Effects of tonic neck reflex on optokinetic nystagmus in rabbits

In this study, rabbits with black-pigmented eyes were suspended in air with none of their limbs touching the ground, and surrounded by a full-field optokinetic drum. The drum was rotated counterclockwise at 60 degrees/s. Horizontal eye movement was recorded by electronystagmograph with the animals in three postures: 1) initial posture, head and trunk aligned along the same body axis, 2) trunk rotated clockwise by 60 degrees with head stationary, and 3) trunk rotated counterclockwise by 60 degrees with head stationary. The recordings were repeated after cervical nerve roots were sectioned unilaterally. The speed of the slow phase of nystagmus was significantly faster in the second than in the initial posture, but significantly slower in the third than in the initial posture. These effects of trunk rotation on the speed of the slow phase were not seen when the side of cervical nerve section was extended. The results indicate that smooth following eye movement is influenced by the neck reflex. It is accelerated when the head turns in the same direction as the visual field, whereas it is inhibited when the head turns in the opposite direction. Proprioceptors of the extended neck appear to account for the effects observed.