Separating the effects of response nonlinearity and internal noise psychophysically

AbstractA psychophysical method is proposed to separate the contrast dependence of internal response and its noise. The resulting contrast relationships represent a signature of the visual processing stage that limits the human observer's performance. The method was applied to contrast discrimination for sustained and transient Gabor patches with a 3 cycle/deg spatial carrier. For both stimulus types the predominant noise was found to be multiplicative with a power exponent of 0.76–0.85 and the source of this noise preceded by an accelerating signal transducer with a power of 2–2.7. These exponents combine to account for the classic compressive power of about 0.4 for the signal-to-noise ratio in contrast discrimination. The estimated transducer acceleration suggests that there is a direct computation of contrast energy in the visual cortex