Blindsight - when blind people can still see

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Blindsight - when blind people can still see

When you see something the visual signal actually goes to two different places. The first is the visual cortex, that renders what we know as sight, the second to a primitive subcortex that also registers some actions even though you don't know about it. So blindness caused by the visual cortex damage can still leave a mysterious type of vision at the subcortex level.

A man blinded in his visual cortex, after a stroke, but with two working eyes, was part of an experience to measure subcortexal vision. He initially 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.

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.

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 independently.”

 http://www.nytimes.com/2008...

Clip from the fascinating two-part documentary "Phantoms in the Brain" in which neurologist V S Ramachandran describes how the study of how the study of patients with certain types of brain damage can give us clues about the nature of consicousness and perception.


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By netchicken: posted on 26-12-2008








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