Sources of visual noise
Noise processes pervade many aspects of visual information processing from photo-transduction on. But it is not known whether all this noise affects the observer's visual sensitivity. To study this issue, I have employed equivalent input noise measurements to characterize the observer's intrinsic noise along several signal dimensions. The equivalent input noise is the power spectral density of white display noise required to double the threshold contrast energy for a given signal. The existence of multiple, frequency-selective channels in the visual system allows us to choose signals to selectively engage channels tuned to different spatiotemporal frequencies, one at a time, and thereby measure the spectrum of the equivalent input noise. The psychophysically measured spectrum can then be compared to the spectra of known intrinsic noise sources: photon noise (generated by the random way in which light is absorbed by photoreceptors), ganglion cell noise, and cortical noise.
The observer's equivalent input noise was measured as a function of letter size (or grating spatial frequency), signal duration, and retinal illuminance both in the fovea and periphery using an identification task. The results indicate that the limiting noise in scotopic vision is indeed the photon noise. Photon noise also limits the visibility of high spatiotemporal frequency signals at photopic and low-mesopic light levels. But, the limiting noise for low spatiotemporal frequency signals, in both the fovea and the periphery, has the characteristics of neural noise.