The importance of contrast for the activity of single neurons, the VEP and perception.

The brightness of a visually perceived object is mainly determined by the average local contrast around the border between object and background. This fact is demonstrated here with several examples of equiluminant objects on nonuniformly luminant backgrounds. Even in Mondrian-like patterns resembling those used by Land and McCann (1971), equiluminant objects may appear to be of unequal brightness. This result does not agree with predictions of the Retinex Theory. The importance of contrast in vision is also suggested by neurophysiological findings, both classical and recent, that reveal the dependence of visual responses on contrast over most of the visual operating range of mean illumination. The dependence on contrast appears to be the result of retinal gain control mechanisms and is not due to center-surround interaction in the receptive field. We have discovered parallel neural channels with high and low contrast gain in the monkey's visual pathway by means of single unit techniques. Visual evoked potential measurements suggest that similar visual pathways, and with high and low contrast-sensitivity, exist in man and monkey.