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increased rod sensitivity at low light intensities. .
Because rods are only weakly coupled to cones, we use the cone as an in-vivo point source
detector of the intensity of the light stimulus. In this way we could account for the profile of
the scattered light in the retina that using a pinhole or slit and a photodiode detector could
not. However, because the cone’s intensity-voltage response relation is nonlinear, we had
to transform the voltage response profile to the light intensity profile using the known cone
I-V response curve to yield the profile of the actual light intensity (figure 5.2A). The light
intensity profile, shown in figure 5.2 C, was narrower than the cone voltage response profile
due to the cone’s increased gain at lower light intensities.
After computing the light intensity profile of the stimulus, we calculated the theoretical
isolated rod response by transforming the light intensity, divided by 10 to account for the
additional ND filter, to rod voltage response'(figure 5.2 D). This transformation was done
using the rod response to whole field flashes of known intensity. Because the background
light from the projector system changes the rod light response, the I-V relationship was
assessed both with and without the background light of the projector (figure 5.2 B). A small
rightward shift in the I-V curve and a lower saturation threshold was seen in the presence
of this background light (figure 5.2 B).
Ideally, the isolated rod response should be the same as the response of a rod to a whole
field stimulus of the same intensity, because whole field flashes “uncouple” the rod network
by homogeneously hyperpolarizing all rods together, preventing current flow between them.
It is this principle that allows us to extract the contribution of rod-rod coupling from the