by Kathleen Cason
Seeing is believing
Processing visual information is a complex undertaking.
Imagine a chipmunk darts in front of your car. That triggers a series of events in your brain before you can understand what youve seen. It may seem instantaneous, but from the time the chipmunks image enters your eye until you think, Im about to run over a chipmunk, better slam on the brakes, 50 to 100 milliseconds have passed. Thats about how long you hear the crack of a fastball hitting a bat.
During that instant, the image enters your eye in the form of light, strikes the retina and is translated into electrical impulses. These impulses flow along two pathways from the retina to a visual-processing center at the back of the brain called V1. From V1, the signal travels along these pathways to other visual centers. One set of ²wiresÓ - the magnocellular (M) or whereÓ pathway - carries information about movement and an object's location. A second set of wiring - the parvocellular (P) or whatÓ pathway - carries information about shape and color. From the various visual centers, the information travels to higher thinking areas of the brain where it is interpreted.
In schizophrenia, it seems theres faulty wiring somewhere along the visual pathway. Previous research indicates that people with schizophrenia and their relatives have a hyperactive M pathway, which may account for the sensory overload and problems filtering stimuli experienced by schizophrenics.
Using a test called backward masking, Bedwell and Miller confirmed that relatives, like schizophrenics, have a deficit in the M pathway.
In the test, letters flash in one of four locations on a computer screen up, down, left or right followed by a mask that covers the entire screen. Researchers vary the lag time between when the subjects see the letter and when the mask appears. Its easy to name the letters without the mask, but harder with the mask. Thats because the mask interrupts the letters image as it travels along the brains visual pathways, causing the signal to stall in one of the visual centers; you dont even know you saw it. The target letter and the mask stimulate a fast M system response and a slower P system response. The test can be used to discriminate the two pathways by asking the test taker to either give the location of the letter relying on the M system or to identify the letter, which uses the P system.
It seems one of the genes that relatives might carry is for a specific type of change in visual processing that is not apparent to them. These very difficult tasks can detect those subtle differences between the groups, Bedwell said.
Diffuse red light selectively suppresses the M system, slowing it down. Knowing that, Bedwell devised the test with two different colored backgrounds either neutral gray or red and compared a persons performance on both backgrounds.
He found that the control group performed worse when the background was red but that relatives performed equally well with both backgrounds. This difference suggests that relatives have a hyperactive M pathway.
If red slows down the magnocellular pathway, then that is what is hurting the controls performance, Bedwell said. And the relatives, as in schizophrenia, have a magno system that goes too fast. Red slows it down to normal levels, so it doesnt really hurt their performance.
Miller and Bedwell are now using functional magnetic resonance imaging or fMRI, (see related story, Mind Reading) to pinpoint the visual areas of the brain that may be responsible for these differences.
Were giving evidence that would suggest where other researchers might want to look to find the genes that turn visual processing on and off, Miller said. The ultimate goal is to find out what causes schizophrenia, what areas are involved, and whether there is any way you can treat it before somebody becomes psychotic and starts seeing things and hearing things and having these strong delusions.
Joan will struggle with her illness for the rest of her life.
I feel Ive settled down now, she said. But she will never complete the dissertation she was writing on Chaucer. And she will probably never hold a job again.
Recovery from schizophrenia like that of Nobel Prize winning mathematician John Nash recently depicted in the movie A Beautiful Mind is rare. The hope for better disease management is earlier detection and a better understanding of how parts of the brain are impacted and change behavior.
Historically, we have made tremendous gains in the understanding of schizophrenia, and yet, we still dont know very much about it, Miller said. Its very difficult for us to see into the mind. We have beautiful pictures of the brain but the mind is still an inference.
more information, contact Stephen Miller at
Kathleen Cason is associate director of Research Communications at the University of Georgia. She has degrees in microbiology and plant pathology and is the former editor of the Georgia Department of Technical and Adult Educations Results Magazine.
Research Communications, Office of the VP for Research, UGA
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