Long-term adaptive changes in primate vestibuloocular reflex. I. Behavioral observations.

1. Experiments were concerned with the long-term adaptive changes that occur in the primate vestibuloocular reflex (VOR) when the visual input associated with head movements is disturbed by various optical devices, including telescopic spectacles (magnification 2.0 and OS), fixed-field spectacles (field of view fixed with respect to the head; hence, equivalent here to “zero power” lenses), and dove prism spectacles (providing left-right reversal of vision). 2. Assessed with sinusoidal oscillations (0. l- 1.0 Hz), the VOR of the normal rhesus monkey measured in the dark was close to ‘ ‘ideal’ ’ for maintaining ocular stability during had turns: 180’ out of phase with the head and gain (slow-phase eye velocity/head velocity) close to one. 3. Telescopic and fixed-field spectacles induced adaptive changes in the gain of the reflex, which proceeded roughly exponentially, eventually achieving up to 75% compensation. No phase changes were observed. Caloric responses in the high-gain state were elevated above normal by an amount corresponding to the increase in VOR gain indicated by passive oscillation. The compensatory eye movements coupled to voluntary head turns in the dark were only slightly greater than would have been predicted solely from the passively assessed VOR gain in both normal and highgain animals; nonvestibular factors continued to play, at best, a minor role in the stabilization of retinal images during head turns in the adapted animals. Recovery after removal of the spectacles also followed a