Contrast enhancement in a Hermann grid with variable figure-ground ratio.

Abstract

Psychophysical experiments on contrast vision were made using a modified Hermann grid which was graded in two directions. This pattern was composed of intersecting (IG) and intersected stripes (ID) representing 15 shades of a gray scale, and was viewed against 5 uniform backgrounds (BGD) ranging from white to black. Illusory light and dark patches at intersections were essentially limited to IG's that ranged in reflectance between BGD and ID. The illusion was most distinct when the contrast ratio between IG and BGD was minimum, while that between IG and ID was maximum; it was much reduced on intersections formed by equireflectant stripes, such as those in the original Hermann grids. “Brighter” responses were evoked by a white BGD and “darker” responses by a black BGD. Gray BGD's elicited both kinds of responses. The contrast effects were more pronounced for vertical and horizontal than for diagonal grid orientation. An optimum grid was devised that shows maximum brightness changes at almost every intersection, even when fixated foveally. The observations are interpreted in terms of Baumgartner's receptive field hypothesis.