Optical Information in Visually Guided Reaching with Perturbation of Optical Direction
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We investigated the optical information used to guide the hand rapidly to a target with monocular viewing. Seated participants reached from the hip to place a stylus in a target located just beyond their knees. We introduce a new method of measuring spatial variability along reach trajectories and use the results to corroborate other evidence that the decelerative portion of the fast phase of reaching is guided visually. What is the visual information used to guide this portion of a rapid targeted reach? We suggest that 2 different 'time-to-contact' variables are used. (See Figure 2.) One is the time to close~, the angle subtended at the eye between the hand and target: 't.,= ~/[d~/dtJ. The other approximates the time for the hand to reach the radial distance of the target from the eye (without regard to direction): 'tR= [1-HIT] HI[ dHldtJ, where T and H are obtained via image size of target and hand, respectively, and dHldt is obtained via hand image expansion rate. Use of this variable requires an appreciation of target size (ST) in hand size (SH) units.