Hearing: the next level of understanding

Abstract

Despite a long history of research involving some of the greater physicists the world has known (Ohm, von Helmholte, von Bekesy), the understanding of the mammalian, in particular human, hearing is still in its infancy. The present state of teaching of how we hear and listen is still centered around the Fourier analyzer partitioning incoming sounds into its frequency components, and that is it pretty much. We will, however see that the strong nonlinearities at work in the cochlea (providing a dynamical range of the signal up to 130 DB), demand this simple view to be dramatically changed. To show this, we follow a complex sound that enters the cochlea and consider what parts of the cochlea are elicited, and what turns out in the end to be responsible for human pitch perception. We do this on the basis of our biophysically detailed model of the cochlea [1-4] (based on Andronov-Hopf small signal amplifying outer hair cells [5]) that has been shown to reproduce all - even the most intricate - measured features of mammalian hearing (eg loudness dependence of pitch, pitch-shift effects, phase properties along the cochlea and much more [4, 6-7]), the measurements originating from laser-interferometry as well as results psychoacoustic roasts (for the latter, it is essential that the signal remains essential unaltered during the signal transduction from the continuous biophysics motions of the basilar membrane to the spike patterns at the upper end of the auditory nerve (to achieve this, biology exploits the potential of stochastic resonance [8]) . We look at this neuronal system (outer hair cells can be regarded as an archetype of such cells) from the angle of criticality, a viewpoint that is presently widely taken by neuroscientists with a physics background. A critical state of a (neuronal) network is characterized by power-law statistics, as the fingerprint of existing long-range correlation within the system at this state. We find, indeed, power-law distributions of links leading from already activated sites to consecutively activated sites within the cochlea, following the nonlinear interaction paradigm of combination tone generation.