Noise suppression in photoreceptors and its relevance to incremental intensity thresholds

Quantal noise is the main limiting factor of visual contrast sensitivity only over an intermediate range of light intensities. At low and high intensities receptor and neural noise determine the bound on sensitivity. In this paper we analyze the effect of receptor nonlinearity on the statistics of visual signals and show its role in the suppression of quantal and receptor noise at high light intensities. This provides adequate physical interpretation of physiological data. Extending this analysis to the behavior of the whole visual system, we arrive at the conclusion that such noise suppression provides a unified explanation of psychophysical incremental threshold data over the whole visual range.