1. Visual transduction in photoreceptors of the ground squirrel, Citellus lateralis, was studied by recording membrane current from individual cones in small pieces of retina. 2. Brief flashes of light produced transient reductions of the dark current; saturating response amplitudes were up to 67 pA. A flash strength of about 11,000 photons microns-2 at lambda max was required to give a half-saturating response. The stimulus-response relation was well fitted by an exponential saturation curve. Responses below 20% of maximum behaved linearly. 3. The response to a dim flash in most cells had a time to peak of 20-30 ms and resembled the impulse response of a series of five low-pass filters. 4. The variance of the dim-flash response amplitude put an upper limit of 80 fA on the size of the single photon response. Estimates based on the effective collecting area suggest the single photon response to be of the order of 10 fA. 5. Flash responses of squirrel cones usually lacked the undershoot observed in primate cones, although in about 1/3 of the cells a small undershoot developed during recording. 6. Background lights slightly shortened the time to peak of the flash response and reduced the integration time. 7. Spectral sensitivity measurements showed two classes of cones with peak sensitivities at about 520 and 435 nm. Rod sensitivity peaked near 500 nm. Spectral univariance was obeyed by all three classes of cells. 8. The shapes of the spectral sensitivity curves of the rod and both types of cones were similar to each other when plotted on a log wave number scale, but differed significantly from similar plots of monkey and human cone spectra. 9. The kinetics and sensitivity of flash responses of the blue- and green-sensitive cones were indistinguishable.
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