Intracellular membrane potential responses were recorded from locust photoreceptors under two stimulus conditions: pairs of flashes to dark-adapted receptors, and white-noise modulated light at a range of background intensities from 500 to 15,000 effective photons per second. Nonlinear analysis of the input-output relationships were performed by estimating the Volterra and Wiener kernels of the system. The Volterra kernels obtained from the double-flash experiments were similar to the Wiener kernels obtained from the white-noise experiments, except for a change of time scale. The structure of the second-order kernels obtained with either method gave evidence for a gain control mechanism acting at an early stage of the cascade. Both feedforward and feedback nonlinearities could account for the observed system behavior at any one background level. The differences in amplitude between the kernels obtained at different background levels could be accounted for by an adaptation process which further decreased the gain of the system, acting on a slower time scale, also at some early stage of the cascade.

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Summary. The response of a locust, Locusta miyratoria,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 ~tm, 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 sublinear summation is caused by the activation of a voltageactivated 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.

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