Does working memory have a single capacity limit?

- Manipulated complexity of stimuli to determine whether there is a single capacity limit.
- Used a computational model to test competing item-limit hypotheses.
- Approximately 1-2 complex items can be stored in visual short term memory.
- Results consistent with flexible allocation of resource and rules out single capacity limit.

Debate continues over whether visual working memory has a single, fixed capacity. Empirically, performance in working memory tasks worsens as the complexity of stimuli increases. However, there exist two explanations for this result. One proposal is that visual working memory is capable of holding fewer complex stimuli. The alternative proposal is that visual working memory can store 3-4 items, irrespective of their complexity. According to this fixed-capacity explanation, performance is worse for complex items because discrimination between complex items is more difficult than discrimination between simple items. These so-called comparison errors are more likely with complex items, and when left unaccounted for, lead to an underestimate of the capacity of working memory. Previous attempts at resolving this debate have relied on clever empirical manipulations of the similarity between stimuli. However, such approaches change the task that is given to the participant, and so may also change the way that participants use their memory. Here, we use a standard change detection task, but use a measurement model to estimate both the capacity of memory, and the probability of comparison errors. We apply the model to two change detection experiments in which we varied the complexity of the stimuli that participants must remember. Critically, we found that capacity estimates, and not comparison error estimates, varied depending upon stimulus complexity. Our results suggest that the number of items that can be stored is dependent on the complexity of the stimuli.