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Relevant to Consciousness lectures

From: Bill M.
Sent on: Tuesday, December 23, 2008 9:23 PM
Blind, Yet Seeing: The Brain?s Subconscious Visual Sense


December 23, 2008
Blind, Yet Seeing: The Brain?s Subconscious Visual Sense 

The man, a doctor left blind by two successive strokes, refused to take
part in the experiment. He could not see anything, he said, and had no
interest in navigating an obstacle course ? a cluttered hallway ? for the
benefit of science. Why bother?

When he finally tried it, though, something remarkable happened. He
zigzagged down the hall, sidestepping a garbage can, a tripod, a stack of
paper and several boxes as if he could see everything clearly. A researcher
shadowed him in case he stumbled. 

?You just had to see it to believe it,? said Beatrice de Gelder, a
neuroscientist at Harvard and Tilburg University in the Netherlands, who
with an international team of brain researchers reported on the patient on
Monday in the journal Current Biology. A video is online at

The study, which included extensive brain imaging, is the most dramatic
demonstration to date of so-called blindsight, the native ability to sense
things using the brain?s primitive, subcortical ? and entirely subconscious
? visual system. 

Scientists have previously reported cases of blindsight in people with
partial damage to their visual lobes. The new report is the first to show
it in a person whose visual lobes ? one in each hemisphere, under the skull
at the back of the head ? were completely destroyed. The finding suggests
that people with similar injuries may be able to recover some crude visual
sense with practice.

?It?s a very rigorously done report and the first demonstration of this in
someone with apparent total absence of a striate cortex, the visual
processing region,? said Dr. Richard Held, an emeritus professor of
cognitive and brain science at the Massachusetts Institute of Technology,
who with Ernst P?ppel and Douglas Frost wrote the first published account
of blindsight in a person, in 1973.

The man in the new study, an African living in Switzerland at the time,
suffered the two strokes in his 50s, weeks apart, and was profoundly blind
by any of the usual measures. Unlike people suffering from eye injuries, or
congenital blindness in which the visual system develops abnormally, his
brain was otherwise healthy, as were his eyes, so he had the necessary
tools to process subconscious vision. What he lacked were the circuits that
cobble together a clear, conscious picture.

The research team took brain scans and magnetic resonance images to see the
damage, finding no evidence of visual activity in the cortex. They also
found no evidence that the patient was navigating by echolocation, the way
that bats do. Both the patient, T. N., and the researcher shadowing him
walked the course in silence.

The man himself was as dumbfounded as anyone that he was able to navigate
the obstacle course.

?The more educated people are,? Dr. de Gelder said, ?in my experience, the
less likely they are to believe they have these resources that they are not
aware of to avoid obstacles. And this was a very educated person.?

Scientists have long known that the brain digests what comes through the
eyes using two sets of circuits. Cells in the retina project not only to
the visual cortex ? the destroyed regions in this man ? but also to
subcortical areas, which in T. N. were intact. These include the superior
colliculus, which is crucial in eye movements and may have other sensory
functions; and, probably, circuits running through the amygdala, which
registers emotion. 

In an earlier experiment, one of the authors of the new paper, Dr. Alan
Pegna of Geneva University Hospitals, found that the same African doctor
had emotional blindsight. When presented with images of fearful faces, he
cringed subconsciously in the same way that almost everyone does, even
though he could not consciously see the faces. The subcortical, primitive
visual system apparently registers not only solid objects but also strong
social signals. 

Dr. Held, the M.I.T. neuroscientist, said that in lower mammals these
midbrain systems appeared to play a much larger role in perception. In a
study of rats published in the journal Science last Friday, researchers
demonstrated that cells deep in the brain were in fact specialized to
register certain qualities of the environment. 

They include place cells, which fire when an animal passes a certain
landmark, and head-direction cells, which track which way the face is
pointing. But the new study also found strong evidence of what the
scientists, from the Norwegian University of Science and Technology in
Trondheim, called ?border cells,? which fire when an animal is close to a
wall or boundary of some kind.

All of these types of neurons, which exist in some form in humans, may too
have assisted T. N. in his navigation of the obstacle course.

In time, and with practice, people with brain injuries may learn to lean
more heavily on such subconscious or semiconscious systems, and perhaps
even begin to construct some conscious vision from them. 

?It?s not clear how sharp it would be,? Dr. Held said. ?Probably a vague,
low-resolution spatial sense. But it might allow them to move around more

Bill Meacham
* [address removed]
* Home office: (512)[masked] 
* Mobile: (512)[masked] 
* http://www.bmeach...­

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