Life Span Changes in Visual Enumeration: The Number Discrimination Task

Participants from 5 groups with mean ages of 6, 8, 10, 22, and 72 years were tested on a series of speeded number discriminations: 1 vs. 2, 3 vs. 4, 6 vs. 7, and 8 vs. 9. The primary measure of interest--response time slope as a function of number size--decreased with age for numbers in the 1-4 range. However, a U-shaped age function emerged in the 6-9 range, with larger slopes for children and senior adults, and the smallest slopes for young adults. These data suggest that different processes are involved in enumerating small and large numbers of items. It is argued that subitizing, the process for small numbers, makes only minimal demands on spatial attention and thus shows developmental improvements without any decline in old age. In contrast, counting, the process for large numbers, requires sophisticated coordination of spatial attention, which has previously been shown to first improve and then decline over the life span.