Changes in the intensity-response function of an insect's photoreceptors due to light adaptation

Summary1.Intracellular recordings from the photoreceptors of the locustsLocusta migratoria andValanga irregularis and the flyCalliphora stygia, were used to examine the differences between theV/logI curve (intensity/response function) determined in the dark adapted and the light adapted state.2.For response amplitudes less than half the maximum the fully light adaptedV/logI curve follows the simple self-shunting model, where the number of channels activated is proportional to the number of photons absorbed.3.We fitted ourV/logI curves with the commonly employed hyperbolic function Eq. (1), and found a consistent deviation of the experimental data from the predicted curves when responses exceeded half maximal amplitude.4.The light adaptedV/logI curves of locusts exhibit a ‘kink’ deformation (Fig. 3) and 4 of the 80 cells recorded from locusts had a small spike superimposed on the rising phase of the photo-response (Fig. 1). No kink or spike was observed in the flyCalliphora.5.Transmembrane recording with a double electrode eliminated the ERG as a candidate for changing the slope and producing the kink or the spike. Stimulation of single ommatidia also ruled out interommatidial interactions.6.Change in pulse duration did not affect the slope or the shape of theV/logI curves.