Shared VSTM resources for enumerating sets and for encoding their colors

Abstract

Several species, including humans, have been shown to posses the ability to nonverbally represent the approximate number of items in a set. Recently, Halberda et al. (2006, Psychol. Sci.) showed that with displays containing multiple spatially overlapping sets, observers can successfully enumerate three such sets (two subsets plus the superset of all items). This three-set limit on enumeration has been argued to converge with previously observed three-item limits of object-based attention and visual short-term memory (VSTM), with each set functioning as an individual entry to attention and VSTM. This proposal implies that the same VSTM resources may be used both for storing sets for enumeration and for storing single object features such as colors and shapes. In the present study, we tested this proposal using a paradigm similar to that of Halberda et al.: participants briefly viewed displays of dots of different colors and were asked to enumerate the approximate number of dots of a specific color (the probe color). The probe color was given either before or after the display was shown. Accuracy on paired ‘probe before’ and ‘probe after’ trials was compared to assess the number of sets that participants could successfully encode. Occasionally, we probed a color that was not present, allowing us to measure the number of colors that participants could encode successfully from the displays. Replicating Halberda et al., we found that participants could successfully enumerate two subsets of the colored dots. Interestingly, participants could only encode about two colors from the same displays. In other words, when participants were able to encode the color of a set, they could also enumerate the number of items in that set successfully. These results indicate that VSTM resources for enumerating sets and for encoding object colors are shared.