Modelling the initial phase of the human rod photoreceptor response to the onset of steady illumination

The initial time course of the change in photoreceptor outer segment membrane conductance in response to light flashes has been modelled using biochemical analysis of phototransduction, and the model has been successfully applied to a range of in vitro recordings and has also been shown to provide a good fit to the leading edge of the electroretinogram a-wave recorded in vivo. We investigated whether a simple modification of the model's equation would predict responses to the onset of steady illumination and tested this against electroretinogram recordings. Scotopic electroretinograms were recorded from three normal human subjects, using conductive fibre electrodes, in response to light flashes (0.30-740 scotopic cd m ^-2 s) and to the onset of steady light (11-1,900 scotopic cd m ^-2). Subjects' pupils were dilated pharmacologically. The standard form of the model was applied to flash responses, as in previous studies, to obtain values for the three parameters: maximal response amplitude r _max, sensitivity S and effective delay time t _eff. A new "step response" equation was derived, and this equation provided a good fit to rod responses to steps of light using the same parameter values as for the flash responses. The results support the applicability of the model to the leading edge of electroretinogram responses: in each subject, the model could be used to fit responses both to flashes of light and to the onset of backgrounds with a single set of parameter values.