Irradiation, border location, and the shifted-chessboard pattern.

The position of a black-white border is perceived somewhat towards the black side of its physical location. New measurements were performed of the magnitude of this effect for foveal vision in the normal observer and it was found to be about 0.4 min of arc at medium photopic luminances. The border shift can be accounted for by postulating that the retinal light spread caused by the optics of the eye is subjected to a compressive nonlinearity of light intensity (Naka-Rushton equation). Such irradiation-induced apparent enlargement of the white elements does not fully account for the shifted-chessboard illusion. When an additional stage of center-surround (DoG) transformation is introduced, a hypothetical excitation distribution can be generated, whose contours, at the appropriate scale, can outline quite well the seen shape deviations from rectangularity of the illusion. Contributing to the final percept, in addition to the light-spread, compressive nonlinearity, and center-surround reorganization, which would be retinal in origin, there are some cortical stages: the generation of the experience of sharp, straight borders and pointed corners, the emergence of a monotonic slope of the demarcation lines made of 'saw-tooth' position and angle shifts, and, finally, when all the retinal effects have been nulled out, a Zöllner kind of orientation deviation, due to offset pattern elements acting as tilted virtual contours.