A, Sketches of receptive field structure. The receptive field center mechanism is thought to derive from a single cone, but the surround may derive from a single cone type or from a mixture of both cone types. The spatial profile is usually considered as a DOG. B, Hypothetical spatial frequency curves for a DOG profile for luminance modulation. The BPI is defined as R0/Rmax. C, With a cone-selective surround, the spatial frequency tuning curve for a cone-isolating grating targeting the center cone class alone is expected to be low-pass in shape. D, With a mixed surround, a bandpass tuning curve is expected for a cone-isolating grating targeting the center cone class, but with a lesser degree of low spatial frequency roll-off compared to a luminance grating.