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How many objects can you hold in mind simultaneously?

June 21, 2011

Neuroscientists at MIT’s Picower Institute for Learning and Memory have found that cognitive capacity limitations (the ability to hold about four things in our minds at once) reflect a dual model of working memory.

The researchers investigated the neural basis of this capacity limitation in two monkeys performing the same test used to explore working memory in humans. First, the researchers displayed an array of two to five colored squares, then a blank screen, and then the same array in which one of the squares changed color. The task was to detect this change and look at the changed square.

As the monkeys performed this task, the researchers 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.

They determined that monkeys (and by extension humans) do not have a capacity of four objects, but of two in each hemisphere. 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; as long as the right side contains only two, the monkeys can easily remember an object on the right side. Or 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.

The fact that we have different capacities in each hemisphere implies that we should present information (for example, in heads-up displays) in a way that does not overtax one hemisphere while under-taxing the other, the researchers said.

Ref.: Earl K. Miller, et al., Neural substrates of cognitive capacity limitations, PNAS, 2011 [DOI:10.1073/pnas.1104666108]

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