Fig. 7. A. Spatial influences on light sensitivity. Mach bands are alterations in perceived brightness induced by spatially juxtaposed surfaces. In this example, the brightness of solid gray bars appears lighter near the edge adjacent to a darker bar. The upper line represents the true distribution of luminance across space for the Mach band stimulus. The lower line represents, in schematic form, the perceived brightness of the bars. The alterations in perceived brightness are predicted by an antagonistic center-surround receptive field, like that of the retinal ganglion cells. B. Scintillating grid illusion. Illustrates influences of spatial configurations, thought to stimulate inhibitory receptive field surrounds and produce the illusion of spots strongly. The illusion of spots is triggered by eye movement and is more pronounced peripherally, where receptive fields are larger. (Schrauf M, Lingelbach B, Wist ER; The scintillating grid illusion. Vision Res 37:1033, 1977.)