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 (40-100 ms/item), effortless, and accurate. "Counting," the process of enumeration when there are more than 4 items, is slow (250-350 ms/item), effortful, and error-prone. Why is there 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. We argue that subitizing exploits a limited-capacity parallel mechanism for item individuation, the FINST mechanism, associated with the multiple target tracking task (Pylyshyn, 1989; Pylyshyn & 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. Ullman, 1984) or to perform feature integration (cf. Treisman & Gelade, 1980), subitizing does not occur (Trick & Pylyshyn, 1993a). Second, the position of the attentional focus, as manipulated by cue validity, has a greater effect on counting than subitizing latencies (Trick & Pylyshyn, 1993b).
Trick
L M
LM
Department of Psychology, University of British Columbia, Vancouver, Canada.
Pylyshyn
Z W
ZW
D000293:Adolescent; D000328:Adult; D001288:Attention; D002648:Child; D002675:Child, Preschool; D004193:Discrimination Learning; D006801:Humans; D007223:Infant; D009949:Orientation; D010364:Pattern Recognition, Visual; D011340:Problem Solving; D011930:Reaction Time