"It's akin to having a jar full of M&Ms and being asked how many of them are in there," says lead study author Jamie Roitman, a neuroscientist at the University of Illinois at Chicago (having moved over from Duke). Identifying the neurons that make these rapid-fire approximations, she adds, provides "a better grasp on the actual functions that underlie our higher cognitive abilities."
Researchers zeroed in on this region of the brain because previous human studies indicated that damage to the intraparietal portion of the brain impairs numerical processing. While two macaque monkeys performed visual and numerical tasks, the researchers monitored the activity of neurons there. A screen in front of the monkeys flashed a series of dot groupings consisting of two, four, eight, 16 and 32 dots. For each set of 100 trials, one of the groupings was made the standard, and that number would appear half of the time. If there were eight dots in the standard set, for example, when they came up, the monkeys were rewarded with a little juice; they would get a whole load of juice if any other cluster flashed on the screen.
After trials featuring each dot grouping as the standard, the researchers found that the subjects' parietal neurons did not show extra activity even if the monkeys expected a sweeter reward (as a result of viewing a nonstandard cluster). "The amount of reward didn't matter [to these neurons]," Roitman says, "it was how many dots were in the array."
Within the intraparietal regions, they did find a group of neurons that routinely became more excited when a higher number like 32 was flashed and then reacted in lower bursts as numbers decreased. A second set of neurons reacted in exactly the opposite way, decreasing their activity with increasing value. Essentially, Roitman says these neurons appear to be working together to make an approximation of total quantity. "They didn't identify the numerical number of the stimulus," she explains. "No neurons [for instance] preferred four or eight."
Roitman speculates that an estimate fed by these intraparietal neurons is then passed to another population of cells in the prefrontal cortex, which then fine-tunes the calculation into an exact value. This portion of the cortex, according to Roitman, is part of the association cortex that connects planning and attention with motor functions. "When this part of cortex is disrupted," as the result of a stroke or brain injury, she says, "the deficits that follow are difficulties in allocating attention."