A comparison of the horizontal and vertical optokinetic reflexes of the rabbit

The horizontal and vertical monocular optokinetic reflexes of the rabbit were measured under closed-loop and open-loop conditions. A random noise, optokinetic stimulus subtending 70×70 deg was presented to the left eye of rabbits placed in front of a rear projection tangent screen. The position of the right eye (nonstimulated) was measured using an infrared light projection technique. During open-loop optokinetic stimulation the eye position signal was fed back to sum with a time-integrated velocity command signal driving the optokinetic stimulus. The dynamics of eye movements evoked by horizontal and vertical optokinetic stimulation were different. Horizontally evoked eye movements never exceeded a deviation of 15 deg before being interrupted by resetting saccades, which returned the eye past the primary position. By contrast, vertical eye deviations greater than 20 deg were often maintained for intervals exceeding 10 s without resetting. The closed-loop gain of optokinetically evoked horizontal eye movements was higher for monocular posterior-anterior optokinetic stimulation than for anterior-posterior stimulation. The vertical optokinetic gain for up-down stimulation was slightly greater than the gain for down-up stimulation. The vertical up-down, open-loop optokinetic gain was greater than the down-up gain over a range of retinal slip velocities of 0.5-5.0 deg/s. Measurement of the horizontal vestibulo-ocular reflex during simultaneous horizontal optokinetic stimulation demonstrated that visual and vestibular information combine linearly to produce reflex eye movements. These data suggest that the higher gain of the horizontal optokinetic reflex may compensate in part for the reduced gain of the horizontal vestibulo-ocular reflex at lower angular accelerations of the head. An equivalent vertical optokinetic gain would be obviated by the contribution of the utricular otoliths to the vertical vestibulo-ocular reflex at low frequencies of head movement.