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been investigated before in rod photoreceptors [30, 5,17, 33, 34, 87,103,115], we show that
cones also exhibit a similar bandpass filter effect, and that the frequency response of rods
and cones varies with input intensity. By injecting hyperpolarizing steps of current into rods
and cones we analyze a circuit analog for photoreceptors (figure 3.6 C), which predicts that
the frequency response of the bandpass filter becomes more peaked and shifts to higher fre-
quencies as stimulus intensity increases (figure 3.7 A, B). These effects are observed within
individual photoreceptors, and are independent of the surrounding network.
3.4.4 HCN1 channels help rods and cones efficiently encode impulses of light
To investigate how HCN channels shape the light response of rods and cones, we use phar-
macology combined with various light stimuli. With chirped light stimuli, we show that
HCN channels act as a compensator, or damper, that normalizes the frequency dependent
decay of the light response in both rods and cones (figures 3.8 A3 and B3). Using Gaussian
white noise (GWN) light stimuli, we estimate the impulse responses of rods and cones at dif-
ferent mean luminances. We find that HCN channels reduce the amount of time needed for
rods to respond to an impulse of information in conditions of low mean luminance (figure
3.9 Al), but that this effect saturates at brighter light intensities (figure 3.9 A2). Conversely,
in cones, HCN channels reduce the time to respond to an impulse of information at brighter
light intensities (figure 3.9 B2), but have little effect at dim light intensities (figure 3.9 Bl).
Previous work has done little to clearly explain the functional advantage of these channels
other than describe the bandpass filtering effect Ih has on the rod network. Our analyses