According to the Barten model, the perceived foveal image is corrupted by internal noise caused by statistical fluctuations, both in the number of photons that trigger photoreceptor excitation and in the signal transport from photoreceptors to the brain. While the former effect is easily identified with the photon shot noise intrinsic to any luminous flux, the latter effect is related to the process of subdivision and recombination of photocurrents generated by each individual cone. These photocurrents create a parallel stream of information that is conveyed through the complex mesh of neural cells (horizontal, bipolar, amacrine, and ganglion) that form the retina-brain connection. Thus, fluctuations in the electrical/biochemical transport through such parallel pathways result in small differences in the image elements arriving at the brain (neural noise).
The overall effect of noise establishes a threshold level below which an image cannot be perceived or distinguished without a high probability of error. The process can be compared to the common experience of image degradation induced by fog in open air, relative to a clear atmosphere, or by flicker in a television image. The disturbance (fog or flicker) plays the role of noise externally added to an otherwise sharp scene, and limits the resolving capability of our visual perception.
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