoidal VOR phase adaptation with normal VOR steps in the light. In this paradigm, the adaptive change in VOR phase 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-position neural integrator can be altered selectively and rapidly depending upon the task required. Such context-specific adaptation is advantageous, because it allows adjustment of the phase of the VOR without degrading the ability to hold eccentric fixation.