Perceiving perpendicular and parallel contours in the frontoparallel plane.

The perception of a pair of contours in a retinal image cannot be understood simply by adding up the perceptions of the individual contours, especially when they form a perpendicular junction, or are parallel to one another. It is the relationship among the contours that determines what is perceived. Note that it is hard to actually compare the perception of such configurations quantitatively. We managed to do this by testing the perception of such configurations in three psychophysical experiments in which the perception was characterized by measuring the orientation threshold of a single contour. This threshold was estimated by using a modified Method of Constant Stimuli based on the assumption that contours forming a configuration are perceived individually, and that they are integrated linearly. This assumption made the quantitative comparison of the perceived configurations possible. We found that changes of the estimated threshold depended on the type of the configuration, specifically thresholds estimated from a perpendicular junction were substantially lower than thresholds estimated from a single contour or from a non-perpendicular junction. The lowest thresholds were observed when the threshold was estimated from a pair of parallel contours. These results suggest that the visual system is sensitive to perpendicular junctions and parallel contours in a retinal image.