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In the transmission of information in the nervous system, all messages are conveyed in a code which is a sort of frequency modulation system of signaling, in which intensity of the nervous sensation is translated into frequency of the nerve impulses. Nervous sensation set up in a receptor organ and transmitted through a nervous system are usually related to a function of the stimulus intensity. But the output signals of some biological transducers are not always a simple function of the input one. In the eye of the cat (FitzHugh, 1957) and the infra-red receptor of the rattlesnake (Bullock & Diecke, 1956; Bullock, 1957) the nervous sensation was accompanied with a fluctuation of impulse frequency, i.e. a noise. In the central nervous system of the crayfish it was difficult to reproduce a particular response to the stimulation of the same region in a given preparation because previously men-tioned fluctuation (Hughes & Wiersma, 1960). In order to know the true response under a given condition, it is convenient to represent the relationship between the input and output signal of the biological transducer by the transfer function (Pringle & Wilson, 1952), which is determine