Extrinsic current and flash sensitivity in turtle cones

The effects of hyperpolarizing current and background light on intracellular responses of red cones in turtle were compared. Even though a background light always reduced response amplitude, hyperpolarizing current did so in only 25% of the cells studied. When hyperpolarizing current reduced response amplitude it also produced changes in response kinetics and the intensity-response relationships, but these changes differed from those produced by background light. Considerably greater hyperpolarization was required with current than with light to produce equivalent reductions in amplitude. The results suggest that current reduces amplitude by activating a membrane conductance, while background light acts through a different mechanism.