Effect of head and body tilt on flicker-induced nystagmus in monkeys

Nystagmus was induced in four normal monkeys ( Macaca mulatta) by monocular stroboscopic stimulation. The direction and frequency of the response were determined in electrooculograms at eight tilts in the frontal plane and four in the sagittal plane. Eight directions, 45° apart, were identified and the prevailing vector was selected as that showing the highest accumulation in histograms. This vector showed a component to the side of the stimulated eye in all positions tested, and varied sinusoidally with the angle of tilt in the frontal plane. It was minimally influenced by tilts in the sagittal plane. The direction of nystagmus at every tilt was strictly opposite when elicited by right or left eye stimulation. Frequency changes with tilts in the frontal plane were similar for both eyes, being maximal in the upside down and minimal in the lateral positions. There was a significant negative correlation between frequency and the deviation of the prevailing vector away from the horizontal plane of the eyes. With tilts in the sagittal plane, frequency was minimal in the prone posture. The changes in the prevailing vector may be attributed to the function of the otolith organ at a central level or through changes in the initial eye position due to ocular counterrolling or both. The changes in frequency can be interpreted as otolith excitatory or strongly inhibitory influences in the upside down or lateral positions, respectively. It could also reflect a property of the oculomotor system whereby it reacts more effectively in the horizontal plane and its efficiency decreases when oblique vectors are introduced. The phenomenon as a whole is discussed in terms of three possible mechanisms, and its modification by static tilts is offered as an experimental animal model of visuootolithic interactions.