Scientists began to debate in the late 1980s whether the complexities of these problems were handled solely by signals from the brain or accomplished via contributions both from the brain and from the eye. The latter group theorized that the "motor plant" of the eye — which includes the eye, the orbit or eye socket and the muscles that pull on it — could handle some aspects of these tasks without input from the brain. The different models suggested very different things about the way the brain controls eye movement.

Angelaki and first author Fatema F. Ghasia, a Washington University postdoctoral fellow, conducted two sets of tests in primates. In the first, the primates tracked a moving target by moving only their eyes; in the second, the bodies or heads of the primates were rotated while their eyes remained fixed on the target, invoking VOR. In both tests, scientists electrically measured the activity of oculomotor neurons, the nerves that control eye muscles. They also measured the vertical, horizonal and torsional (toward the shoulders) movement of the eyes.

The oculomotor neurons changed their firing activity in the test that included head and body movement,

Dora Angelaki

 demonstrating the brain's involvement in control of VOR. But in the first test, oculomotor nerves did not significantly change their firing patterns as the primates tracked the target by moving their eyes, suggesting some of the guidance for the eye's movements was coming from the eye itself and its surrounding tissues.

"It appears that the motor plant of the eye is equipped to solve the problem on its own, and then whenever you need to step in and override that process, the brain has a way to take over," Angelaki explains. "Better understanding of how this ability is naturally engineered into the motor plant of the eye is going to be very important for clinical applications, because every time a surgeon manipulates the muscles around the eye it might interfere with these abilities."

Published on 14^th December 2005