Single photon responses in locust photoreceptors: the effects of stimulus location on amplitude and time course

1. We recorded intracellular membrane potential responses from a superfused, sliced preparation of the locust (Locusta migratoria) eye which allowed localized stimulation of the photoreceptors. 2. The average amplitude of bumps was affected very weakly by stimulus location, in a way which could be accounted for by cable attenuation between the stimulating and recording sites. 3. The time to peak of bumps was found to decrease in an approximately linear fashion as the stimulus site was moved distally. 4. The relative width of bumps (a parameter proportional to the ratio between width and time to peak) was not significantly affected by stimulus location. 5. In all cases, regional variations only made minor contributions to the total variability of bump parameters. The major sources of variability were intrinsic and were not eliminated by restricting the stimulus to a small region of the photoreceptor. 6. Within each stimulus site, the correlations between bump parameters suggested that a single photon absorption initiates independent generating and terminating processes for each bump.