94
5.4.5 Network input impedance
2D input impedance
The input impedance measured from a point in the two-dimensional network Rιn2D is given
by the steady-state voltage response of a cell divided by the value of the current step injected
into it. Solving the discretized equation 5.14 numerically for the voltage response of a unit
point source of current injected into that cell gives the input resistance wo,o = Rin2D∙
The analytical form is [69] :
2 ( 7 ʌ Γ 4
(5.15)
Rin2D ~ ¾0 ^^ Rm I 7 I ɪɔ I T
π ∖y + 4/ I Lt + 4
where D is the elliptic integral of the first kind [69]. For this work, the numerical solution
for υss was used to calculate the input impedance.
ID input impedance
The one-dimensional input impedance is given by:
RiniD =V0 = ——§ (5.16)
It is important to note that the one-dimensional input impedance RmiD is different from
the two-dimensional input impedance Rin2D∙ The one-dimensional impedance is not as
useful for a two-dimensional network because it would be very hard to measure experimen-
tally. It would be the input impedance to a current injection into a column of photoreceptors
§We derive this relationship in appendix A.5 ,
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