Non-Linearity and light adaptation in the fly photoreceptor

Summary

• 1. Short, dim flashes were used to investigate the properties of the photoreceptor response in the compound eye of the flyMusca domestica.

• 2. In a dark-adapted receptor cell, the latency of the response and the response peak amplitude were determined. The response amplitude increases linearly up to an intensity where 10 photons are absorbed in one cell. Above this intensity the gain of the transduction decreases.

• 3. However, even below this flash intensity the response as a whole is not linear because a significant reduction of the time-to-peak is observed in responses produced by the absorption of only four photons, and possibly even less.

• 4. These results show that a very dim adapting light also affects the time course of the response to a flash. This has been verified experimentally. Some previous authors did not detect any changes in latency through light adaptation. This anomaly is explained because these authors' lowest adapting light was bright enough to light-adapt the cell nearly completely with respect to the time-to-peak.

• 5. The implications of the ‘prebump’ hypothesis (Hamdorf and Kirschfeld 1980) are discussed and the possibility of a common mechanism responsible for the non-linearity in the dark-adapted cell and the light adaptation effects is suggested.