Extraordinary perception deficit sheds light on how we see

To the casual observer, the student seemed absolutely normal. Though she often made mistakes in spelling and math, those were usually ascribed to carelessness. After all, the girl - known here as 'AH' to protect her anonymity - was a top student in history at The Johns Hopkins University.

In reality, though, AH had an extraordinary deficit in visual perception that caused her often to see objects on the opposite side from where they actually were. It was a deficit that Michael McCloskey, professor of cognitive science at Johns Hopkins, discovered when working with the student back in the late 1990s.

'When AH looks at an object, she sees it clearly and knows what it is, but she's often dramatically wrong about where it is,' said McCloskey, who has spent years studying AH's fascinating case. 'For example, she may reach out to grasp a coffee cup that she sees on her left, but miss 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 of the screen.'

'There is nothing wrong with AH's eyes,' McCloskey said. 'The problem is with how her brain processes the signals from her eyes.'

The result of McCloskey's 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 was 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,' said McCloskey, who formerly focused his research on language processing, but shifted that focus when he discovered AH and her fascinating deficit.

Through their work with AH, McCloskey and his colleagues also learned about subsystems within the brain's visual system. For instance, they discovered that when an object was stationary and remained in view for a least a second or two, AH often would see it in the wrong place. However, if an object was shown to her very briefly, or if the object was put in motion, the student was able to see its location accurately.

'These results tell us that the visual system has separate pathways, one for perceiving stable, non-moving objects, and the other 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,' said McCloskey, who contends that we can learn much about how healthy human visual systems work by studying a system like AH's that works differently.

McCloskey said that one of the most important lessons learned 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 quite literally see something different from what is actually there, he said.

'The whole thing was absolutely extraordinary,' said McCloskey. '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.'

AH's deficit is extremely rare. Only one other similar case has been reported since McCloskey and his colleagues published their first findings about AH: In 2007, Swiss researchers described a 33-year-old woman who suffered brain damage after her brain was temporarily deprived of oxygen and thereafter made perceptual errors very similar AH's. What makes AH's case even more interesting is that she was apparently born with her deficit, and in fact did even not know she had it until McCloskey discovered it in the laboratory as they were working together.

'She approached me one day after a lecture during which I was talking about a patient who had difficulty spelling after a brain-damaging stroke, and she mentioned that she wasn't a very good speller,' McCloskey remembered. 'I offered to give her the same spelling test I routinely use in research, and was surprised to find that this obviously bright student misspelled nearly half of the words. That was a clue that something was going on here.'

McCloskey discovered exactly what was going on through further tests. He said that the student was 'startled' to learn about her deficit, but that in the end, it probably helped explain certain challenges she had faced in her life.

According to McCloskey, it was AH's ability to compensate for this deficit that allowed her to be such a successful, high achiever.

'First of all, under some conditions - most notably, when objects are moving - she can perceive location and orientation normally. It's largely for this reason that she is able to drive,' McCloskey explained. 'Second, she perceives sound and touch normally, and this helps. For instance, if a person is talking to her, she uses the sound - and the person's motion - to locate him or her. Third, AH is extremely bright, which has helped her do well in school and elsewhere despite the deficit.'

And fourth, over the years, AH deliberately devised approaches to help her compensate for this weakness. For example, she had trouble with phone numbers, because she mixes up the digits. So she learned to place her two index fingers together on the page so she could see a single digit at a time. In addition, because the deficit means she cannot read digital or analogue clocks (she sees the numbers or hands in the wrong places), she stopped wearing a wristwatch. This allowed her an excuse for asking other people for the time.