Abstract
Contextual influences on fine spatial discrimination were assessed in two 2AFC signal detection rating experiments. Patches of sinusoidal gratings that differed slightly in orientation were discriminated in the absence and presence of modulated surrounds of the same contrast (0.1). Differences to be discriminated were determined individually for each observer to yield a d′ of approximately of 1.5 and held constant thereafter. In Experiment 1, orientation judgments were made on a 3 cpd vertical test grating alone (control) or in the presence of a 3, a 15, or 3+15 cpd vertical surround. Test and surround were either in-phase or out-of-phase and either abutting or separated by a 30 min gap of mean luminance. Each condition was run in a separate block of 80 trials. Results suggested that relative to control, performance was only suppressed when 3 or 3+15 cpd surrounds were in-phase with no gap. In Experiment 2, we tested the hypothesis that perceptual segregation mechanisms play a role in eliminating lateral suppression, and asked whether segregation between center and surround by a difference in luminance would reduce or eliminate suppression. We replicated the in-phase, 3 cpd, abutting surround condition of Experiment 1, but now with a luminance mismatch between center and surround components. Contrast of center and surround were still held constant at 0.1. Preliminary results suggest that segregation by a luminance mismatch does indeed reduce or eliminate suppressive effects. We conclude that perceptually segregating a scene into parts interrupts lateral influences, and the results suggest that lateral influences affecting discrimination tasks only occur within a segmented area, not across areas.