Attention and perceptual learning modulate contextual influences on visual perception.

Brightness discrimination thresholds and facilitation by lateral interaction were measured in five human observers and two monkeys. The subjects judged the brightness of one of four peripherally seen lines against a reference. This experiment was performed both when the observer was cued to the position of the test line (focused attention) and when there was no cue (distributed attention). Discrimination was better with focused than with distributed attention. When the test line had a collinear flank, its brightness was enhanced; this enhancement was four times more prominent with distributed than with focused attention. After training, thresholds improved and collinear facilitation decreased under distributed but not under focused attention. The findings show that there are fewer benefits from contextual interaction once attention is directed toward a visual location, and that the attentional effects are subject to training.