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 ful l-held optokinetic drum. The drum was rotated counterclockwise at 60 d egrees/s. Horizontal eye movement was recorded by electronystagmograph with the animals in three postures: 1) initial posture, head and trun k aligned along the same body axis, 2) trunk rotated clockwise by 60 d egrees with head stationary, and 3) trunk rotated counterclockwise by 60 degrees with head stationary. The recordings were repeated after ce rvical 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 ini tial posture. These effects of trunk rotation on the speed of the slow phase were not seen when the side of cervical nerve section was exten ded. The results indicate that smooth following eye movement is influe nced 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 h ead turns in the opposite direction. Proprioceptors of the extended ne ck appear to account for the effects observed.