Cones excite rods in the retina of the turtle.

The intracellular responses of rods in the retina of the turtle, Chelydra serpentina, were studied with brief flashes of monochromatic light. 1. Flashes of red or green light applied over an area 25 mum in diameter produce responses with the same shape. With such restricted stimuli, the spectral sensitivity of a rod agrees well with the absorption spectrum of the porphyropsin pigment contained in its outer segment. 2. With stimulating spots more than 500 mum in diameter, dim flashes of red or green light produce responses having different shapes. When the spectral sensitivity of a rod is tested using dim lights of large diameter, the sensitivity to red light is much greater than predicted by the absorption spectrum of porphyropsin. 3. The shape of the response produced by large diameter spots of dim, red light resembles that of cones. 4. Increasing the diameter of a dim, red spot beyond 500 mum markedly alters the amplitude and shape of responses from horizontal cells but does not significantly affect the response of rods. It is concluded that rods receive an excitation from neighbouring cones. This interaction is unlikely to be mediated by type I luminosity horizontal cells but may be mediated by either direct connexions between cones and rods or by an interneurone with a small receptive field.