People can only hold about four things in our minds at once, medical research has shown. But researchers could not completely understand how the brain reaches this limit.
To understand the neural basis of this capacity limitation, Earl Miller, the Picower Professor of Neuroscience in MIT's Picower Institute for Learning and Memory, and Timothy Buschman, a post-doctoral researcher in his lab, tested two monkeys (monkeys also have the same working memory capacity as humans) in a simple task.
As the monkeys performed task, Buschman recorded simultaneously from neurons in two brain areas related to encoding visual perceptions (the parietal cortex) and holding them in mind (the prefrontal cortex). As expected, the more squares in the array, the worse the performance.
"But surprisingly, we found that monkeys, and by extension humans, do not have a general capacity in the brain," said Miller.
"Rather, they have two independent, smaller capacities in the right and left halves of the visual space. It was as if two separate brains - the two cerebral hemispheres - were looking at different halves of visual space," added Miller.
This study resolves two long-standing debates in the field. Does our working memory function like slots, and after our four slots are filled with objects we cannot take in any more; or does it function like a pool that can accept more than four objects, but as the pool fills the information about each object gets thinner? And is the capacity limit a failure of perception, or of memory?
"Our study shows that both the slot and pool models are true," said Miller.
"The two hemispheres of the visual brain work like slots, but within each slot, it's a pool. We also found that the bottleneck is not in the remembering, it is in the perceiving," added miller.
"The fact that we have different capacities in each hemisphere implies that we should present information in a way that does not overtax one hemisphere while under-taxing the other," said Buschman.
"For example, heads-up displays (transparent projections of information that a driver or pilot would normally need to look down at the dashboard to see) show a lot of data. Our results suggest that you want to put that information evenly on both sides of the visual field to maximize the amount of information that gets into the brain," added Buschman.
The study appears online in the Proceedings of the National Academy of Sciences.