Todd and Marois used functional magnetic resonance imaging (fMRI), a technique that reveals the brain regions active in a given mental task by registering changes in blood flow and oxygenation in these regions, to identify where the capacity limit of visual short-term memory occurs.
The brains of research participants were scanned with fMRI while they were shown scenes containing one to eight colored objects. After a delay of just over a second, the subjects were queried about the scene they had just viewed.
While the subjects were good at remembering all of the objects in scenes containing four or fewer objects, they frequently made mistakes describing displays containing a larger number of objects, indicating that the storage capacity of visual short-term memory is about four.
The fMRI results revealed that activity in the posterior parietal cortex strongly correlated with the number of objects the subjects were able to remember. The magnitude of the neural response in this brain area increased with the number of objects viewed up to about four and leveled off after that, even when additional objects were presented.
Importantly, the posterior parietal cortex did not respond differently to the number of objects presented in a scene if the participants were not asked to remember what they had seen. In contrast, regions of the visual cortex in the occipital lobe did respond differently to the number of objects even in the absence of the memory task. Thus, it appears that while some regions of the visual cortex register how much visual information we see, the posterior parietal cortex represents how much of that information we can hold in mind.
“The results suggest that the posterior parietal cortex is a key brain area associated with our impoverished ability to hold a representation of a visual scene in our mind once the scene is out of sight,” Marois said.
Marois is a member of the Kennedy Center for Research on Human Development, Vanderbilt Vision Research Center and the Center for Integrative & Cognitive Neuroscience. The work was supported by a grant from the National Science Foundation.