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 micrometer, 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, irrespective of whether the flash response precedes or follows the current response. These results indicate that the sub-linear summation is caused by the activation of a voltage-activated shunting conductance. This conductance is not blocked by low intracellular concentrations of tetraethyl-ammonium chloride, and is therefore different from the conductance which causes the decrease of the receptor's step response from transient to steady-state.