Head Movements During Optokinetic Stimulation in the Alert Rabbit

Pigmented rabbits with their heads free to move about the vertical axis were seated inside a rotating optokinetic drum in order to evoke the optocollic reflex (OCR). At drum velocities below 5 degrees/s, head movements were inconsequential, and eye velocity generally matched drum velocity. At velocities between 5-15 degrees/s head movements were irregular and slight; head velocity was less than 20% of drum velocity, and gaze was undercompensatory by 1-3 degrees/s (retinal image motion of 1-3 degrees/s). At drum velocities above 15 degrees/s, and especially above 30 degrees/s, head movements were substantial (more than 20% of the drum velocity), but gaze was undercompensatory by 60-70% of the stimulus velocity. In the same rabbits in the same test periods and conditions, the vestibulo-collic reflex (VCR) was evoked with vision with minimal gaze undercompensation relative to a stationary surround; however, when deprived of vision the VCR gain dropped. The present results support the notion that with vision, the OCR does not contribute significantly to the improvement of the VCR response, since massive undercompensation of the gaze relative to the rotating drum was required in OCR testing to evoke head movements similar to those seen in VCR tests. Due to many differences in operating characteristics of the vestibular and optokinetic systems, and due to the nature of OCR testing, there were several unexpected results: in some cases head movements did not result in summation of vestibular and optokinetic reflexes, and with sinusoidal drum rotations of about 2 degrees/s2 peak acceleration there was overcompensation (gaze moves faster than the drum) for intervals up to 20 s. Thus, optokinetically generated active head movements could produce behavior strongly contrasting with passively induced head movements in visual-vestibular tests. It is tentatively concluded that in mammals there is a vestigial and specific optokinetic control of gaze and that the optokinetic control of the head is weak (relative to the eyes). However, other non-reflex mechanisms controlling head movements-such as stimulus entrainment and temporal asymmetries in the vestibular and optokinetic reflexes-must also be considered to explain all facets of the data.