Noise sources in background-adapted rod photocurrent

Purpose: It has been known for some time that at certain adapting background intensities, the major source of noise seen in the rod photocurrent is caused by the random superposition of single photon responses(Baylor'79). Here we identify dominant noise sources in the rod photocurrent, over a wide range of adapting background intensities, and investigate to what extent the above linear relation holds true. Methods: We recorded photocurrents under dark and light-adapted conditions in isolated rods of tiger salamander using the suction electrode technique(Baylor'79). Power spectra estimation and matched filtering were used to estimate spectral components of noise and event frequency. Results: Dark-adapted photocurrents contained bumps, whose kinetics were similar to single photon responses, occurring at a rate of about 1 event every 57sec at 21°C; because stray light levels are significantly lower than this rate, we conclude that this noise is caused by thermally activated Rh*. Light-adapted photocurrents exhibited marked fluctuations about their respective mean amplitudes. Data show that for a range of backgrounds below 10 Rh*/sec, 30 secs after stimulus onset, the mean amplitude for each light-adapted intensity and the variance of fluctuations about this mean remained constant for adaptation periods as long as 400sec. The power spectra of adapted photocurrents could be fitted by power spectra of single photon responses which scale with background intensity. Conclusions: Our data indicate that during adapted states (I_b<10Rh*/sec), the mean photocurrent is stationary and the dominant noise source is photon noise; this is consistent with previous reports in toad. Analysis of the data allows us to evaluate how noise constrains the rod's ability to signal photons reliably and how adaptation mechanisms (manifested in the rod's decreased flash sensitivity and quickened response kinetics) contribute to overcoming these noise constraints.