ABSTRACT
Spatiotemporal Response of the Photoreceptor Network
by
Andrew Barrow
The retina is a specialized part of the central nervous system adapted to encoding images into
electrical signals. Images are formed on the back of the eye by the lens and cornea, and photons
that make up those images are absorbed by light sensitive pigments in the photoreceptors. Photon
absorptions by these pigments generate a current, the photocurrent, which is modified by voltage-
gated ion channels and electrical connections to adjacent photoreceptors. A voltage change in the
photoreceptor is transformed into a chemical signal to downstream cells by its modulatory effect on
the calcium concentration at the synapse. This thesis examines two important elements in photore-
ceptor function other than the photocurrent: the Ifl current and electrical coupling between rods.
Here, using the tiger salamander (Ambystoma tigrinum) as a model, we investigate the kinetic
properties of the HCN channels responsible for the Ih current in photoreceptors, and show that
they are similar in rods and cones, which in turn are similar to the known properties of the HCN1
isoform. With western blot and immunostaining, we show that the HCN1 isoform is present in retina.
We also demonstrate how HCN channels modify the kinetics of the rod and cone light response
to make it faster. This thesis integrates this and other data from photoreceptor ion channels into
physiology-based models of rod and cone photoreceptors. Through simulation, the model of the
rod demonstrates that conductance changes from the h and Kx currents largely cancel one another
during the rod light response. The cone model is used to demonstrate the feasibility of two proposed
mechanisms for horizontal cell to cone negative feedback.
Finally, this work presents measurements of electrical coupling between rod photoreceptors in
the salamander retina using both light and electrical stimuli. Using measured parameters for the
coupling resistance, a model of the electrically coupled network of rod photoreceptors is developed.
We use this model to demonstrate how rod-rod coupling decreases noise at the expense of attenuating
sharp contrasts in visual scenes. The model predicts the tradeoff between these two factors results
in an overall improvement in the signal-to-noise ratio for most perceptible stimuli. Results suggest
that photoreceptor coupling is especially helpful in the perception of images with statistical qualities
similar to natural scenes.