Visual grasp, evidence for object-based attention from letter row displays

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Epp, Delmar B.
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A series of 6 experiments attempted to specify the characteristics of a proposed attentional phenomenon dubbed "visual grasp." It was hypothesized that a visual grasp makes the external contours or elements of a briefly-presented display available for identification before more internally-located elements become accessible. Tests involved having observers report a single letter, cued at display offset, from a horizontal row of letters. Experiments 1 and 1A demonstrated that a visual grasp occurs when observers are required to process all items in a display. Outermost letters in a row were identified more accurately than interior letters. The failure of differential instructions to affect identification scores led to the suggestion that visual grasp involves the exogenous capture of visual attention. Experiment 2 provided evidence that visual grasp is directed toward perceptual objects rather than regions of visual space. Letters in identical retinal positions were recalled with greater accuracy when those positions appeared at row ends than when they appeared within rows. Experiments 3, 3A, and 3B indicated that visual grasp may be directed toward individual perceptual groups within a larger display. Color groups were created within letter rows, letters appearing at the edges of color groups were, in some cases, identified more accurately than letters in identical retinal positions that were not at the edge of a color group. It is suggested that visual grasp represents an early and fundamental aspect of object identification.