The mechanisms involved include retinal interaction, adaptation at the level of the bipolars in the retina and eye movements. Adaptation at the bipolar layer introduces a non-linear relation between the photoreceptor response and the ganglion cell response. It also sets an upper limit to the response of the bipolars and were it not for eye movements the visual field would be reduced to a uniform brightness and were it not for eye movements the visual field would be reduced to a uniform brightness at high luminance levels. The eye movements prevent a difference in adaptation on the two sides of a border and thus provide for a difference in the response on the two sides of a border. Lateral inhibition has a greater spread than excitation and thus also accentuates the brightness at borders. A special frequency equalizing mechanism is postulated to explain the perceived uniformity of a surface between two borders. Surface contrast and border contrast involve the same mechanisms.