The importance of visual short-term memory is clear to anyone who has ever played the children's card game that requires players to identify identical face-down cards at different locations. Visual short-term memory is the temporary buffer that stores visual information. Behavioral studies indicate that this buffer can store up to four objects, but more recent evidence indicates that the maximum number of objects that can be stored becomes smaller as object complexity increases. It is therefore unclear whether visual short-term memory capacity is limited to a fixed number of objects or if it is variable.

In a paper in Nature ('Dissociable neural mechanisms supporting visual short-term memory for objects', doi:10.1038/nature04262), Yaoda Xu and Marvin Chun resolve this controversy by using functional magnetic resonance imaging (fMRI) to dissociate object representations in parietal and occipital cortices. Observers were asked to detect a change in a simple or complex shape feature in the same set of objects. The number of objects in a set was varied. Observers did better when they had to detect a change in a simple feature and also when the number of objects was small. The authors found a similar interaction in the superior intraparietal sulcus (green in the picture) and the lateral occipital cortex (red), which tracked behavioral performance, but only for simple shape features, not complex ones. In contrast, activation in the inferior parietal sulcus (orange) tracked overall performance based only on the number of objects seen, regardless of whether observers judged simple or complex shape features. In control experiments, the authors ruled out perceptual processing limitations and spatial location as an explanation for these results, and also correlated the observed activity with the encoding and maintenance phases of visual short-term memory.

These results indicate that there are differing representations for visual short-term memory in the brain. Whereas the inferior parietal sulcus representation is fixed by the number of objects, object representation in the superior parietal sulcus and the lateral occipital cortex varies according to the complexity of the objects being held in visual short-term memory. The inferior parietal sulcus representation is thus likely to be the mechanism determining the maximum number of objects that can be held in visual short-term memory and may determine capacity limitations in tasks such as subitizing and multiple object tracking. The superior parietal sulcus and lateral occipital cortex representation are more likely to contain detailed representations of objects. These results demonstrate that visual short-term memory capacity is determined both by object number and by object complexity.