Three stages and two systems of visual processing.

Three stages of visual processing determine how internal noise appears to an external observer: light adaptation, contrast gain control and a postsensory/decision stage. Dark noise occurs prior to adaptation, determines dark-adapted absolute thresholds and mimics stationary external noise. Sensory noise occurs after dark adaptation, determines contrast thresholds for sine gratings and similar stimuli, and mimics external noise that increases with mean luminance. Postsensory noise incorporates perceptual, decision and mnemonic processes. It occurs after contrast-gain control and mimics external noise that increases with stimulus contrast (i.e., multiplicative noise). Dark noise and sensory noise are frequency specific and primarily affect weak signals. Only postsensory noise significantly affects the discriminability of strong signals masked by stimulus noise; postsensory noise has constant power over a wide spatial frequency range in which sensory noise varies enormously. Two parallel perceptual regimes jointly serve human object recognition and motion perception: a first-order linear (Fourier) regime that computes relations directly from stimulus luminance, and a second-order nonlinear (nonFourier) rectifying regime that uses the absolute value (or power) of stimulus contrast. When objects or movements are defined by high spatial frequencies (i.e., texture carrier frequencies whose wavelengths are small compared to the object size), the responses of high-frequency receptors are demodulated by rectification to facilitate discrimination at the higher processing levels. Rectification sacrifices the statistical efficiency (noise resistance) of the first-order regime for efficiency of neural connectivity and computation.