14
background noise levels with a giga-seal patch were 400-600 fA RMS at 4 kHz bandwidth.
To measure the Ih IV curve, activation curve, and activation time constants, a voltage-
clamp protocol that pulsed cells from a holding potential to command potentials between
-134 mV and -24 mV and then to a tail potential of -4 mV was used. Leak currents were
subtracted with a standard P/4 protocol [91].
For chirped current stimulus recordings, stimuli were modulated by exponentially in-
creasing frequencies (equation 2.1) in order to increase the signal component in the low
frequency range, where the response of the cell is more interesting. Frequency range was
.5 to 20 Hz, and delivered over a period of 20 seconds. MATLAB (Mathworks) was used
to generate frequency-chirped sine wave stimuli (equation 2.1) and gaussian white noise
stimuli, which were then fed into Patchmaster. Frequency chirped stimuli were sampled at
500 Hz. Current amplitudes were approximately 50 pA with a 50 pA offset, but were deter-
mined at the time of experiment by the voltage response to steps of current input. Stimuli
were calibrated to have voltage responses no less than -100 mV in order to avoid dielectric
breakdown.
Istim ~ lamp ' COs(2τΓʃstart ' ∙ t) + !offset
k - ln(-^r-^-}∣tmax
(2.1)
∖ If / I / / t∕U'J-
Jstart
Recordings of whole cell currents from both rods and cones were made using the whole
mount retina. To control for the potential effect of Ih activation in adjacent cells Contribut-