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Other than a more detailed analysis of the photoreceptors themselves, there are two pos-
sible directions in which studies of noise in the retina can be extended: to upstream noise
in the stimulus source, and to downstream noise elements such as the synapse, bipolar cells,
and ganglion cells.
While photoreceptors generate their own intrinsic noise from fluctuations in the pho-
tocurrent and ion channel activity, it has been shown that the major determinant to rod
noise in response to dim visual scenes is the variability in the light stimulus itself [105]. The
quantal nature of light means that in dimly lit conditions, fluctuations in the number of pho-
tons arriving in a given time interval may make up a significant fraction of the mean arrival
rate. Photon emission, and thus arrivals, are probabilistic events, described by a Poisson
distribution, which means that a variable numbers of photons will be absorbed in a given
time interval. This variability is seen by the photoreceptor when the response to a single
photon makes up a significant fraction of the total response in a given time interval. While
it could be argued that variability in the incident light stimulus is an intrinsic part of the
signal rather than a noise phenomenon, it nevertheless represents an uncertainty that the
animal must deal with in order to perceive its surroundings.
In other systems where this type of variability is present it is often called shot noise. In
low light or high speed photography, it is called photon noise, and can be seen in the noisy
appearance of dim areas of images. In semiconductor devices, the particle nature of electron
charge carriers manifests itself as small fluctuations in current flowing across a charge barrier
which is called shot noise. Rice’s analysis on the effect of shot noise in linear devices such as