The name is absent

95

in the network. This could be assessed with the bar light stimulus by dividing the voltage
response by the equivalent photocurrent generated, but would require making several as-
sumptions.

A plot of the network impedance vs 7 = R_c∣R_m for a ID and 2D network is shown
in figure 5.7 B. As the R_c∣R_m grows, the network becomes essentially uncoupled, and the
input impedance approaches R_m. The impedance in the one-dimensional network is greater
because there are fewer paths for the current to take.

Figure 5.7 A shows the normalized length constant D∕λ as a function of R_c∣R_m. The
length constant decreases as R_c∣R_m increases because it becomes increasingly difficult for
current to flow laterally. For the ID steady-state solution, the length constant describes an
exponential decay as in equation 5.4. Although the 2D steady-state solution (equation 5.7
involves a Bessel function, the discrete solution is well approximated by an exponential decay
with length constant λ2D> shown in figure 5.5. Figure 5.4 C shows the relationship between
length constant and cell input impedance. This is a useful relationship because the length
constant can be measured experimentally, whereas 7 cannot.

5.4.6 Network impulse response and frequency response

Figure 5.8 B shows the impulse response (Green’s function) for a 2D network with R_m =
- 0.7 GΩ, R_c = 0.9 GΩ, and C = 50 ■ IO^-3 nF. The corresponding frequency responses of
the indicated cells are shown in figure 5.8 C. The cutoff frequency in the Origninating rod
(0,0) is around 1 Hz. The cutroff frequency in more distant rods decreases further from the

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