Brightness versus apparent contrast. 1: Incremental and decremental disks with varying diameter.

This study describes the matching of the brightness and of the apparent contrast of foveal disks, presented as an increment or decrement with varying diameter against a 300 cd.m-2 background. If the brightness in the centre of the disks is matched with a constant reference brightness, the well-known spatial Broca-Sulzer phenomenon is obtained. This effect is not found if the apparent contrast of the disks is matched instead. All matching results and detection thresholds indicate that luminance increments and decrements are processed asymmetrically by the visual system: for decrements the threshold curve and curves of isobrightness as well as apparent iso-contrast are all shifted towards larger disk radii relative to those for increments. Two nonlinear single-channel models are tested against the data.