Pattern discrimination with increment and decrement Craik-Cornsweet-O'Brien stimuli.

Previous studies have reported that the magnitude of induced brightness in Craik-Cornsweet-O'Brien (CCOB) figures is greater for decrement compared with increment figures. Moulden and Kingdom (Spatial Vision, 1990, 5, 101-121) suggested this was due to larger Off-centre compared with On-centre receptive fields. Such an explanation would also predict an increment-decrement difference in the contrast threshold for discriminating a CCOB stimulus from its step-edge equivalent. To test this prediction contrast thresholds were measured for discriminating 'cusp' from 'square-bar' stimuli, for both increment and decrement forms. Contrary to prediction however, no difference was found between increment and decrement discrimination thresholds. These findings suggest that On- versus Off-centre receptive field size differences are unlikely to underlie the polarity-based asymmetry in induced brightness in CCOB figures. More generally they demonstrate that the magnitude of induced brightness in CCOB figures does not have a direct parallel in the ability of observers to discriminate those stimuli from their step-edge equivalents. The significance of these findings for models of brightness coding is discussed.