Brain has two slots for working memory

Like side-by-side computer RAM cards, the left and the right hemispheres of the brain store information separately, a new study finds. The results help explain why people can remember only a handful of objects at one time, and suggest that people may be able to maximize their cognitive power by delivering information in equal doses to both sides of the brain, researchers suggest online the week of June 20 in the Proceedings of the National Academy of Sciences.

On average, people can hold about four things in their working memory at once, such as the location of four cards in a game of Concentration. Though many studies have linked this memory capacity to intelligence, scientists still don’t completely understand how the brain reaches this limit.

“Why can’t you think about 100 things simultaneously, or 50 things simultaneously? Why only four?” says study coauthor Earl Miller of MIT. “If we understand something about that, we’ll understand something very deep about how the brain represents information and how thoughts are made conscious.”

Miller and his colleagues tested two monkeys (monkeys also have a four-item working memory capacity) in a simple task. First, the monkeys saw two to five colored squares flash on a computer screen for a little less than a second. The screen went blank for about the same amount of time, and then the squares reappeared — but one was a different color. The monkeys were rewarded for spotting the change.

As the number of squares increased, the monkeys got worse at finding the color change. But Miller and his colleagues noticed a curious twist to this limit: Adding an extra square to the left side of the computer screen didn’t affect a monkey’s ability to remember squares on the right side of the screen, and vice versa. The two hemispheres were operating independently.

Each side of the brain handles visual information coming in from the opposite side. Since the monkeys could track about two objects on each side of the screen, this means the magic number of four is really a sum: two objects tracked by the left brain hemisphere and two objects tracked by the right hemisphere.

While the monkeys were doing the tasks, the researchers also eavesdropped on their nerve cell activity using electrodes. Certain changes in the nerve cell behavior told researchers when and where the monkeys’ brains were overloaded, and even which specific squares were causing trouble.  

The electrodes showed a distinctive split in working memory performance. Packing one side of the screen with squares caused nerve cells to go haywire. But adding even more squares to the more sparsely populated side of the screen made little difference.

This split-brain finding may lead to techniques for boosting working memory capacity, Miller says. For instance, dashboard information projected evenly onto the left and right sides of a car’s windshield might be more effective than a display confined to one side of the driver’s visual field. Or, Miller suggests, security personnel screening bags at an airport might be more efficient if the X-ray displays scrolled vertically rather than horizontally, since objects moving sideways squander the individual powers of the two hemispheres.

Figuring out how the brain handles objects is important because working memory ability reflects the cognitive power that is measured by IQ scores, SAT scores and the ability to learn a second language, says neuroscientist Edward Vogel of the University of Oregon in Eugene. “The more we understand about these basic capacity limits, the more that’s going to tell us something deep about the core cognitive abilities that differ from individual to individual.”