Neurophysiology of Saccade Adaptation: The Role of Superior Colliculus

University Of Washington, Seattle WA

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Abstract

DESCRIPTION (provided by applicant): Motor adaptation refines our movements during development, adjusts them when we attempt to acquire a new skill, and repairs them if they are compromised by injury. Our long-term goal is to identify the neural mechanisms of the motor adaptation that maintains the accuracy of saccadic eye movements. Most, if not all, neurophysiological studies on saccade adaptation have used targeting (reactive) saccades made in response to the sudden appearance of a target that falls off the fovea, called here the retinotopic goal of the saccade. For targeting saccades, the vectors of the retinotopic goal and the initial motor error (the difference between the target and current eye positions [also called the desired saccade vector]) are congruent. However, saccades often must be coordinated with other saccades or other types of eye movement, which may intervene before a saccade to an intended goal is executed. Such interruptions dissociate the vector of the saccade to be executed and its retinotopic goal, so the brain must perform a spatial updating to re-compute the vector of the intended saccade. Our preliminary experiments using a double-saccade task (DST) revealed that (1) the saccadic burst of saccade-related neurons in the superior colliculus (SC) are modulated by the retinotopic goal, and (2) unexpectedly, adaptation of targeting saccades does not generalize i.e., transfer, to the second saccade of a DST, even when the two saccades have identical initial motor errors. These findings lead us to hypothesize that the SC uses different processing for targeting saccades and saccades that require spatial updating. We will examine the effect of this separate processing on saccade adaptation. We will use three approaches to test this hypothesis: 1. Behavioral experiments to infer the adaptation sites of targeting saccades and the second saccade of a DST on the basis of both the characteristics of each adaptation and its transfer to the other type of saccade. 2. single unit recording to reveal any changes in SC activity specific to the two different saccade types and their adaptations and 3. microstimulation and focal inactivation to test the difference of SC topographic activity during the execution of saccades in the two tasks . Because of the similarities of simian and human saccadic eye movement behavior, the results of this project should have considerable relevance in the diagnosis, treatment and rehabilitation of patients with saccadic eye movement disorders. Our proposed experiments aim to provide both behavioral and neurophysiological evidence that adaptation of saccades with the same vectors, but generated in different goal contexts, do not transfer to each other and hence involve different adaptation pathways. Understanding the limitation of the transfer of adaptation between saccades in different contexts may help to design a more specific repertoire of rehabilitation strategies for patients with saccade disorders, and perhaps general motor deficits.