Spatial structure of contextual modulation.

Contextual effects are ubiquitous in vision and provide a means for detectors with localized receptive fields to encode global properties of a stimulus. Although the nature of the neural connections is complex, the majority of evidence supports the Gestalt idea of collinearity; interactions are greatest when the target and surround orientations are spatially aligned to form a contour. Here we create a novel stimulus that simultaneously probes all areas around a detector to determine which spatial positions influence perception in human observers. We find that the surrounding spatial areas that contribute most to contextual effects for our perception of orientation and motion are not confined to a specific location. Rather our results reveal that human perception displays some interobserver variability in the weighting of detector interactions that is largely independent of collinear structure. We propose that these more extensive surround stimuli reveal how complex visual structure may modulate performance in a manner that is not easily predictable using more conventional stimuli.