Blindsight

In the early 1970s, a patient known in the scientific literature as DB underwent surgery that removed part of the visual area at the back of his brain. As a result, he lost sight in a large part of his visual field. When an object was placed there, he reported seeing nothing at all.

The psychologist Lawrence Weiskrantz and his colleagues decided to test him in an unusual way. Instead of asking what he saw, they required him to guess. A line would appear in the blind region, and DB had to say whether it was horizontal or vertical; in other trials, whether a shape was an X or an O. DB found the request pointless, since he could see nothing to guess about. Yet his answers were correct far more often than chance would allow, and on some tasks he was right almost every time. When shown his results, he was astonished.

Weiskrantz named this ability blindsight. Its most likely basis lies in the structure of the visual system. Although most signals from the eye travel to the region DB had lost, some take other routes to different parts of the brain, and these routes appear to be enough to guide a response.

Since the first reports on DB, published in 1974, blindsight has been documented in other patients with similar damage. The condition has become central to research on consciousness because it separates two things usually assumed to go together: responding to what is in front of the eyes, and being aware of seeing it.