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light, see the optimal contribution of HCN channels in these conditions, whereas cones,
which are sensitive to brighter light, see the optimal effect of HCN channels at brighter light
intensities.
The Fourier transform of the impulse response gives the estimated frequency response
from the GWN stimulus. This is shown for the rod and cone where HCN channels are
effective-at dim and bright light intensities, respectively (figure 3.9 A3 and B3). These esti-
mates of the rod and cone light mediated frequency responses are similar to our estimates
using chirped light stimuli (figure 3.8 A3 and B3), with a frequency dependent decay at low
frequencies in both rods and cones with HCN block. The similarity in the frequency re-
sponses predicted from the GWN technique and those from the chirped light stimuli con-
firms that, photoreceptors have a mostly linear response component [79], and supports our
use of GWN estimated impulse responses to approximate rod and cone function. It also
shows how a the changes in the rod and cone impulse response functions with HCN block
correspond to the low-pass roll off characteristic seen with the frequency chirped light stim-
uli.
3.4 Discussion
3.4.1 Ih in salamander rods and cones is mediated by HCN1 channels
In this study, we systematically investigate the biophysical and physiological properties of
HCN channels in salamander rod and cone photoreceptors. We show that the f⅛ current in
rod and cone photoreceptors is alike with respect to the whole-cell current magnitude, acti-