The Attentional Capacity of Visual Search Under Flicker Conditions
Ronald A. Rensink, Cambridge Basic Research, Cambridge, MA 02142 USA.

Perception, 25(suppl.):2. 1996. [ECVP 1996; Strassburg, France.]

High-level allocation of attention is required to detect changes in images under "flicker" conditions , i.e., when an original and a modified image are repetitively alternated, with a blank field between each display (Rensink, O'Regan, and Clark, ECVP '95). But how many items can attention "grab" and compare during each display? To determine this capacity, search experiments were carried out under flicker conditions, using displays formed of arrays of rectangles. In half the trials, displays differed by the position of one of the rectangles; in the other half, displays remained the same. Subjects were asked to determine if a change was occurring between the two displays.

Performance was measured for several durations of displays and blank fields. In all cases, search showed a clear serial deployment of attention. Comparing the rate of this deployment against the rate of display alternation yielded the number of items compared at a time. Results indicate that for display durations of less than 100 ms, average capacity is about 1.5 items. Capacity increases somewhat with longer display times, indicating that more items can be stored with increasing display time, or that more items are being grouped into each attentional "operand".

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