CAMBRIDGE, MA--For decades, scientists believed that particular areas of
the visual cortex in primate brains were each responsible for
recognizing a certain attribute of a viewed object, such as color or
shape. However, a study by MIT's Professor Peter Schiller published in
today's 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 in rhesus monkeys called the V4 area. 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"
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.