Eye-head coordination and lateral canal block in squirrel monkeys.

Eye and head nystagmus during horizontal optokinetic stimulation was analyzed in squirrel monkeys. Slow phase speed of coupled head and eye nystagmus showed highly significant negative correlations in normal subjects; correlation coefficients were -0.81 at 60 degrees/sec and -0.72 at 100 degree/sec stimuli. Bilateral semicircular canal block resulted in a reduction of the negative correlation. The postoperative reduction of the correlation coefficient value ranged from 0.17-0.28 (significant at P less than 0.01 level) at 60 degrees/sec stimulus and 0.06-0.13 (non significant) of 100 degrees/sec stimulus. Gaze speed was calculated by summing slow phase speeds of coupled head and eye nystagmus. Mean error of gaze in normal subjects was 1.5% at 60 degrees/sec stimulus and 7.8% at 100 degrees/sec stimulus. Mean gaze speed showed an overshoot (8% at 60 degrees/sec stimulus and 6.5% at 100 degrees/sec stimulus) in the tree to four weeks postoperative period; however, this subsided two to three months after the operation. Measurement of head-turning performance (in yaw plane) in humans may provide a clinically useful index for the assessment of certain types of vestibular function.