Plasticity in the vestibulo-ocular and optokinetic reflexes following modification of canal input.

The vestibulo-ocular reflex (VOR), the optokinetic reflex (OKR), and their interactions were studied before and after unilateral horizontal canal (HC) plug. Two independent changes occur in the VOR following HC plug: (a) a drop in VOR gain which recovers rapidly, provided that monkeys are permitted normal visual-vestibular interaction (VVI); and (b) a deterioration in response characteristics at low frequencies, consistent with a decrease in effective VOR time constant. The latter occurs regardless of VVI and VOR gain recovery and progresses over several days. The time constant may recover, but only after several months. OKR responses are in keeping with the notion that they represent a combination of fast and slow processes. Results after HC plug suggest that the gain of the slow OKR process rises along with that of the VOR. Response characteristics of synergistic and antagonistic VVI behave as linear combinations of VOR and OKR response characteristics both before and after HC plug. Changes in VVI arising after HC plug simply reflect modifications in VOR and OKR response characteristics.