Dolphins’ echolocation strategy of target identification: Is it determined by the peripheral auditory encoding?

Abstract

It is well known that the auditory system of dolphins is capable of analyzing the fine frequency—time structure of short high-frequency echo signals. This can be explained by the burst principle, according to which the auditory peripheral encoding of signals is determined by the collective effect of a multitude of synchronously excited auditory nerve fibers. Encoding is efficient when the sensitivity of the majority of the fibers corresponds (is close) to the level of the synaptic potential evoked by the stimulus. Under the effect of long signals, such a correspondence is achieved as a result of auditory adaptation, which adjusts the sensitivity of the fibers to the synaptic potential level. However, dolphins identify a target by analyzing a single short echo signal, but, in the course of echolocation, they vary the intensity of the probing pulse over wide limits. In this paper, a model of the high-frequency auditory periphery is used to test the hypothesis that the intensity variation is an adaptive mechanism serving for the adjustment of the intensity of echo signals to the fiber sensitivity, which is formed under the effect of ambient noise. It is found that a necessary condition for the realization of the burst principle is the presence of the differentiating properties inherent in the response of the multitude of auditory nerve fibers. The identification of short signals is interpreted in terms of a model that includes a model of the auditory periphery and units serving for the minimization of signal description and for making decisions. A model algorithm of small-target identification by echolocation, which realizes the object identification strategy used by dolphins, is discussed.