Study Finds Why We can Only Hold About 4 Things in Our Minds at a Time

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 capacity as human 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?