Stochastic constancy, variability and adaptation of spike generation: Performance of a giant neuron in the visual system of the fly

The stochastic structure of the spike activity generated by a movement processing wide-field element in the visual system of the fly has been studied over the whole performance area of the neuron. The structure of this discharge is described in terms of an Adaptive Integrate-to-Threshold model for a wide variety of spatio-temporal stimuli as well as steady-state stimuli. In order to reproduce the experimental results it is shown that the source of randomness in the model (e.g. the threshold) behaves like a random variable which is distributed according to a two-state Markov renewal process. In the case of stationary discharges generated by moving sinewave patterns the shape of the interspike interval distribution (which, in the Integrate-to-Threshold model, reflects the shape of the threshold distribution) changes continuously from a two-state distribution at low firing rates to a one-state distribution at high firing rates. In dynamic conditions of the discharge, generated by temporal dynamic stimuli, the experimental results show that the shape of the (demodulated) interval distribution of the discharge is determined by the highest instantaneous firing rate with an adaptation time constant of a few seconds. The physioligical origin of this intriguing behaviour remains — up till now — out of the picture.