Vertical optokinetic nystagmus in the split-brain monkey

Electrooculograms during optokinetic stimulation in the vertical plane were recorded from seven monkeys (Macaca mulatta) before and after section of pregeniculate visual pathways or midsagittal division of brain commissures, or both. Section of one optic tract or of the chiasma did not alter the response. Additional cutting of the corpus callosum resulted in a nystagmus of lower frequency and perverted direction with an oblique component toward the side of the visually "deprived" hemisphere. The deficits in the monkeys with section of the optic chiasma appeared only on monocular stimulation. Additional division of massa intermedia, anterior, posterior, habenular, and intercollicular commissures, completely abolished the nystagmus upward. This defect was also present on binocular stimulation of animals with section of the optic chiasma. These findings support the concept that both sides of the brain must receive adequate visual input, either through the classic optic pathway or through the commissures, to result in normal optokinetic nystagmus in the vertical plane. Subcortical commissures, particularly the posterior commissure, may be more related to the oculomotor output since their section affects the response even on binocular stimulation of monkeys with division of both the optic chiama and the corpus callosum. Since optokinetic nystagmus is a most sensitive indicator of the integrity of the oculomotor system, the possibility of defective eye movements should be considered in the interpretation of the results of visual tests in split-brain monkeys.