Eye-Head Coordination during Optokinetic Stimulation in Squirrel Monkeys

Head and eye movements in the yaw plane were recorded during and after optokinetic stimulation in squirrel monkeys. 1) Phasic or tonic head deviations to the side of the ocular quick phase occurred in 94% of total recordings (n = 50) during the perstimulus period, and in 75% of recordings (n = 49) during the poststimulus period. Magnitude of mean head deviation was significantly different between perstimulus and poststimulus periods. 2) Head nystagmus associated with eye nystagmus was consistently observed in seven of nine squirrel monkeys during optokinetic stimulation. Squirrel monkeys are thereby less prone to display head nystagmus than either guinea pigs, pigeons or chickens. 3) Slow phase speeds of coupled head and eye nystagmus were subjected to statistical analysis. A highly significant negative correlation was found between slow phase head and eye speeds. The correlation coefficient was −0.81 at 60°/sec stimulus (n = 119) and −0.72 at 100°/sec stimulus (n = 131). The gaze speed, calculated by summing the head and eye speeds, was 59.1 ± 6.8/sec at 60°/sec and 92.2 ± 11.4 at 100°/sec stimulus. There was no significant difference between the gaze speed in a free head condition and the eye speed when the head was fixed.