By Lisa De Nike
Michael McCloskey is a scientist, so he doesn't often use words like "serendipity." But perhaps he should, as few other terms so accurately describe how an after-class encounter with a student 18 years ago would unearth a profound and unique deficit in visual perception that changed forever the course of the cognitive scientist's research.
McCloskey had given a lecture on his research specialty — language deficits in brain-damaged individuals — and had described a person who could no longer spell words correctly after a stroke. The student, described by her initials "AH," was intrigued and approached her professor, saying that she was a bad speller herself. McCloskey offered to give her the same spelling test that he routinely used in his research, and he was surprised to find that this obviously bright student, a history major, misspelled nearly half the words.
Much more surprising, however, was the conclusion McCloskey reached after many additional tests: AH had an extraordinary visual deficit that caused her often to see objects on the opposite side from where they actually were.
"When AH looks at an object," McCloskey said, "she sees it clearly and knows what it is, but she's often dramatically wrong about where it is."
For example, she may reach out to grasp a coffee cup that she sees on her left but misses it completely because it is actually on her right; and when she sees an icon at the top of her computer screen, it may really be at the bottom.
"AH doesn't feel confused about where an object is," McCloskey said. "She sees an object in a particular place just like anyone else does, but in her case the object is often somewhere else."
To the casual observer, AH seemed normal. She had no obvious difficulty in school or daily life. She often made mistakes in spelling or math or everyday tasks such as watering plants (she sometimes missed the flowerpot and poured the water on the floor), but her family, friends and teachers attributed the mistakes to "carelessness" or "clumsiness."
"It was absolutely extraordinary," said McCloskey, a professor in the Department of Cognitive Science at Johns Hopkins. "Here was this intelligent high achiever who was apparently born with an amazing perceptual deficit and learned to compensate for it. I was extremely interested, to say the least, so much so that it changed my research focus from words to visual perception."
The result of that research — which now spans almost two decades — is a book titled Visual Reflections: A Perceptual Deficit and Its Implications, recently published by Oxford University Press. In the book, McCloskey discusses AH's deficit and explains how she is able to adapt and compensate so well. The book also describes how AH's perceptual errors, combined with many other clues, led McCloskey to some very interesting conclusions about how we perceive the world.
"Studying AH has taught us about how the brain codes where things are. Some parts of the visual brain use codes very much like the x and y coordinates we learned about in algebra class," he said.
Through their study of AH, McCloskey and his colleagues also learned much about subsystems within the brain's visual system. They noted that when an object remained stationary, and in view for a least a second or two, AH would often see it in the wrong place. However, they observed that if an object was shown very briefly, or if it moved, she was able to pinpoint its location accurately.
"These results tell us that the visual system has separate pathways, one for perceiving stable, nonmoving objects, and another for objects that are moving or otherwise changing. AH's pathway for stable objects is abnormal, but her pathway for moving or otherwise changing objects is normal," McCloskey said. "We can learn a lot about how healthy visual systems work by studying a system like AH's that works, well, differently. The truth is, we can often learn more about a process like visual perception when something goes wrong than when it functions perfectly. This is a perfect example."
The cognitive scientist says that one of the most important lessons from the study of AH is that vision is not as simple as we are inclined to assume.
"The signals sent from our eyes to our brains must undergo complex processing in several brain regions before we can see the scene in front of us. If that processing malfunctions, as in AH, we may quite literally see something different from what is actually there," he said.
Source: JHU Gazette