The perception of brightness of the grey field depends on the environment - in bright surroundings, an identical grey appears darker than in dark surroundings.

The fact that a medium grey area will appear light grey if it is bordered in black, or dark grey if it is bordered in white. This can be explained by the fact that the stimuli perceived are processed directly - brightness is perceived as a result of the lightness contrast between the grey area and the immediate surroundings. What we are considering here is a visual impression that is based exclusively on sensory input which is not influenced by any criteria of order linked with our intellectual processing of this information.