![]() ![]() The slope of the RT x set size function is a standard measure of search efficiency, since it gives an estimate of search throughput in terms of items per unit time. ![]() RT increases in a roughly linear manner with set size. The time to make a response ( reaction time, or RT) is measured, as well as the accuracy of the answer. Typically, the target is present on half of the trials, and absent on the others. The number of items in the scene (set size), and thus the number of distractors, is varied from trial to trial. Observers are asked to perform several hundred trials of the search task. ![]() Such an artificial scene might subtend a region of the visual field measuring 20 degrees of visual angle (dva) by 20 dva. How well can we perform a specific search task? In a standard laboratory search task, observers are asked to search for a target in an image on a computer monitor. This means that either such objects are processed in parallel, or we can make several covert attentional shifts per fixation. However, with stimuli that do not require direct foveation, 4-8 objects can be searched during each fixation. Overt movements of the eye and covert deployments of attention are closely related (Kowler, Anderson, Dosher, & Blaser, 1995). Under real world conditions, a new point of fixation is selected 3 or 4 times per second. Under laboratory conditions, many search tasks can be performed entirely with covert attention. Attentional shifts made during a single fixation are termed covert, because they are inferred rather than directly observed. If the relevant items in the visual scene are large enough to be identified without fixation, search can be successfully performed while the eyes are focused on a single point. Overt search refers to a series of eye movements around the scene made to bring difficult-to-resolve items onto the fovea. Most visual searches consist of a series of attentional deployments, which ends either when the target is found or the search is abandoned (see section 6.3). This subset may be an array of locations, but more likely it is an object, or a small group of objects (Goldsmith, 1998). Visual attention is used to control the selection of the subset of the scene. Accordingly, mechanisms like those subserving object recognition process only a restricted part of the visual scene at any one time. Many visual scenes contain more information than we can fully process all at once (Tsotsos, 1990).
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