Abstract
Abstract: :
Purpose:To determine the range of orientations over which lateral interactions among patterned stimuli affect orientation discrimination. Methods:The patterns to be discriminated were near–vertical sinusoids of 4 cpd (approx. ± 0.5 deg), displayed in a sharp–edged 40–minute circular window at a mean luminance of 19.8 cdm–0.5. In control conditions, no surround modulation was present. Modulated surrounds were simple vertical gratings that varied in orientation from vertical to horizontal in 9 steps. Test and mask contrasts were held constant at 0.1. Each condition was run in a separate block of 80 trials. Differences to be discriminated in either orientations or spatial frequencies were adjusted individually for each observer to yield a d' of approximately 1.5 in control conditions. All observers were well–practiced. Difference thresholds (d’ = 1.2–1.8) were in the hyperacuity range and fixed for all conditions. A two–alternative signal–detection rating procedure was used to measure how discrimination performance changed as a function of center–surround similarity. Results: The magnitude of interactions did not monotonically decrease with increasing differences between center and surround organization. Instead, performance was minimal with vertical surrounds; rapidly recovered or was enhanced over control levels at small (2–5 deg) orientation differences, but was again reduced considerably at 10 deg, rising slowly to near control values at 90 deg. Iindividual differences were observed, further suggesting that more than a single factor underlies lateral interaction effects in discrimination. Conclusions: The orientation dependence data of Cannon & Fullenkamp (Vis. Res.,1991, 31, 1985) suggested that there were two orientation–dependent mechanisms underlying contrast suppression; one of which disappeared by approximately 15 deg, and another that decreased slowly over 90 degrees. Our discrimination data, which differ from apparent contrast data in many other respects, also suggest two underlying suppressive mechanisms, and closely resemble the data of Cannon and Fullenkamp.
Keywords: scene perception • discrimination • perception