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December 26, 2008

BehindTheMedspeak: Explaining Tommy — Blindsight Unmasked


A remarkable report published this past Monday in the journal Current Biology documents as never before the phenomenon "... of so-called blindsight, the native ability to sense things using the brain's subcortical — and entirely subconscious — visual system," wrote Benedict Carey in an article in this past Tuesday's New York Times Science section.

The graphic above, captioned "A patient whose visual lobes in the brain were destroyed was able to navigate an obstacle course and recognize fearful faces subconsciously," led the Times piece, which follows.

    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.

    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 Pöppel 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 independently.”


You can watch a video of the blind individual navigating a complex maze here.

The abstract of the Current Biology paper follows.

    Intact navigation skills after bilateral loss of striate cortex

    A patient with bilateral damage to primary visual (striated) cortex has provided the opportunity to assess just what visual capacities are possible in the absence of geniculo-striate pathways. Patient TN suffered two strokes in succession, lesioning each visual cortex in turn and causing clinical blindness over his whole visual field. Functional and anatomical brain imaging assessments showed that TN completely lacks any functional visual cortex. We report here that, among other retained abilities, he can successfully navigate down the extent of a long corridor in which various barriers were placed. A video recording shows him skillfully avoiding and turning around the blockages. This demonstrates that extra-striate pathways in humans can sustain sophisticated visuo-spatial skills in the absence of perceptual awareness, akin to what has been previously reported in monkeys. It remains to be determined which of the several extra-striate pathways account for TN's intact navigation skills.

December 26, 2008 at 04:01 PM | Permalink


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You need to multitask Clifyt. There's no reason that you can't enjoy your cocktails while reading bookofjoe.

Posted by: Rocketboy | Dec 27, 2008 12:20:38 PM

This is very similar to how those with anterograde amnesia can actually acquire new information. While they may not be able to access it through their standard memory models, when presented with a novel puzzle that they had never experienced before the damage occurred, it is a proven fact that through repetition and practice, they will learn to do the task. They will act as if they were never given this, and act as though their ease of doing it is only because it is a simple task not having any memory of doing it before, but you can measure this phenomenon and it has a very similar learning curve as most other learning tasks.

The mind is an interesting thing...once you start understanding the neurological systems, you find that there is always multiple areas of the brain that make up any process...if one area is damaged, one can still access the information even if it is not in a conscious sort of way.

Anyhoo...why am I sitting here wasting valuable cocktail time on this blog?

Posted by: clifyt | Dec 26, 2008 5:47:31 PM

With the sighted guy behind him, I'm not convinced that this is a valid test.

Posted by: Rocketboy | Dec 26, 2008 4:48:55 PM

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