Contour-integration deficits on the intact side of the visual field in hemianopia patients

Visual impairments in hemianopia are thought to be exclusively caused by the reduced visual field size. However, the primary lesion may affect the contralateral hemisphere through damage of interhemispheric projections. The question therefore arises if the presumed "intact" hemifield is perceptually impaired.

Three hemianopia patients and three matched controls carried out a Yes/No figure detection task with their intact side of the visual field. The figure (square) contours were composed of non-contiguous Gabor patches embedded in a random patch array of different background densities (low, delta=2; high, delta=1). Response accuracy and reaction times were recorded.

A temporal-parietal patient revealed figure detection impairments, with accuracy rate, 77% (delta=2) and 53% (delta=1), below compared control values. An occipital patient was comparable to his match: 99% (delta=2); 84% (delta=1). Both patients exhibited frequent false alarms to random patterns and required longer presentation times to perform the task. In the third patient, with optic tract lesion, figure detection was nearly normal at low density (92%, delta=2) but impaired at noisy background (62%, delta=1).

The "intact" visual field in hemianopes is impaired in detection of incomplete figures embedded in a noisy background. This deficit may be caused by damage to higher visual centers and/or loss of interhemispheric interactions.