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Chapter 5
Electrical Coupling in the Rod Network
5.1 Introduction
Electrical coupling between photoreceptors has been demonstrated in many species, in-
cluding the turle, the tiger salamander, the ground squirrel, and primates [69, 5, 35, 56].
Although the pattern of photoreceptor coupling varies between species— there are vary-
ing combinations of rod-rod, rod-cone, and cone-cone coupling in different animals, the
purpose of this coupling appears to be the same across species. Coupling between alike
photoreceptors (rod-rod, cone-cone) is thought to reduce noise by averaging out fluctua-
tions in dark current activity in these photoreceptors. On the other hand, coupling between
rods and cones is thought to provide a way for cones to make use of the rod bipolar cells at
light intensities where rods approach saturation.
In transforming light signals into electrical signals, the retina creates a representation
of the underlying information while minimizing contamination with noise. The first stage
in this process, and the limiting factor in the baseline noise in the retina, is the conversion
of photons into electrical signals by the photoreceptors. Noise intrinsic to photoreceptors
comes from several sources including the photocurrent and ion channel activity. The pho-
tocurrent has two sources of noise: spontaneous thermal activation of rhodopsin molecules,
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