Optic Ataxia

Patients with optic ataxia cannot accurately reach for visually presented targets. They move their hand to the vicinity of the target and then start searching movements until they hit the target. Visual misreaching contrasts with fast and accurate pointing to parts of own body and to auditorily presented targets. Many patients also have difficulties when asked to explore, compare, and estimate spatial positions without reaching for them, but there is no correlation between the severity of this general visuospatial disorder and the severity of visual misreaching. Single patients with optic ataxia pass all tests of visuospatial estimation and exploration perfectly, and many patients with severe visuospatial problems can accurately reach for visually presented targets.

A plausible interpretation of optic ataxia is that it affects the transformation ofretinotopic locations into body-centered reference frames necessary for movement planning, leaving intact motor control not requiring visual input as well as visuospatial processing not requiring transformation from visual to body-centered coordinates.

Optic ataxia can be restricted to one hand or one hemifield or even to a specific hand-hemifield combination, and manual misreaching may contrast with accurate fixation by saccades. These dissociations indicate that transformations from retinotopic to body-centered coordinates are made by mechanisms dedicated to single body parts or sectors of the visual field.

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