Rods respond linearly to stimuli whose dynamic contrast is similar to that of natural scenes

Purpose: The reflectances of natural objects fall within a narrow range (Krinov, '54) suggesting that the range of contrasts in a natural scene is similarly limited, (Shapley et al. '83). Small incremental flashes superimposed on any background elicit linear current responses in rod photo receptors. We test the hypothesis that when presented with light that is dynamically modulated about a mean intensity and whose contrast varies within the range encountered in a natural scene, the rod photocurrent is a linear transformation of the stimulus. Methods: We systematically measured light intensities in a variety of natural environments using a photometer equipped with a narrow-field detector. We then exposed solitary rods, isolated mechanically from the tiger salamander retina, to dynamic stimuli whose contrast varied within the measured limits, while recording the rod's circulating current with a suction pipette, (Baylor et al. 79). The light source was a narrow-bandwidth green LED modulated either by a Gaussian white noise or by a random waveform whose power spectral density varied as I/o)2. Results: We calculated the contrasts, (C = 1(1 - ImeanXImaan)l). in the ten different natural environments we examined. Of 220 samples taken, 89% had a contrast, ICI, <0.5. Higher contrasts were usually associated with specular reflections of sunlight. When exposed to white-noise-modulated light of contrast 0 - 0.65 and a wide range of mean intensities, the rod current response consisted in small deviations of 1 - 4 pA about its mean. When exposed to either white-noise-modulated or 1/u2modulated light of a given mean intensity, the transfer function, (the ratio of the Fourier Transforms of recorded current and stimulus), was similar to the amplitude spectrum of linear-range responses elicited by brief flashes during exposure to a steady background of the same mean intensity. Conclusions: These results demonstrate that the dynamic contrasts likely to be encountered in a natural scene elicit linear responses in the retinal rods. These responses are no more than a few picoamps in amplitude suggesting that the retina must be functionally organized to transmit small cinnale reliably, despite the presence of intrinsic sources of noise.