The Effect of Local Target Surround and Whole Background Constraint on Visual Search Times

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Abstract
Two experiments are described in which the effect of nontargets in positions immediately adjacent to the target (target surround) on visual search times was examined under two background conditions. The target in the first experiment had a higher luminance than the background elements; in the second experiment the target was a “double-dot”, i.e., two background elements immediately adjacent. The backgrounds used were statistically generated using the parameter probability of transition (Pt) from the state “dot to no dot” to determine presence or absence of nontargets in the display matrix. When Pt= 0.5, a random background was generated and when Pt = 0.25, a constrained background was produced. Constrained backgrounds are those in which some grouping is imposed on the elements of the background. For both background conditions, mean search times increased sigmoidally as a function of the number of nontargets in the target surround. Search times were longer for unconstrained than for constrained backgrounds. A search strategy is suggested which would produce these results; further experiments are suggested to test the validity and generality of these concepts in visual search experiments.

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