The a-wave of the human dark-adapted ERG is thought
to derive from activity of rod photoreceptors. However, other
sources within the retina could potentially perturb this simple
equation. We investigated the extent to which the short-latency
dark-adapted rod a-wave of the primate ERG is dominated
by the rod photoresponse and the applicability of the
phototransduction model to fit the rod a-wave.
Dark-adapted Ganzfeld ERGs were elicited over a 5-log-unit
intensity range using short bright xenon flashes, and the
light-adapted cone responses were subtracted to isolate the
rod ERG a-wave. Intravitreal 4-phosphono-butyric acid
(APB) and cis-2,3-piperidine-dicarboxylic acid (PDA) were applied
to isolate the photoreceptor response. The Hood and Birch version
of the phototransduction model, Rmax[1 −
e−I·S·(t−teff)2] , was fitted
to the a-wave data while allowing Rmax and
S to vary. Three principle observations were made: (1) At
flash intensities ≥0.77 log sc-td-s the leading edge of the normalized
rod ERG a-wave tracks the isolated photoreceptor response across
the first 20 ms or up to the point of b-wave intrusion.
The rod ERG a-wave was essentially identical to the isolated
receptor response for all intensities that produce peak responses within
14 ms after the flash. (2) The best fit of sensitivity (S) was
not affected by APB and/or PDA, suggesting that the inner retina contributes
very little to the dark-adapted a-wave. (3) APB always
reduced the maximum dark-adapted a-wave amplitude (by
15–30%), and PDA always increased it (by 7–15%).
Using the phototransduction model, both events can be interpreted
as a scaling of the photoreceptor dark current. This suggests
that activity of postreceptor cells somehow influences the rod
dark current, possibly by feedback through horizontal cells
(although currently not demonstrated for the rod system), or
by altering the ionic concentrations near the photoreceptors,
or by neuromodulator effects mediated by dopamine or melatonin.
