Influence of passive and active pendular head rotation on horizontal optokinetic nystagmus

The influence of pendular head rotation on optokinetic nystagmus was examined using a vestibulo-optic stimulator (pendular rotating chair with an optic cylinder) to study passive head rotation, and an optic cylinder which was rotated by a motor fixed to the head to study active head rotation. Pendular head rotation and optic stimuli were simultaneously and independently applied horizontally. The optic cylinder consisted of 12 vertical stripes rotating at a uniform velocity of 30 degrees/s or 90 degrees/s. Passive pendular head rotation was applied at a frequency of 0.1 Hz and a peak angular velocity of 30 degrees/s. Active head rotation was applied for a period of approximately 10 s, and at an amplitude of approximately 50 degrees. Optokinetic nystagmus was enhanced when the head was rotated in the opposite direction to the optic cylinder. However, when the head and the optic cylinder were rotated in the same direction, optokinetic nystagmus was inhibited. There was little difference between the effects of passive and active head rotation on enhancement. However, during active head rotation, optokinetic nystagmus was less inhibited than during passive head rotation.