The Oculomotor Response to Small Target Replacements

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Abstract
The speed and accuracy with which a small target (from 7·5 min to 15 min) is tracked by visual axis has been investigated using an after-image method of observing eye-movements (27 subjects). Complementary measurements (using a contact lens and mirror) were made on accuracy of fixation and time of response (two subjects). A range of tracking performance is found: at one extreme (called Type A subject) a target displacement of 7·5 min is compensated in 1 sec by a saccade. At the other extreme (which we call Type B) compensation requires up to 10 sec and only the larger target displacements stimulate a compensating saccade. For subjects of Type B, the time required for compensation increases rapidly as the size of saccade is reduced so as to indicate a 'dead-space' within which target movements are not compensated. The size of the dead. space was measured for two subjects, yielding values of 5·5 min and 9·8 min. Mechanisms that maintain the direction of the visual axis are discussed in the light of these findings.

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