Correlation Hypothesis to Explain the Fine Frequency Discrimination of the Ear

Abstract

Earlier suggestions to reconcile the observed broad frequency-displacement function of the basilar membrane and the subjective high discrimination of the hearing mechanism seek an explanation in possible neural interaction and mutual suppression of less strongly stimulated neurons in the vicinity of more strongly stimulated neighbors. This explanation leaves questions on the hearing of combinations of tones in doubt. A mechanism is postulated based on the traveling-wave nature of the disturbance of the membrane. In the vicinity of the maximum of the displacement, the wavevelocity of the disturbance on the membrane is a sharp function of frequency. Many hair cells are assumed to be excited in phase with the local displacement, and signals passing from a receptor closer to the stapes are delayed in time with respect to the signal from the more apical receptors. Delays due to neuron pathlength are assumed to be an exact replica of the time delay of the signal along the membrane, for a specific frequency. Different delay-line characteristics in a given region of the membrane would, therefore, produce simultaneity of pulses at separate synapses for slightly different frequencies. Plausibility of this hypothesis is greatly enhanced by the observation that the phase velocity of the traveling wave on the basilar membrane at the point of maximum displacement is very close to the velocity of signals along unmyelinated nerve fibers, so that the latter could act in exactly the manner postulated. [This work was supported by the Electromagnetic Warfare and Communications Laboratory, Aeronautical Systems Division.]