Abstract
How does the visual system maintain an active representation of a visual scene after it can no longer access that information directly? Two key theories dominate the current understanding of how the brain deals with the challenges of maintaining information from its rich visual surroundings. Slot models predict that up to 3-4 discrete items can be simultaneously maintained in working memory. By contrast, resource models assume a limited capacity that can be flexibly distributed to remember a few items very well, or to store many items with less precision. Our study evaluates these models by measuring the precision and capacity of visual working memory for orientations. We used orientation because this continuously varying feature can be precisely represented by the human visual system. Subjects were briefly presented with 1-6 randomly oriented gratings; after a 3-second delay subjects were cued to report the orientation of one of the gratings by method-of-adjustment. This design allows the dissociation of two components of visual short-term memory performance: precision of a remembered item and likelihood of forgetting (Zhang & Luck, 2008). Rigorous psychophysical testing indicated that each of our subjects was able to accurately maintain a representation of orientation across set sizes (5-20° SD). Precision of this memory declined steadily when subjects had to remember more items, which is consistent with the resource model. However, not all our results were inconsistent with the slot model. The likelihood of forgetting sharply increased when more items had to be remembered, indicating the difficulty of maintaining over 4 items in memory. Finally, we found only a weak effect of distracter orientation on target response, contrary to some recent reports (Bays et al. 2009). Our results suggest that people can maintain more than 4 visual items simultaneously, albeit with some loss of precision and a greater likelihood of forgetting.
NSF BCS-0642633, NIH R01-EY017082, P30-EY008126.