A Comparison of Two Methods of Producing Adaptation of Saccade Size and Implications for the Site of Plasticity. Charles A. Scudder, Ekatherina Y. Batourina, George S. Tunder. Department of Otolaryngology, University of Pittsburgh, 200 Lothrop St., Pittsburgh, PA. 15213.
APStracts 4:285N, 1997.
ABSTRACT
Saccade accuracy is known to be maintained by adaptive mechanisms that progressively reduce any visual error that consistently exists at the end of saccades. Experimentally, the visual error is induced using one of two paradigms. In the first, the horizontal and medial recti of trained monkeys are tenectomized and allowed to reattach so that both muscles are paretic. After patching the unoperated eye and forcing the monkey to use the "paretic eye", saccades initially undershoot the intended target, but gradually increase in size until they almost acquire the target in one step. In the second, the target of a saccade is displaced in mid-saccade so that the saccade cannot land on target. Again saccade size adapts until the target can be acquired in one step. Since adaptation using the latter paradigm is very rapid but adaptation using the former is slow, it has frequently been questioned whether the two forms of adaptation depend on the same neural mechanisms. We show that the rate of adaptation in both paradigms depends on the number of possible visual targets, so that when this variable is equated, adaptation occurs at similar rates in both paradigms. To demonstrate further similarities between the result of the two paradigms, an experiment using intrasaccadic displacements was conducted in order to show that rapid adaptation possesses the capacity to produce gain changes that vary with orbital position. The relative size of intrasaccadic displacements were graded with orbital position so as to mimic the position-dependent dysmetria initially produced by a single paretic extraocular muscle. Induced changes in saccade size paralleled the size of the displacements, being largest for saccades into one hemifield, and being negligible for saccades into the other hemifield or in the opposite direction. Collectively, the data remove the rational for asserting that adaptation produced by the two paradigms depends on separate neural mechanisms. We argue that adaptation produced by both paradigms depend on the cerebellum. 

Received 15 May 1997; accepted in final form 14 October 1997.
APS Manuscript Number J403-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 29 October 1997