Contrast Enhancement and the Negative Afterimage

An optical-lever technique was used to stabilize the retinal image, in order to determine spatial contrast-transfer functions for several modes of presentation of rectangularly and sinusoidally modulated bar patterns. These modes include a sudden replacement of a uniform field by a bar pattern, the replacement of a bar pattern by a uniform field, the intermittent presentation of a bar pattern at several repetition rates, and the sudden superposition of a uniform field on a bar pattern. Average retinal illuminances of 50 and 5 td were used. The outcomes of these experiments enable us to decompose the spatial contrast-transfer function for the case of normal vision into a branch due to the positive primary image and a branch due to a negative afterimage. The interaction of the primary image and the afterimage causes spatial transients in the image to be enhanced.