The Independence of Visual Number and Area Processing: Evidence from Psychophysics, Development, and Eye-Tracking
61.24, Wednesday, May 15, 8:15 - 9:45 am, Royal Ballroom 4-5
Darko Odic1, Justin Halberda1; 1Psychological and Brain Sciences, Johns Hopkins University
Although human and non-human animals can nonverbally estimate number, it remains unknown how the visual system extracts the number of objects from a scene. One reasonable and intuitive proposal is that number extraction begins by estimation of the total surface area that the items subsume, and then proceeds by combining this estimate with information from other dimensions, such as density (Durgin, 1995; Dakin et al., 2008). Here, we directly compare performance on number discrimination and surface area discrimination in order to test whether the encoding and representation of number and area is shared or independent. First, we tracked eye movements of observers who viewed identical displays of blobs and judged which color was greater in either number or cumulative surface area (Fig. 1a). Both number and area discrimination obeyed Webers law, but area performance was superior to and uncorrelated with number performance (Fig. 1b,c). Critically, we found distinct eye movement patterns for the two tasks: while number trials included many saccades and short gazes, area trials involved few saccades and long gazes. This suggests that observers seek out different visual information during number discrimination than during area discrimination, even though the displays are identical. Second, we found that number and area remain distinct throughout development. A cross-sectional sample of 3-6 year olds and adults discriminated both number (with an array of dots) and area (with a blob image; Fig 2a). Once again, both number and area obeyed Webers law, but area performance was superior to and uncorrelated with number performance (Fig. 2c). Overall, these results suggest that number and area are independent representations, and that number encoding is not learned or exclusively derived from area encoding. Instead, it would appear that the encoding of number and surface area relies on unique visual routines in both identical and non-identical visual displays.