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Unlike a camera, which records everything it sees, the brain can focus on one part of an image, as when you look into someone's eyes and ignore their other facial features. Scientists call this 'visual attention.'
"The relationship of visual attention to eye movements is controversial," says Maurizio Corbetta, M.D., assistant professor of neurology, radiology and neurobiology at Washington University School of Medicine in St. Louis. "Behavioral data suggest that you can keep your eyes very steady while moving your attention around, so some people have predicted that different parts of the brain are used in the two tasks. But other behavioral data suggest that the two processes are functionally linked. Our imaging data demonstrate that visual attention and eye movement systems share the same areas of the brain and probably use similar neural mechanisms."
Corbetta and colleagues report their results in the October issue of Neuron. They determined which parts of the brain became active when subjects fixed their gaze on a particular spot but paid attention to their peripheral field of vision. They also imaged the brain while the subjects moved their eyes across their field of view. Therefore they were able to directly compare attention shifts with eye movements.
They used functional magnetic resonance imaging to obtain the images. Lying in the scanner, six volunteers viewed a row of boxes on a computer screen. In the 'shifting attention' task, they fixed their gaze on the center of the display while shifting their attention to each of the boxes left of center to detect a visual stimulus ( a star ) in a box. In the 'eye movement' task, the subjects moved their eyes sequentially from one box to another, center to left, to detect the star.
The images revealed which parts of the brain were active during each task. To get a better view, the researchers superimposed the data on flattened maps of the brain. Made by David C. Van Essen, Ph.D., the Edison Professor of Neurobiology and head of anatomy and neurobiology, and Heather A. Drury, research scientist of neurobiology, these 2-D maps show regions of the brain that normally are hidden in folds of tissue.
The researchers mapped regions that became active during the 'shifting attention' task in red. They mapped regions that became active during the 'eye movement' task in green. Then they superimposed the two maps to show the common areas in yellow. Surprisingly, 60 percent to 80 percent of the activated regions were yellow. They were in the frontal, parietal and temporal lobes of the brain.
"Such a tight overlap between attention and eye movements was a little surprising," says Gordon L. Shulman, Ph.D., research scientist of neurology and psychology. "It suggests that common processes are involved in moving the eyes and shifting attention."
In light of this finding, Corbetta speculates that eye movements and attention may not have been independent in early mammals. "But in primates, there may have been the need to segregate direction of gaze from attention in space. That would allow you, for example, to pay attention to the dominant male in your group without looking directly at him."
Corbetta M, Akbudak E, Conturo TE, Snyder AZ, Ollinger JM, Drury HA, Linenweber MR, Petersen SE, Raichle ME, Van Essen DC, Shulman GL. A common network of functional areas for attention and eye movements. Neuron, Oct. 1998.
Grants from the NIH, NASA and the Charles A. Dana Foundation supported this study.