Effects of visual noise

Random fluctuation in luminance, over time or space or both, is luminance noise. Squared threshold contrast is found to be proportional to the spectral density of luminance noise, plus a constant. The constant is an estimate of the observer's "equivalent noise". The observer's near-threshold performance--at detection and discrimination (and possibly apparent-contrast matches) is consistent with a description of the observer as noise-free except for the equivalent noise at the visual field. Effects of varying noise spectrum in all three spatia-temporal-frequency dimensions support the hypothesis that we detect a grating by means of a linear spatia-temporal filter, a "channel", so that squared threshold contrast is proportional to the total noise power (including equivalent noise) passed by the linear filter. A hypothesis of intrinsic uncertainty leads to the channel-uncertainty model, which is consistent with the noise effects and predicts summation and facilitation effects as accurately as accurately as other theories which do not explain the effects of noise. The uncertainty hypothesis was confirmed by direct test: threshold contrast for one of ten-thousand possible signals was only a factor of 1. 5 higher than threshold contrast of a fixed signal