Interactions among line orientation detectors

The thresholds for identifying the orientation of line segments were studied with the following stimuli: a) single line segment (S1); b) two parallel line segments (S2) with or without longitudinal displacement; c) three parallel line segments (S3) with or without longitudinal displacement. The thresholds for the stimuli S2 and S3 are lower in comparison to those for the single line segment S1. When the segments are considerably apart (21') the decrease in the thresholds coincides with the theoretically predicted ones under the assumption for probability summation and could be explained by suggesting and independent processing in the orientation selective units (orientation detectors). With closely located line segments the threshold changes for S2 and S3 deviate from the expected values for probability summation and so an interaction among the detectors could be postulated.