Electrical responses of single cones in the retina of the turtle.

1. Intracellular recordings have been made from single photoreceptors in the retina of the turtle. Histological sections of the retina made after injection of dye through the recording electrode reveal dye in the inner segments of single cones.2. Following a brief flash of light the cone undergoes a hyperpolarization which is graded with the intensity of the flash.3. The excitatory receptive field of a receptor is probably as small as the cross-section of a single cone, but accurate measurements are rendered difficult by scattering of light within the retina.4. The voltage drop produced by a current injected into the cell is increased during the response to light. Steady hyperpolarizing currents increase the size of the response to light; depolarizing currents of increasing strength reduce and then reverse the response.5. The results are consistent with the hypothesis that light activates the visual cell by decreasing the permeability of membrane channels which in darkness act as a shunt of the membrane.