What's the News: The left and right halves of the brain have separate stores for working memory, the information we actively keep in mind, suggests a study published online yesterday by the Proceedings of the National Academy of Sciences. People can, on average, hold only four pieces of information in working memory---say, where four strangers are seated in a room. The current study suggests that, in fact, working memory capacity is two plus two---two items stored in each side of the brain---rather than four items stored anywhere. This understanding could be used to design learning techniques and visual displays that maximize working memory capacity. How the Heck:
The researchers had two monkeys---which, like humans, can on average hold four pieces of information in mind---do a standard working memory task used in both primates and people. The monkeys were shown between two and five squares on a screen. The squares disappeared for a moment, and when they reappeared, one square had changed color. To get a reward, monkeys had to look at the square that had changed.
While the monkeys did the task, the researchers used electrodes to record their brain activity in two areas: the parietal cortex (which helps us take in what we see) and the frontal cortex (the seat of working memory).
As in earlier studies, monkeys made more mistakes as the number of squares increased. (To go back to the strangers sitting in a room, it's as if one of the people had wandered into the hall and someone else had taken his seat while your back was turned. If there were only four people, you'd probably realize what had happened; if there were ten people, or thirty, it would be much harder to notice the change.)
But the team noticed that it wasn't the total number of squares onscreen that mattered so much as how many squares were on each half of the screen. (Each half of the brain processes visual information from the opposite side of the visual field.) The monkeys were just as good at remembering items on the right side of the screen when more squares were added to the left side, and vice versa.
"[The monkeys] 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," said neuroscientist Earl Miller, one of the researchers, in a prepared statement.
Looking at the neural recordings let the scientists suss out when and how the brain was overloaded with information. They found that the limitation on working memory came from the monkeys' ability to take in information rather than to remember it. Too many squares overloaded the cells in the parietal cortex; they simply couldn't encode that much information.
What's the Context:
Researchers have long known that people can keep only a handful of items in working memory. (The limit was initially put at 7, plus or minus 2---often cited as the reason phone numbers are 7 digits long---but further research bumped it down to 4.) How that limit plays out in the brain, however, has remained something of a mystery.
People can boost their working memory by using certain mnemonics, or memory tricks. A common way to expand working memory is chunking, combining individual items into one larger, often more meaningful unit when you remember them (remembering the digits 0-7-0-4 as "the 4th of July," for instance). Intensive training and use of these techniques can lead to huge increases in working memory capacity.
Earlier work has shown that working memory is strongly linked to intelligence.
Working memory has different stores for visual and auditory information (say, repeating the digits a phone number to yourself so you don't forget them). The results of this study apply only to visual information.
The Future Holds:
These findings suggest better ways to present important information, Miller says. X-ray screening at security checkpoints could scroll vertically, rather than horizontally, to maximize screeners' ability to notice objects on the scans, for instance, and heads-up displays could be designed to show equal amounts of information on the right and left side of the windshield, helping drivers or pilots to process it better.
Reference: Timothy J. Buschman, Markus Siegel, Jefferson E. Roy, and Earl K. Miller. "Neural substrates of cognitive capacity limitations." Proceedings of the National Academy of Sciences, published online before print June 20, 2011. DOI:10.1073/pnas.1104666108
Image: Wikimedia Commons / Patho