Abstract
While separating sounds into frequency components and subsequently converting them into patterns of neural firing, the mammalian cochlea processes signal components in ways that depend strongly on frequency. Indeed, both the temporal structure of the response to transient stimuli and the sharpness of frequency tuning differ dramatically between the apical and basal (i.e., the low- and high-frequency) regions of the cochlea. Although the mechanisms that give rise to these pronounced differences remain incompletely understood, they are generally attributed to tonotopic variations in the constituent hair cells or cytoarchitecture of the organ of Corti. As counterpoint to this view, we present a general acoustic treatment of the horn-like geometry of the cochlea, accompanied by a simple 3-D model to elucidate the theoretical predictions. We show that the main apical/basal functional differences can be accounted for by the known spatial gradients of cochlear dimensions, without the need to invoke mechanical specializations of the sensory tissue. Furthermore, our analysis demonstrates that through its functional resemblance to an ear horn (aka ear trumpet), the geometry of the cochlear duct manifests tapering symmetry, a felicitous design principle that may have evolved not only to aid the analysis of natural sounds but to enhance the sensitivity of hearing.
Highlights
While separating sounds into frequency components and subsequently converting them into patterns of neural firing, the mammalian cochlea processes signal components in ways that depend strongly on frequency
Mirroring the variation in frequency tuning, response latencies—whether assessed using the wave-front delay of the traveling wave, the mechanical group delay at the wave peak, or the acoustic delay of sounds evoked from the ear—decrease from base to apex when measured in periods of the local CF5–7
We demonstrate that the shape of the cochlear duct, in its apt resemblance to an ear horn, enables traveling pressure waves to propagate to their best places with negligible geometric attenuation
Summary
While separating sounds into frequency components and subsequently converting them into patterns of neural firing, the mammalian cochlea processes signal components in ways that depend strongly on frequency. (Because the wavenumber κ is essentially real in this region, the complex exponential term contributes only a phase shift but neither power gains nor losses.) the prefactor is not exactly constant in the apex (or in the base of the chinchilla), so that the driving pressure decreases with position, the attenuation space constant (∼ 8l ) is larger than the length of the cochlea.
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