Filling in the brain's line of sight.

A curious visual quirk sometimes arises after damage to one or the other side of the brain, usually to tissue near the brain's midpoint. A single object lying in either the right or left visual field is visible, but an item placed in the visual field opposite the ailing hemisphere vanishes if it's shown at the same time as a different item in the adjacent visual field.

Psychologist Jason B. Mattingley of the University of Cambridge in England directed experiments focusing on a 66-year-old female stroke victim. The results suggest that the one-sided disappearing act occurs only after her brain conducts an automatic, relatively thorough three-dimensional analysis of the "invisible" item. A disturbance of brain processes that then direct conscious visual inspection causes her to pay attention to only one of two items positioned in separate visual fields, Mattingley's group contends in the Jan. 31 Science.

The woman they studied had suffered right-brain damage and lost sight of items in her left visual field. Yet when shown a pair of illusory rectangles-each consisting of four open-mouthed Pac-man shapes placed at the corners and facing inwards, yielding the eerie sensation of a bounded rectangle-she usually reported seeing the rectangle in both visual fields. She had similar success when shown a pair of drawings, each of which creates the illusion of a rod running behind a cube.

When her brain performed the complex task of filling in the incomplete image, the woman's conscious detection of items to the left rebounded, the scientists theorize. The perception of an illusory surface was striking enough to engage her attention in both visual fields, they hold.