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Study shows complexity of primate visual cortex

For decades, scientists believed that particular areas of the visual cortex in primate brains were each responsible for recognizing certain attributes of a viewed object, such as color or shape. However, a study by MIT's Professor Peter Schiller published in the July 27 issue of Nature casts further doubt on this hypothesis and indicates that the capacity to recognize images is much more complicated than previously thought.

Dr. Schiller, professor of medical physiology in the Department of Brain and Cognitive Sciences, has been studying a part of the visual cortex called the V4 area in rhesus monkeys. Work in the early 1970s postulated that this area was dedicated to the analysis of color. Other areas in the visual cortex were similarly thought to perform other aspects of vision such as analysis of motion or depth perception-the "one area, one function" hypothesis.

In Professor Schiller's experiment, monkeys were trained to recognize a shape on a screen matching one that had already appeared (recognition was detected by monitoring the animals' eye movements). After surgery to remove part of the V4 area, the monkeys could still recognize a matching object-but only if that object was exactly identical to the first one presented. If the second object differed in size or contrast or was partially obscured by a grid, the monkey's ability to recognize it as identical was greatly reduced. These results indicate that the V4 area is involved in the recognition of transformed objects within the visual field as well as the perception of color or form, providing more evidence against the "one area, one function" hypothesis.

Professor Schiller's work also indicates that the ability to recognize objects under varying conditions of distance, size or lighting conditions-an ability that is central to the vision of all higher animals-originates at an earlier stage of visual processing than was once thought. Thus, the temporal lobe (believed to be the central region in the brain for object perception) is not solely responsible for integrating and analyzing data; the V4 area and others along the way play important and complex roles as well.

Rather than handling individual components of visual information, neurons in the visual cortex "engage in complex, higher-level analysis. they can perform several jobs and provide information about several aspects of a visual scene," Professor Schiller said. In lower animals, there is more evidence for "dedicated systems" rather than the more versatile, multi-tasking abilities of primate visual systems, which would be analogous to a typewriter versus a computer. "It's a more networked way of looking at things," he said.

Scientists have identified more than 30 different areas in the visual cortex with at least 300 interconnections, indicating a complexity in visual processing that is only now being realized. Though researchers have moved beyond the earlier, simpler models, "one doesn't have a ready scheme (to replace it); the mechanisms have not been identified," Professor Schiller said.

The work was funded by the National Eye Institute, part of the National Institutes of Health.

A version of this article appeared in MIT Tech Talk on August 16, 1995.

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