A Simplified Model of Activation and Deactivation of Human Rod Phototransduction—An Electroretinographic Study

PURPOSE: To test the hypothesis that a simple model having properties consistent with activation and deactivation in the rod approximates the whole time course of the photoresponse. METHODS: Routinely, an exponential of the form f = α·(1 – exp(–(τ·(t – t(eff))(s)(–1)))), with amplitude α, rate constant τ (often scaled by intensity), irreducible delay t(eff), and time exponent s–1, is fit to the early period of the flash electroretinogram. Notably, s (an integer) represents the three integrating stages in the rod amplification cascade (rhodopsin isomerization, transducin activation, and cGMP hydrolysis). The time course of the photoresponse to a 0.17 cd·s·m(−2) conditioning flash (CF) was determined in 21 healthy eyes by presenting the CF plus a bright probe flash (PF) in tandem, separated by interstimulus intervals (ISIs) of 0.01 to 1.4 seconds, and calculating the proportion of the PF a-wave suppressed by the CF at each ISI. To test if similar kinetics describe deactivation, difference of exponential (DoE) functions with common α and t(eff) parameters, respective rate constants for the initiation (I) and quenching (Q) phases of the response, and specified values of s (s(I), s(Q)), were compared to the photoresponse time course. RESULTS: As hypothesized, the optimal values of s(I) and s(Q) were 3 and 2, respectively. Mean ± SD α was 0.80 ± 0.066, I was 7700 ± 2400 m(2)·cd(–1)·s(–3), and Q was 1.4 ± 0.47 s(–1). Overall, r(2) was 0.93. CONCLUSIONS: A method, including a DoE model with just three free parameters (α, I, Q), that robustly captures the magnitude and time-constants of the complete rod response, was produced. Only two steps integrate to quench the rod photoresponse.