Light-dark asymmetries in the Craik-Cornsweet-O'Brien illusion and a new model of brightness coding.

With the aid of a matching technique, the magnitude of induced brightness in bars bordered with Craik-Cornsweet-O'Brien (CCOB) edges was investigated as a function of the width and amplitude of those edges. Data were collected for stimuli with the sloping part of the edge on both the inside and outside of the bar, and also for stimuli with both positive-going and negative-going edges. The results confirmed previous reports that induced brightness was greater for CCOB stimuli with negative-going, as opposed to positive-going, edges and greater for CCOB stimuli whose edges contained outer, as opposed to inner gradients. A model of brightness coding is offered to provide an explanation for the specific anisotropies observed, as well as the general effects of stimulus amplitude and width on induced brightness. The model assumes that a symbolic description of brightness is generated separately from each of a number of different-sized 2DG (second difference of a Gaussian) filters, and the resulting brightness profile obtained by averaging across the separate descriptions. The ability of other brightness models to account for the data is also discussed.