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Keeping Tabs on Roving Eyes

Knowing whether a baseball is headed for the strike zone--or wide outside--is no easy task. Now scientists have found the brain region that allows everyone from schoolchildren to major leaguers to track objects, even though the eyes themselves are moving. These neurons, described in today's issue of Nature, appear to be first in line to process incoming visual signals into a spatial map, suggesting that one of the brain's first steps toward making sense of the outside world is to compensate for the body's own motion.

To simplify the problem of studying how the brain makes a spatial map, a team lead by Jean-René Duhamel and Werner Graf of CNRS-Collège de France in Paris trained two macaque monkeys to focus their eyes on a spot of light on a screen; the dot would appear for a few seconds, disappear, then appear a few seconds later somewhere else on the screen. The monkeys' heads were held still, so they had to find the dot by moving only their eyes. At the same time, the researchers monitored nerve firing in a part of the cortex--the ventral intraparietal area--that serves as an interface between the brain's visual and motor processing regions.

The researchers found that certain neurons would fire only in response to light shone in a specific area--suggesting that these neurons were responsible for keeping track of set coordinates in the field of vision. "The same point on the screen will always excite the same neuron," says Duhamel, no matter what direction the eyes are looking in. This suggests that other neurons receive visual signals, compensate for eye movement, then send the processed signal to the ventral intraparietal area.

This is "an important result," says Michael Graziano, a psychologist at Princeton University who studies how the brain tracks objects. Duhamel now plans to look for the neurons that adjust visual maps to the head's motion.