Separating the effects of response nonlinearity and internal noise psychophysically.

A 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.