Changes in the dynamic properties of locust photoreceptors at three levels of light adaptation

The frequency response function of phototransduction was studied in the locust compound eye at three levels of dim light adaptation. The eyes were stimulated with light from a green light emitting diode (LED) and the resulting changes in membrane potential were measured with intracellular electrodes. Absolute sensitivities and light adaptation levels were established by counting the arrivals of single photons in dark adapted eyes. Frequency response functions for phototransduction could be well fitted by a model developed earlier for fly compound eyes, which includes underdamped second order poles and a pure time delay. However, the locust data requires longer time constants for the poles and a longer delay than were used to fit the fly data, reflecting the slower response characteristics of the locust. Changing the level of light adaptation at dim levels caused changes in both the sensitivity and the dynamic properties of the photoreceptors. Sensitivity was reduced and significant decreases were seen in the delay and several time constants. The behavior also became much more damped, with one of the second order poles decomposing into first order poles.