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
"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
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
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
"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.