Interaction of Smooth Pursuit and the Vestibuloocular Reflex in Three Dimensions. MISSLISCH, H., D. TWEED, M. FETTER, J. DICHGANS and T. VILIS. Department of Neurology, University of Tubingen, 72076 Tubingen, Germany, Departments of Physiology and Ophthalmology, University of Western Ontario, London, N6A 5C1, Canada.
APStracts 2:0364N, 1995.
SUMMARY AND CONCLUSIONS
1. What is the neural mechanism of vestibuloocular reflex (VOR) cancellation when a subject fixates a target moving with the head? One theory is that the moving target evokes pursuit eye movements that add to and cancel the VOR. A recent finding with implications for this theory is that eye velocity vectors of both pursuit and the VOR vary with eye position, but in different ways, because pursuit follows Listing's law while the VOR obeys a "half-Listing" strategy. As a result, pursuit cannot exactly cancel the VOR in most eye positions, and so the pursuit superposition theory predicts an eye-position- dependent pattern of residual eye velocities during cancellation. To test these predictions, we measured eye velocity vectors in humans during VOR, pursuit and cancellation in response to torsional, vertical and horizontal stimuli with the eyes in different positions. 2. For example, if a subject is rolling clockwise (CW, frequency 0.3 Hz, maximum speed 37.5 /s) while looking 20õ up, the VOR generates an eye velocity that is mainly counterclockwise (CCW), but also leftward. If we then turn on a small target light, located 20 up and moving with the subject, then pursuit superposition predicts that the CCW component of eye velocity will shrink and the horizontal component will reverse, from leftward to rightward. This pattern was seen in all subjects. 3. Velocities depended on eye position in the predicted way; e.g. when subjects looked 20õ down, instead of 20õ up, during CW roll, the reversal of horizontal eye velocity went the other way, from rightward to leftward. And when gaze was 20 right or left, analogous reversals occurred in the vertical eye velocity, again as predicted. 4. Analogous predictions for horizontal and vertical stimulation were also borne out by the data. For example, when subjects rotated rightward while looking 20õ up, the VOR response was leftward and CCW. When the target light switched on, the torsional component of the response reversed, becoming CW. And analogous predictions for other eye positions and for vertical stimulation also held. 5. For all axes of stimulation and all eye positions, eye velocity during cancellation was roughly parallel with the gaze line. This alignment is predicted by pursuit superposition, and has the effect of reducing retinal image slip over the fovea. 6. The fact that the complex dependence of eye velocity on the stimulation axis and eye position predicted by pursuit superposition was seen in all subjects and conditions suggests strongly that the VOR is indeed additively canceled by pursuit. However, eye velocities during cancellation were consistently smaller than predicted. This shrinkage indicates that a second mechanism, besides pursuit superposition, attenuates eye velocities during cancellation. The results can be explained if VOR gain is reduced by (30%, and if, in addition, pursuit is driven by retinal slip rather than reconstructed target velocity in space. 

Received 12 June 1995; accepted in final form 28 November 1995.
APS Manuscript Number J374-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 23 December 95