Sublinear summation of responses in locust photoreceptors

The response of a locust, Locusta migratoria, photoreceptor to a simultaneous pair of dim flashes is smaller than the sum of the responses to the individual component flashes, even when the photon absorption sites are separated by a distance of 120 μm, which should prevent them from interacting biochemically. A similar depression is observed when a flash is paired with a depolarizing current instead of a second flash, irrespecive of whether the flash response precedes or follows the current response. These results indicate that the sublinear summation is caused by the activation of a voltage-activated shunting conductance. This conductance is not blocked by low intracellular concentrations of tetraethylammonium chloride, and is therefore different from the conductance which causes the decrease of the receptor's step response from transient to steady-state.