The linear dynamic properties of phototransduction in the fly compound eye.

1. Photoreceptor cells in the compound eye of the fly Phormia regina were stimulated with a green light emitting diode whose output was modulated by a pseudo-random noise generator while the resulting fluctuations in membrane potential were measured with intracellular microelectrodes. 2. The responses to repeated sequences of identical pseudorandom patterns were averaged together to give frequency response functions having a significantly improved signal to noise ratio at frequencies where the response has not previously been characterized accurately. 3. Measured frequency response functions could not be fitted by a set of simple exponential time constants but could be well fitted by a model including two under-damped second order time constants. 4. The phase data had no detectable asymptote and could only be accounted for by a model which includes a pure time delay. 5. Residual fluctuations in the phase data with frequency suggest that a small portion of the recorded signal arises from a delayed interaction with lateral or proximal cells.