The Eyes Don't Have it All

What you see isn't always what you get

THURSDAY, Aug. 30, 2001 (HealthDayNews) -- When it comes to processing what you see, there's more than meets the eye.

Experiments by scientist at the California Institute of Technology (Caltech) show two kinds of visual afterimages. One is what the retina of the eye records, like the dots after a flashbulb goes off in front of your eye. The other seems to be an image controlled by the visual cortex of the brain based on your own perceptions rather than on physical reality.

The finding reveals a "new mechanism underlying conscious experience," says Shinsuke Shimojo, one of the study's authors.

His research with Caltech colleagues appears in the Aug. 31 issue of Science.

Consider that black dot you see when you close your eyes after staring at a flashbulb. That afterimage, called "local," is directly related to the light to which you were exposed and is a simple recording by the retina of what you have actually seen.

But Shimojo says another afterimage, called "global," seems to be controlled by the visual cortex and is unrelated to the local afterimage. For instance, he says people shown cards with color only on the corners reported seeing the whole card as colored, a perception of the image rather than the image itself.

Shimojo says these illusory afterimages may mean the cortex of the brain adapts to the perceptual rather than physical attributes of what we see.

"There could be biological benefits to this, giving the brain more flexibility in adapting visual images," he says.

In a series of experiments designed to separate local from global afterimages, the researchers showed 20 participants slides and movies that induced both local and global afterimages. In addition to the square with colors in the corners, participants were shown moving pictures of lines and dots. Movement enhances the perception of global afterimages and minimizes the strength of local afterimages, Shimojo says.

Shimojo says his findings have no specific medical applications but is important as groundwork for future study.

He says, "What is so good, ecologically speaking, is that the brain can adapt … , is constantly adapting. Overall, there are interesting implications with regard to how our conscious experience of perception is brought about by neural processes."

Nigel Daw, an ophthalmology professor at Yale University. says, "If [this report] is proof that some component of the brain is involved with the afterimage, that is very interesting."

What To Do

The Steen-Hall Eye Institute has a simple explanation of how the eye works. For information about the brain, Neuroscience for Kids is great for grownups, too.

To try a few visual experiments yourself enter the hall of illusions and try not to stare too hard -- your eyes will water.

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