CAMBRIDGE, Mass.--Scientists at the Massachusetts Institute of Technology have discovered that an area of the brain previously thought to process only simple visual information also tackles complex images such as optical illusions.
The research, conducted with animals, also provides evidence that both the simple and more complex areas of the brain involved in different aspects of vision work cooperatively, rather than in a rigid hierarchy, as scientists have believed to date.
"Because half of the human brain is devoted directly or indirectly to vision, understanding the process of vision provides clues to understanding fundamental operations in the brain," said Professor Mriganka Sur of MIT's Department of Brain and Cognitive Sciences. The research, which will appear in the December 20 issue of the journal Science, was conducted by Professor Sur, graduate student Bhavin R. Sheth, and postdoctoral fellows Jitendra Sharma and S. Chenchal Rao, all of the same department.
"We've found that even supposedly simple parts of the brain are doing complex, sophisticated processing of such things as visual illusions," said Mr. Sheth. "By knowing what various parts of the brain do, we can make predictions about how the brain will function if parts of it have to be removed or if there is some sort of trauma."
Mr. Sheth compares vision to an orchestra, where clusters of cells in different parts of the brain cooperate to process different components of visual information such as vertical or horizontal orientation, color, size, shape, movement, and distinctions between overlapping objects.
The MIT research focused on an area of the cerebral cortex--the outer layer of gray matter that envelops the entire brain--called the primary visual cortex, also known as V1 and Area 17 of the brain. In humans that area is about five centimeters in diameter--the size of four postage stamps--and a couple millimeters deep on both sides of the rear of the head, just below the crown.
The V1 area is the first point of entry in the brain's cortex of visual information from the eye's retina. V1 has to date been thought to be involved only in processing very simple spatial orientations, such as whether an object is placed vertically or horizontally, but not whether that object is a pencil or a finger.
Using optical imaging techniques to record visual responses in cats over the past two-and-one-half years, the researchers found that V1 can also process optical illusions and other complex images. The researchers said the same is likely to be true in the V1 area of the human brain.
For example, if a person takes a sheet of notebook paper with horizontal lines and places an identical sheet as close as possible to the right of it and slightly lower, the lines on both pages won't connect in a continuous straight line. Yet the brain's visual processing system will try to fill the space between the two sets of real lines by creating an optical illusion known as a subjective contour.
Subjective contours are higher-level visual functions that involve the brain's understanding the context and relationship of the images, not just the static placement of one set of lines next to another. Another example is a telephone: a handset may obscure part of the phone base under it, but the brain's visual processes will see both the handset and the entire phone base as two distinct objects that belong together.
"We are just beginning to understand the brain mechanisms that underlie complex cognitive processes in vision," Professor Sur explained. "Our work is the first and most important step in showing that right in the earliest stages of the visual cortex, where visual input first enters the brain, there are groups of cells that break down these stimuli and respond to them. That leaves open the question of how higher-order visual cortex areas further process these kinds of stimuli."
The discovery of complex subjective contour processing in the V1 area is bolstered by earlier work in Professor Sur's laboratory with Louis Toth, a former MIT graduate student (MIT PhD '95) now conducting brain research at Harvard Medical School.
In a September 1996 paper in the Proceedings of the National Academy of Sciences, Professor Sur, Dr. Toth and colleagues reported that V1 could also be the site of "filling-in," another function traditionally thought to be high-level. "Filling-in" is when the brain compensates for a lack of information in one area of the visual field by making an educated guess from information elsewhere in the visual field. It explains why patients with small lesions don't see black spots, and why you are not aware of your "blind spot."
The knowledge gained from both experiments can be applied to other brain areas and functions, Dr. Toth said. For example, Dr. V.S. Ramachandran from the University of California, San Diego, who has also studied "filling-in" phenomena in vision, is interested in how the similar wiring of the part of the brain that detects touch may explain why amputees perceive "phantom limbs."
"The way the visual cortex is 'wired' is similar to the way the rest of the brain's cortex is 'wired,'" said Dr. Toth.
Professor Sur said this is a very important concept in understanding the brain, because from merely an anatomical or structural study of the brain, different areas of the cortex look remarkably similar. What distinguishes the different areas of cortex is the inputs they get and how these inputs get processed and then farmed out to different areas.
"So if one knows that an area of the brain with its connection and circuitry does a certain kind of thing, one immediately sees the possibility that all areas of the brain can do similar things to their respective inputs," Professor Sur said. "This is a very powerful idea."
The work was supported by the National Institutes of Health.
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