What are the units of visual short-term memory, objects or spatial locations?

We investigated whether the capacity of visual short-term memory (VSTM) is defined by number of objects or number of spatial locations. Previous work is consistent with either alternative. To distinguish these factors, we use d overlapping stimuli that allowed us to independently manipulate the numbe r of spatial locations while holding constant the number of objects and fea tures to be encoded (Duncan, 1984; Vecera & Farah, 1994). In Experiment 1, the number of spatial locations had no effect on VSTM, suggesting that VSTM is object based. Experiments 2 and 3 ruled out alternative explanations ba sed on perceptual segregation difficulty or decision noise factors. Our res ults provide additional support to Luck and Vogel's (1997) demonstration th at integrated objects form the units of VSTM capacity.