See Inside April/May 2008

Subconscious Sight

People with "blindsight" can correctly deduce the visual features of objects they cannot see. Such visual intuition can even exceed what is possible with normal vision

DB is a 67-year-old man whose view of the world is dark from the center of his gaze leftward. He has been blind to this left part of his visual scene since age 33, when he had surgery to remove an abnormal tangle of blood vessels at the back of his brain. Unfortunately, while taking out the tangle, surgeons destroyed an important center of visual processing called the primary visual cortex, or area V1, which relays information from the eyes to higher-level brain areas dedicated to sight.

DB lost just the right half of V1. Because the right part of the brain processes visual information from the left visual field (and vice versa), his doctors were not surprised that DB became blind to the left portion of his view. But they were astounded that although DB denied seeing anything to the left of center, he was nonetheless able to accurately “guess” many properties of targets, such as shape and specific location, presented in this perceptually dark field.

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