Abstract
We investigated how the presence of an intervening gap influences the effects of a surround on fine discriminations performed on a center test patch. We used a 4-degree, 4 cpd sinusoidal grating surround with a center grating 40 min in diameter. Discriminations were based on differences in contrast (C) spatial frequency (SF), or orientation (O). Test contrast was held constant at 0.1. Surround contrast varied from 0.025 to 0.5. Test and surround were in-phase or 180 degrees out-of-phase, and either abutted or were separated by a 4-minute gap of mean luminance. Each condition was run in a separate block of 80 trials. Differences to be discriminated were adjusted individually for each observer to yield a d′ of approximately 1.2 in no-surround control conditions and fixed for all conditions. A two-alternative signal-detection rating procedure measured how performance changed with condition, and results were plotted as contrast-versus-performance functions. When no gap was present and center and surround were in phase, a small enhancement of performance was seen at low contrast levels. Performance dropped to a minimum when surround and test contrast were equal; it remained low for C and SF judgments but rose again at higher surround contrasts for O judgments. When center and surround were adjacent and out-of-phase, performance declined gradually with contrast for all tasks. At 50% contrast, in-phase and out-of-phase performance was approximately equal. Introduction of the gap significantly reduced or eliminated effects due to both surround contrast and phase. Performance remained at or near control levels. We will explore several alternative explanations for these findings.