How the brain chooses sides

The brain almost always has a plan B, even when deciding which hand to use to press a button, a new study finds.

A part of the brain called the left posterior parietal cortex plans button-pressing movements for both hands simultaneously, shows the study, published online September 27 in the Proceedings of the National Academy of Sciences. After a very brief neural tussle, one hand wins the competition and the other’s movement is suppressed, Flavio Oliveira, a neuroscientist at the University of California, Berkeley, and his colleagues demonstrate.

Scientists actually know very little about how decisions such as which hand to use for a task are made in the brain, says Scott Frey, a cognitive neuroscientist at the University of Oregon in Eugene. While he may quibble with some of the details of the new study, “It helps to address a pretty blatant gap in the literature and it does it in an elegant way,” he says. “I think it’s one we’re going to be citing for a long time.”

Oliveira and his collaborators studied right-handed people, as most such studies do. The volunteers placed their hands on a table containing a motion-tracking system. When a target was illuminated, the participants were supposed to reach as quickly as possible to hit the target. At first the volunteers were instructed to use only the right hand or left hand for the task. Then the participants were given a choice of which hand to use. Having to decide slowed the volunteers’ reaction times by about 30 milliseconds, especially when the target was about equidistant from both hands. The participants reached for an equidistant target more often with their right hands.

In a separate experiment, participants performed the same task, but this time researchers used magnetic pulses to momentarily alter electrical activity in some parts of the brain.

When the researchers briefly disrupted activity in the posterior parietal cortex on the left side of the brain while participants were deciding which hand to use, the volunteers reached for the target with their left hands about 13.5 percent more often. Stimulating other parts of the brain, including the right posterior parietal cortex, did not produce a switch in hand preference. The result suggests that the left side of the brain is specialized for planning movements.

Researchers suspect that the left posterior parietal cortex will also decide hand choice in left-handers, but won’t know for certain until tests are repeated on a group of them.