Spontaneous, subthreshold fluctuations of membrane potential are recorded in the eccentric cell body or dendrite of the dark-adapted Limulus ommatidium. These slow potential fluctuations (SPF's) are random in amplitude and in time of occurrence. The relation between average frequency of SPF's and light intensity is linear for low light intensities and becomes a non-linear saturation for higher intensities. The occurrences of SPF's have Poisson statistics in the dark but are non-Poisson with light stimuli. Light-adapting the ommatidium greatly decreases the SPF amplitude, and it increases the average frequency of SPF's in the dark and in response to light (facilitatory action). The shape (time course of response) of the SPF does not change at different light intensities and it is the result of a concurrent and conterminous change in membrane resistance. The functional properties of the SPF's are analyzed in terms of a stochastic model based on the summation of random events in time ("shot effect").

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