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Second sight

Avoiding obstacles in our path is so easy for most of us that we do it almost without noticing them. Now, researchers have reported the ability of a blind man to negotiate obstacles completely subconsciously, providing another dramatic demonstration of 'blindsight'.

The man, known as T. N., has healthy eyes and optic nerves but is thought to have lost all function in both visual cortices following two strokes. Previous investigations with other people made blind by such damage uncovered the phenomenon of blindsight — showing, for example, that automatic physical reactions to emotional faces can remain intact. T. N. displayed similar reactions, and so the researchers decided to test his ability to sense and avoid objects.

To their astonishment, he successfully walked the length of a corridor filled with obstructions, side-stepping and swerving around items he was not consciously aware were there. Lead author Beatrice de Gelder, of Tilburg University, said: “He walked much faster than we had expected, without hesitation or any kind of exploration.” ( New Scientist , 22 December 2008.)

Blindsight is thought to result from the projections of the optic nerves to subcortical areas, including the superior colliculus and the amygdala. According to Richard Held of the Massachusetts Institute of Technology, evidence now indicates that “...there is a form of vision which is not dependent ... upon the primary areas responsible for processing inputs from the eyes.” ( Washington Post , 23 December 2008.)

“It's a part of our vision that's for orienting and doing rather than for understanding,” said de Gelder ( The Times , 23 December 2008). “One would hope ... it could be capitalized on,” she added, proposing that people with damage to the visual cortices might be able to restore certain capacities “...such as the ability to move around the house...” through training (Washington Post).

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Nicholson, C. Second sight. Nat Rev Neurosci 10, 86 (2009). https://doi.org/10.1038/nrn2585

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