Detectability of power fluctuations of temporal visual noise

The following psychophysical functions were obtained with a homogeneously illuminated field (20 degrees, retinal illuminance 750 trolands) of white light the illuminance of which was a function of time: (1) The sensitivity to sine wave modulations (the De Lange curve). (2) The sensitivity to a white noise signal. (3) The incremental sensitivity to periodical power fluctuations in supra-threshold white noise signals as a function of the modulation frequency and the mean noise power. (4) The sensitivity to noise bursts as a function of the duration. We find that the incremental (or decremental) sensitivity to power fluctuations of noise signals follows Weber's law. If the duration of the noise signal is less than one third to half of a second then the detection threshold is reached if the noise energy (noise power integrated over time) exceeds a certain fixed level. The results can be quantitatively understood if we assume that for our stimuli the visual system functions as a variance estimator characterized by a bandwidth that is essentially given by the De Lange function, and a time constant of one third to half of a second. The visual sense processes these input signals in chunks of about two dozen statistically independent samples. This scheme functions both for liminal and supra-liminal signals. The limits of validity of the scheme are discussed. Relations to previous work are indicated.