Reduced spatial sensitization on nonuniform backgrounds

Spatial sensitivity (Westheimer) functions, when measured on nonuniform backgrounds made up of light dots of 12 min arc, were found to differ in shape, depending on the polarity of the central area on which the test spot was placed. When thresholds were measured on the dark centre between light dots, ie on the adapting-field illumination, the resulting curve was similar to the control curve, measured on a uniform background equated for flux. In comparison, thresholds measured on a central light dot, serving as a pedestal, peaked at larger background diameters and showed much less sensitization compared to the control function.