Global spatiochromatic mechanism accounting for luminance variations in contrast sensitivity functions.

Psychophysical luminance and chromatic spatial contrast sensitivity functions (CSF's) exhibit a number of variations that depend on the average luminance of the stimulus grating. The contrast sensitivity at each spatial frequency and the maximum resolvable spatial frequency decrease with decreasing average luminance. Most prominently, luminance CSF's have peaks that shift toward lower frequencies, broaden, and disappear as luminance decreases. Chromatic CSF's are flat at low frequencies and do not exhibit luminance-related variations in shape. In this paper, we account for the luminance-dependent variations of CSF's by incorporating them into an earlier model that accounts for the basic high-luminance shapes of psychophysical luminance and chromatic spatial CSF's. These variations are modeled by increasing the spatial extent of a global center mechanism while decreasing the inhibitory effect of a global surround on that center. The results support the hypothesis that the global center mechanism changes size as a means of maintaining the dynamic range and controlling the signal-to-noise ratio as luminance varies.