The world you see around you appears perfectly stationary, even though your eyes dart back and forth two to three times every second in little hops called saccades. For more than a century researchers have assumed that the brain must keep track of the impulses that cause these tiny motions, so as to subtract their effect from our visual awareness. Now researchers have identified a circuit in the monkey brain that seems to play this role.
Ignoring the motion of our eyes allows us to focus on changes in our environment. The alternative would be chaos, says brain researcher Robert Wurtz of the National Institutes of Health in Bethesda, Md. "It's almost as if you have a movie camera on top of a bronco and it's jumping around," Wurtz says. "If you watched the movie it would make you sick." Researchers believe the brain solves this problem through a process called corollary discharge. Every time the brain sends the eyes a signal to twitch, it sends a copy, or corollary signal, to another location in the brain, sort of like the way your e-mail client sends copies of your e-mails to their own folder, Wurtz explains.
Wurtz and his colleague Marc Sommer, now at the University of Pittsburgh, stumbled onto the presumed corollary discharge pathway while stimulating the brain region that controls eye movements in live monkeys. Sommer noted that a current applied to this area, called the superior colliculus, elicited a delayed response in the frontal cortex, which is associated with attention and decision making, Wurtz recalls. The delay suggested a relay of neurons ending at the frontal cortex.
Researchers had already observed that brain cells in this region seem to anticipate where the eye's center of focus will move to after an impending saccade, making it a reasonable place for corollary discharges to end up. By probing the animals' brains with electrodes, Wurtz and Sommer discovered that the signals from the superior colliculus to the frontal cortex passed through a portion of the thalamus. "That really gives us a switch we can turn on and off," Wurtz says. To see if the thalamus is the source of corollary discharges, they injected a small part of it with a compound that blocks neuron firing. As would be expected for the corollary pathway, the frontal cortex of the injected monkeys consistently displayed less of the saccade-anticipating activity, the group reports in a paper published online November 8 in Nature.
Other experts find the result convincing. "For a long time we've known that mechanism had to be there, and they've shown how it works," says neurophysiologist Douglas Munoz of Queens University in Ontario. Besides solving this puzzle, adds James Lynch of the University of Mississippi, the group's "imaginative and exceedingly difficult" experiments also mark a new step in the ability to pinpoint the flow of information in the brain. Sommer says future experiments may inactivate more of the thalamus to see if monkeys have a harder time distinguishing their own saccades from changes in their environment.