1. We examined signal, noise and response properties of salamander rod photoreceptors by measuring: (a) the circulating current of rods which were adapted to darkness and to a wide range of backgrounds; (b) contrasts of natural environments; (c) the effect of adaptation on the linear response range of rods; and (d) the behaviour of rods responding to dynamically modulated stimuli having a range of contrasts found in nature. 2. In the dark, the circulating current contained two noise components analogous to those described in toad. A discrete noise component consisted of events occurring at a rate of 1 event per 32 s (21°C) and had a variance of 0.036 pA2. A continuous noise component contributed 0.022 pA2 to the dark current, roughly equal to the discrete noise variance. 3. Exposure to a wide range of steady backgrounds (suppressing up to 80% of the circulating current), elicited a sustained fluctuating photocurrent having a power spectrum which resembled those of single photon responses and was consistent with the linear summation of single photon events; this indicates that the primary source of noise in the current is caused by the light. 4. Eighty-nine per cent of the contrasts (C) measured in natural environments had |C| < 50%, where C = |I-I(mean)|/|I(mean)|. The linear response range elicited by brief flashes expanded with brighter backgrounds, well-encompassing flash contrasts of 100%. 5. Dynamically modulated stimuli and incremental flashes having contrasts similar to those in natural scenes elicited small currents which deviated by a few picoamps about the mean and the transfer functions computed from each type of stimulus-response pair closely corresponded to one another. These results indicate that in natural environments, rods behave as linear small-signal transducers of light.
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