Effects of prolonged optokinetic stimulation on oculomotor and locomotor balance functions.

Squirrel monkeys were exposed to optokinetic stimulus (90 degrees/sec constant speed, unidirectional) for 60 min. Eye movements during and after the stimulus exposure were recorded. Comparison of the data between very early and late stages of exposure showed the oculomotor gain increase and the nystagmic frequency decline. Slow phase eye velocity of bilaterally labyrinthectomized squirrel monkeys in the late exposure stage could reach almost to the level of normal animals. Post-stimulus analysis in normal monkeys showed that amphetamine enhanced the optokinetic after-nystagmus duration, the maximum slow phase eye velocity, and the time constant. In contrast, the effect of amphetamine on reversed optokinetic after-nystagmus was not at the significant level in all parameters studied. The manifestation of directional reciprocity of optokinetic after-nystagmus was inconsistent. In bilaterally labyrinthectomized animals, ipsilateral optokinetic after-nystagmus did not appear after the stimulus cessation. Instead, immediate reversed optokinetic after-nystagmus appeared. When the normal animal was kept in the light after the stimulus cessation, slow phase eye velocity of reversed optokinetic after-nystagmus declined relatively rapidly. Reversed optokinetic after-nystagmus and vestibular evoked nystagmus were summated or deducted, in velocity domain, depending upon the direction. Optokinetically induced system imbalance did not depict when the monkey's spinal locomotor function was measured by the platform runway test with the availability of vision.