Allowing all three parameters to vary, the
P3 model was fitted to the leading edge of the rod-isolated ERG a-wave recorded to a probe flash presented without a preceding test flash (probe only,
Fig. 1A ). The parameters
S and
t d were then fixed, and only parameter
R max
P3 was allowed to vary to fit the
P3 model to the rod-isolated ERG a-wave at a given ISI (
Fig. 1A , dashed lines). The derived
R max
P3 at each ISI (
R max
ISI) was normalized in relation to
R max
P3 derived from the isolated probe presented without a preceding test flash (
R max
IPR) and plotted against ISI
(Fig. 1B) . The normalized recovery of ERG a-wave amplitude was analyzed in terms of a double-exponential recovery function
\[R\ \mathrm{max}_{\mathrm{ISI}}/R\ \mathrm{max}_{\mathrm{IPR}}\ {=}\ d\ {\cdot}\ {\{}1\ {-}\ \mathrm{exp}\ {[}{-}(t\ {-}\ T_{\mathrm{c}})/{\tau}_{\mathrm{1}}{]}{\}}\ {+}\ (1\ {-}\ d)\ {\cdot}\ {\{}1\ {-}\ \mathrm{exp}{[}\ {-}\ (t\ {-}\ T_{\mathrm{c}})/{\tau}_{2}{]}{\}}\ 0\ {\leq}\ d\ {\leq}\ 1\]
where
R max
ISI is the derived maximum a-wave amplitude at ISI time (
t);
R max
IPR is the maximum a-wave amplitude from the isolated probe;
T c is the critical delay before the initiation of recovery; and τ
1 and τ
2 are the time constants of the two exponentials. The parameter
d is a weighting factor that scales the relative contribution of the two exponentials to the recovery function. In
Figure 1B , the solid curve shows the fit of
equation 2 to a representative set of normalized recovery data. A further parameter,
T 50, defined as the time taken from
T c to reach 50% of full recovery, was derived from the fit of
equation 2 (
Fig. 1B , inset).