Imagine that you can read each letter in the small row of the eye chart. And that you can correctly name any single color of the rainbow. Now imagine being shown two letters of different colors and being unable to see both letters at the same time. You likely won't see each letter in its proper color either. In fact, in your mind, the shape of the letter is in one place and its color is in another. This is the visual world of an unusual stroke patient whose unique impairment has given researchers from the University of California, Davis, and Princeton University new clues about how the human brain "glues" together many visual pieces so we can see a coherent image. UC Davis graduate student Stacia Friedman-Hill, Veterans Administration research scientist Lynn Robertson and Princeton University professor Anne Treisman report their findings in the Aug. 11 issue of the weekly journal Science. "Often it takes a long time for this patient to tell us what he sees," says Robertson, who is also a neuropsychologist at the UC Davis Center for Neuroscience. "He says things like, 'it keeps shifting colors,' or 'the color is floating off to one side.' My perception of what he sees is that it is total chaos. Things moving in and out of view. Colors appearing and disappearing. Colors and shapes changing in size." In the first known clinical case of its kind, the patient has trouble "binding" images together. Until now, the problem of binding had been thought to be only theoretical; because the brain processes different aspects of vision in different places, researchers had known the image had to come together somewhere, somehow in the brain. The specific area of brain damage in this case seems to have widespread implications for this process of visual processing. In the case of the person known only as "R.M.," the researchers found that damage to the part of the brain that processes where objects are impairs a person's ability to see completely what an object is -- for example, its correct shape, size, color and texture combined. Because of medical confidentiality, the most that can be said about R.M. is that he had his first stroke in 1991 at 54 after being employed for 20 years at the same company. He was treated for the stroke for as long as his insurance held out, about 6 months. One month later he had the second stroke. That is when he came to the attention of Robertson, a member of the UC Davis East Bay neurology group. R.M.'s rare brain lesions came from two strokes caused by bloodclots. "It's unusual to have damage in isolation and symmetrically," Robertson says. In the annals of cognitive neuroscience, a few rare patients stand out for changing the way neuroscientists think about how the brain does certain things. R.M.'s strokes selectively damaged an area known as the "posterior parietal cortex." "People have known for a long time that this area of the brain is involved in spatial processing," Friedman-Hill says. "We've shown the connection that, in order to see an object totally, you need to know where the features are located in space." Just as processing images requires a lot of computer power, vision uses a large part of the human brain -- about half of the cortex (the bulky outer layer of the brain). The eye is only the first step in vision. The images travel to the back of the cortex, where the picture is sorted into up to 30 more manageable components. For example, a blue car on the freeway may break down into color, brightness, contrast, direction of movement and more. These components are thought to be processed along two main pathways in the brain. One pathway is believed to handle what objects look like, such as color, size, shape. Other aspects of the picture -- location and direction of movement, for example, are thought to be routed along the where pathway. Somewhere further along in the brain, the big picture all comes together again, in a process known as "feature binding." "This study is important because it confirms what vision scientists have long theorized," Friedman-Hill says, "if the neural mechanisms that bind objects together are damaged then vision can become unglued." This study suggests that the two pathways are more interdependent than previously known. R.M.'s experience also shows that binding is a practical problem for persons with impairment to the spatial processing area, not just a theoretical issue of interest to researchers. In the early 1980s, Treisman had predicted that spatial processing would be found to be important for binding. To better understand R.M.'s rare neural damage and visual problems, which had surfaced in extensive neurological examinations, the researchers conducted a series of cognitive perceptual tests. In one of the tests, for example, R.M. is shown a red "X" and a yellow "O." He says he sees a yellow "X." The feature of yellow from one object is bound improperly to the feature of shape of the other object. This also happens with size and shape. The Science report underscores the need to understand how brain damage can create basic problems so that clinicians can design effective interventions to help stroke victims lead better lives in the future, Robertson says. "One of the more perplexing problems for people with spatial deficits is that they do not typically receive rehabilitation," she says. "Unlike people with language deficits caused by stroke who may receive years of speech therapy, there has been little appreciation for rehabilitation for people with spatial problems." Now that the researchers know that spatial information plays a fundamental role in visual perception, they would like to confirm their results by studying other patients as well as through functional imaging of neurologically "normal" subjects. Further study is also necessary to better characterize the kind of spatial information that contributes to object vision and to pinpoint the interacting brain regions on a finer scale, Friedman-Hill says. The research was funded by the Medical Research Council of the Veterans Adminstration, the National Science Foundation, the Air Force Office of Scientific Research, the Office of Naval Research and the McDonnell-Pew Foundation.