Abstract
Center-surround interactions decrease the apparent contrast of a center stimulus. Most models assume the cause is lateral interactions among low-level mechanisms, and generally (e.g., Solomon, Sperling & Chubb, 1993) assume that the interactions are reciprocal between center and surround. As part of our investigations of lateral interactions affecting the ability to make fine spatial judgments, we tested the reciprocity assumption. Observers discriminated between two highly similar grating patches (4 cpd) in each block of eighty trials. Half discriminated on the basis of orientation; half on spatial frequency. Observers made decisions either on a center circular patch of grating with and without an annular masking surround or on an annular surround ring with and without a center masking disk. Four test/mask size ratios were used (surrounds varied from a width of 10 to 80 minarc when judgments were on the 20 minarc diameter center patch; center diameter varied from 10 to 80 minarc when judgments were made on the annular ring (width of ring 20 minarc). Contrast of the test was 0.10. Mask contrast varied in six steps between 0 and 0.50. Each of the 48 conditions was run in a separate session using a signal detection rating paradigm. Sessions were replicated five times following extensive practice and individual determination of differences needed to yield unmasked d's of approximately 1.2. Interactions were found not to be reciprocal. Surrounds masked centers, but not vice-versa. These results strongly suggest that mechanisms of figure-ground organization play an important role in mediating these lateral interactions.