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.
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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.
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