Why are small and large numbers enumerated differently? A limited-capacity preattentive stage in vision.

"Subitizing," the process of enumeration when there are fewer than 4 items, is rapid (40–200 msec/item), effortless, and accurate. "Counting," the process of enumeration when there are more than 4 items, is slow (250–350 msec/item), effortful, and error-prone. Why there is a difference in the way the small and large numbers of items are enumerated? A theory of enumeration is proposed that emerges from a general theory of vision, yet explains the numeric abilities of preverbal infants, children, and adults. It is argued that subitizing exploits a limited-capacity parallel mechanism for item individuation, the FINST mechanism, associated with the multiple target tracking task (Z. W. Pylyshyn, 1989; Pylyshyn and R. Storm, 1988). Two kinds of evidence support the claim that subitizing relies on preattentive information, whereas counting requires spatial attention. First, whenever spatial attention is needed to compute a spatial relation (cf. S. Ullman, 1984) or to perform feature integration (cf. A. Treisman and G. Gelade, 1980), subitizing does not occur (L. M. Trick and Pylyshyn, 1993). Second, the position of the attentional focus, as manipulated by cue validity, has a greater effect on counting than subitizing latencies (Trick & Pylyshyn, 1993). (PsycINFO Database Record (c) 2010 APA, all rights reserved)