Kybernetik 14, 117--126 (1973) 9 by Springer-Verlag 1973 Analog Modelling of Cochlear Adaptation J. J. Eggermont Academisch Ziekenhuis, E.N.T.-Department, Leiden, Netherlands .Received: June 1, 1973 Abstract Adaptation phenomena in the peripheral hearing organ ori- ginate from the properties of the haircell-neuron synaps as revealed by their temperature dependency. In order to verify this hypothesis a model experiment is set up. The model consists of programming on an analog computer the equations describing the reaction kinetics from the frogs myoneural junction. A model neuron is attached to the synapsmodel in which two independent noise sources are incorporated. This noise addition serves to simulate the siochastic behavior of the transmitter release in the synaps resulting in a fluctuating generator potential and independ- ently thereof to reflect the varying threshold of the nerve fiber. The reaction rate constants in the synaptic model were modified with respect to the original ones in order to get a coincidence of in vivo- and model results. The compound model primarily is used to simulate the quite different synchronization between the auditory nerve fibers occurring during intensity changes and during changes of the stimulus repetition rate. These results were known to be different in the animal experiments and were also generated by the model. It is shown clearly that neither synaptic noise alone nor membrane noise alone can account for the observations made in the animal experiments. Therefore a combination of both types of noise is used in this model. The model experiment also visualizes that amplitude changes for compound AP's during adap- tation can be explained in toto by a decreasing synchronization, i.e. the broadening of the latency distribution function for the nerve fibers. 1. Introduction Adaptation processes in the peripheral hearing organ result, when observing single nerve fibers: 1) in a decrease of the firing rate with time after the onset of a long toneburst or 2) in a decrease in the number of time-locked spikes per click with in- creasing click rate (Kiang et al., 1965). When re- cording gross nerve potentials from the round window adaptation is observed 1) as a decrease of the N 1- amplitude with increasing period number after the onset of a long low-frequency (below 2000 Hz) tone- burst or 2) as a decrease of the Nl-amplitude in. response to clicks or short high-frequency-tone-bursts with increasing repetition rate (Eggermont and Spoor, 1973). When observing cochlear microphonics which are generated at the hair bearing surface of the hair cells in the cochlea one does not find an amplitude dependency on the repetition rate of tonebursts or on the time since the onset of the long toneburst. While some authors believe that the CM is just an epiphenomenon to the cochlear excitation process (Grundfest, 1965) most investigators state that it is an intermediary in the peripheral hearing processes (Davis, 1965). Therefore one may conclude that the adaptation mechanisms must be located central to the site of CM generation (see also Coats, 1971). By observing the temperature dependency of adaptation phenomena in the cochlea it was deduced that the adaptation mechanism could not be located in the axon part of the first auditory neurones (Eggermont, 1973). It was stated that the haircell- neuron synaps is the most appropriate place for the location of the adaptation mechanism. This in fact results also from the by Stange et al. (1964) derived dependency of adaptation on metabolic processes occuring in the haircells. The study of adaptation phenomena thus gives information about the transfer function of this synaps. The responses to long tonebursts give the step response of the system, while the dependency of repetition rate gives evidence about the frequency response thereof. The role of adaptation in the in- formation processing of the auditory system is discussed earlier by Keidel et al. (1961) and Keidel (1965). It is not our aim to compute the synaptic transfer- function but to test if known synaptic mechanisms as investigated in myoneural junctions (Stevens, 1968) are capable of generating transferfunctions as provided by our experiments (Eggermont and Spoor, 1973). It was shown by Eggermont and Spoor (1973) that the adaptation phenomena observed in round window recordings of the AP represent weighted