One of the classic images used in introductory biology and psychology courses is the motor homunculus: a deformed map of the body drawn on the primary motor cortex, showing which brain areas control different body parts. But that map may need to be redrawn now that researchers have discovered a second, fundamentally different type of map in the primary motor cortex. The findings, published in the 30 May issue of Neuron, have mystified and intrigued neuroscientists.
Researchers mapped out the motor homunculus decades ago by applying brief--50-millisecond or so--pulses of electricity to different parts of the primary motor cortex in humans who were undergoing brain surgery. Such pulses cause muscle twitches, allowing researchers to associate a particular stimulation with a particular part of the body. Lots of neurons control the hands and the face, they found, so these features of the homunculus are exaggerated, while less nimble body parts, such as the torso, look relatively scrawny.
But when an animal makes a movement, activity in the primary motor cortex lasts longer than 50 milliseconds. To see what happens with longer artificial stimulations, Michael Graziano and colleagues at Princeton University in New Jersey gave half-second bursts to brain sites in two monkeys. Rather than just twitching a muscle, the monkeys carried out coordinated, well-timed, determined movements. For instance, stimulating one spot caused a monkey to clench its fingers, move its hand near its face, and open its mouth. It didn't matter where the monkey's hand started out--every stimulation of the same cortical spot caused the monkey to assume the same final position. Zapping nearby spots lead the hand to a slightly lower position or slightly farther from the middle of the body--suggesting that the primary motor cortex is organized according to final positions in space. Of 324 stimulation sites in two monkeys, some 86% evoked a distinct posture.
The results might mean that the "map of motor cortex will have to be redrawn," says neuroscientist Larry Abbot of Brandeis University in Waltham, Massachusetts. "It will be a much more deep map than just this picture of a body." "The challenge is to figure out just what this tells us about the organization of the cortex," adds cognitive neuroscientist Larry Snyder of Washington University in St. Louis.