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6.2 Future Directions
After talking with and reading about many scientists, I realize that every paper published is
never really finished—there are always more experiments to be done, more in depth analyses
to be undertaken, and more to be discovered. If I were to say that I am truly done with the
work in my thesis, I think it would mean I would be ready to retire, not graduate. That being
said, the work in this thesis is able to stand on its own, and should be a useful and interesting
contribution to visual neuroscience. However, even as I write this I have ideas about how to
extend the work I presented in chapter 5.
While the steady-state analysis of photocurrent generated noise in section 5.6 is valid
for the photocurrent, whose frequency components are below the filter cutoff several pho-
toreceptors distant from the source, it is not true of other noise sources in the rod. For
instance, HCN channels, which have a time constant around 60 ms, would fluctuate with
a cutoff frequency of 2.6 Hz, above the cutoff frequency for any rod in the network (figure
5.8). Other ion channels such as voltage gated potassium channels, with even faster kinetics
may also contribute to receptor noise. Accurately characterizing the rod network’s response
to these noise sources would require using the transient impulse response function. Higher
temporal frequency noise sources would contribute less and less to noise in more distant
photoreceptors. On the other hand, in studies of photoreceptor noise in the turtle cones, it
appears that the photocurrent noise is the largest determinant of the receptor voltage noise
[70]. If this also applies to salamander rods, this would imply that our analysis of rod-rod
couplings effect on photoreceptor noise is a valid approximation.