Distribution of visual attention over space.

Subjects viewed 3 X 3 grids in which different subsets of the nine squares were designated as "figure," either by physical shading of those squares or by a verbal instruction to imagine those squares as shaded. The time taken by participants to respond "on" or "off" the figure was measured for single or multiple probe dots, which all appeared on or off the figural subset together, and which had already been shown to be equally detectable against shaded or unshaded squares and in all nine locations within the grid. In contrast to the set-size effect generally found in experiments on memory scanning, reaction time did not necessarily increase with the number of squares in the figural subset. Instead, the critical variable, which in previous research may often have adventitiously covaried with set size, was the spatial compactness of the subsets (as indexed by square-root-area over perimeter): Probes of less compact figures required more time to classify correctly. Subjects were evidently more successful in confining their attention to sets of mutually proximal items. Reasons are given for believing that this principle may also apply in the more abstract representational or semantic spaces that determine reaction times and errors in various other cognitive tasks.