Abstract
Four seems to be a magic number in vision and in memory (see Cowan, 2001 and subsequent commentaries). We seem to have a capacity that is limited to about 3–4 items in several visual tasks: subitizing (Kaufman et al., 1948), simple change detection (Luck and Vogel, 1997) and in multiple object tracking (Scholl and Pylyshyn, 1999). Is it a mere coincidence that these tasks have the same capacity (Miller, 1956), or does a single capacity limit all of these tasks? We specifically explored the relationship between subitizing capacity and visual short-term memory (VSTM) capacity. We hypothesized that if subitizing and VSTM share a single capacity, then subitizing capacity would decrease as VSTM load increases. Alternatively, if they are independent, then increasing VSTM load would not affect subitizing capacity.
On every trial, we asked participants to enumerate briefly presented Gabor patches (0 through 9) during the delay period of a VSTM change detection task. In separate blocks, participants detected a change in color, location or identity within a grid of 1,2,3,4 or 6 letters.
We found that increasing the number of gratings to be enumerated decreased performance on both enumeration and change detection accuracy. We also found that increasing VSTM load impaired change detection accuracy as well as enumeration accuracy. However subitizing capacity was preserved when participants were asked to remember 1–4 letters (i.e., accuracy was high and independent of numerositty up to three gratings). When VSTM load was increased to 6 letters subitizing capacity decreases to 2 gratings. Our results suggest that subitizing capacity and VSTM capacity are neither one and the same, nor are they completely independent. When VSTM capacity is exceeded, our ability to subitize is impaired.