"But surprisingly, we found that monkeys, and by extension humans, do not have a general capacity in the brain," says 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."
In other words, monkeys, and by extension humans, do not have a capacity of four objects, but of two plus two. If the object to remember appears on the right side of the visual space, it does not matter how many objects are on the left side. The left may contain five objects, but as long as the right side contains only two, monkeys easily remember it. Conversely, if the right side contains three objects and the left side only one, their capacity for remembering the key object on the right is exceeded and so they may forget it.
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 object 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," says 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." That is, when the capacity for each slot is exceeded, the information does not get encoded very well. The neural recordings showed information about the objects being lost even as the monkeys were viewing them, not later as they were remembering what they had seen.
|Contact: Marta Buczek|
Massachusetts Institute of Technology