Differential sensitivity for luminance differences existing in two spatially separated homogeneous fields was measured under three viewing conditions. In the first the comparison fields were separated by darkness; in the second the fields were joined by a transition area in which luminance increased linearly from the value presented in the darker field to that presented in the lighter field. A third condition controlled for the decrease in size of the fields which occurred with increasing separation. In all viewing conditions the darker field was presented to the left and to the right of the lighter field equally often; and the observer's task was to detect the darker side of the display. Detection was significantly better with the gradient present than with the fields separated by darkness. The results are discussed in terms of the suggested ability of the visual system to utilize the distribution of light in an intervening gradient to emphasize the difference in the luminances of separated fields.
HAROLD W. HAKE and EMANUEL AVERBACH, "Spatial Effects in Foveal Brightness Discrimination," J. Opt. Soc. Am. 46, 274-277 (1956)