How Blind Can See Again Without Their Eyes

A new study offers 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.

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BLINDSIGHT A patient whose visual lobes in the brain were destroyed was able to navigate an obstacle course and recognize fearful faces subconsciously.

Scientists have previously reported cases of blindsight in people with partial damage to their visual lobes. This 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.

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, but also to subcortical areas. 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 patient 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.

Sources:
The New York Times December 22, 2008
The New York Times January 4, 2009
Current Biology December 23, 2008;18(24):R1128-9

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