Visual Search in Complex Fields: Size Differences between Target Disc and Surrounding Discs

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Abstract
An adaptable technique for performing search experiments, enabling extensive studies to be undertaken with well-practiced observers, is described. In each trial a single target disc was presented. Cumulative distributions of the times taken to locate 6 solid disc targets of varying size in a display containing 99 larger standard discs arranged in a regular fashion, and 3 disc targets in a display of 107 larger discs arranged irregularly, are presented. Three practiced observers were used with each display. Sixty readings per observer, per target, per display were obtained. It is suggested that for the targets most different in size from the background discs, the distributions of times to locate are largely dependent on response time factors; and for the targets closest in size the distributions are largely dependent on search factors. Some support is lent to theoretical work that suggests search times are exponentially distributed. The shortest time required to locate a particular target is used as an estimate of response time. Response times are found to be inversely proportional to both the difference between the log of the target and nontarget disc diameters, and to the difference between the diameters. An amendment, taking response time into account, is suggested for exponential search equations.

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