Age differences in enumerating things that move: implications for the development of multiple-object tracking.

The attentional theory of spatial enumeration (Trick & Pylyshyn, 1994) predicts that subitizing, the rapid process (40-120 msec/item) used to enumerate 1-4 items, employs the same mechanism that permits individuals to track 4-5 moving items simultaneously, whereas enumerating more items requires moving attentional focus from area to area in the display. To test this theory, enumeration of static and moving items was investigated in 8-, 10-, 12-, and 20-year-old participants using a number discrimination task. As was predicted, random independent item motion did not substantially impede enumeration of 1-4 items regardless of age. However, even movement within a 1.14 degree square area slowed enumeration of 6-9 items, although on average the interference decreased with age from 788 msec for the 8-year-olds to 136 msec for the 20-year-olds. The relevance of this finding for theories of enumeration, multiple-object tracking, visual working memory, and object-based attention is discussed.