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

Summary

• 1. 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.