Nonlinearities in locust photoreceptors during transduction of small numbers of photons

1. Photoreceptors in the compound eye of the locust Locusta migratoria, were stimulated with a range of dim light flashes containing only small numbers of effective photons. Flash intensities were calibrated by counting single photon arrivals in each cell. 2. Nonlinear growth of the voltage response and changes in its time course were seen when two or more photons were transduced together. 3. The time courses of the nonlinear interactions were investigated by stimulating photoreceptors with pairs of flashes having variable temporal separation. Each flash contained an average of one effective photon. 4. Two types of nonlinearity were observed. A small facilitation in the leading edge of the response could only be seen with flash separations of a few milliseconds or less. A stronger depression of the peak response was maximal at 6 ms flash separation but persisted with flash separations of more than 100 ms. 5. No evidence could be found for nonlinear interactions between flashes of light and simulated flash responses produced by electric current injections. 6. The results are discussed with reference to models of phototransduction and the spatial spread of excitation in photoreceptors.