Enumeration by location: Exploring the role of spatial information in numerosity judgments

Abstract

Enumerating a set of visual objects requires the individuation of these items, which inherently relies on location information. To examine the role of location memory in small-number judgments (subitizing), we devised a task that presented observers a brief display of small discs and then required them to mark the location of each disc on a blank screen. In doing so, observers provided an indirect measure of their representation of the numerosity of the display. Observers were tested on three stimulus durations (50, 200, 350 ms) and eight numerosities (2-9 discs); the black discs were approximately 1 degree in visual angle and placed randomly on a gray screen. Following a full-screen mask, observers marked the disc locations on a blank screen by using a mouse pointer to place markers (“X”) for each disc. This provided a measure of recall for object locations and display numerosity. ANOVAs on enumeration performance revealed significant main effects for numerosity and display duration (with interactions). High enumeration accuracy was observed for displays containing up to six discs (>90% of trials with perfect recall); error rates increased rapidly for larger numerosities. When observers made counting errors, they were generally underestimates. In the location analysis, error was measured as the distance between a stimulus disc and a paired response disc (discs were paired using nearest-neighbor methods). Location errors were significantly worse in the 50-ms presentation duration and for larger numerosities. We speculate that the process of adding markers for each object provided a way to keep track of which objects had already been counted and thus improved enumeration accuracy. The methodology for this new subitizing task and the implications of the current findings will be discussed.