A cochlea with three parts? Evidence from otoacoustic emission phase in humans

Abstract

The apical and basal regions of the cochlea appear functionally distinct. In humans, compelling evidence for an apical-basal transition derives from the phase of otoacoustic emissions (OAEs), whose frequency dependence differs at low and high frequencies. Although OAEs arising from the two major source mechanisms (distortion and reflection) both support the existence of an apical-basal transition—as identified via a prominent bend (or “break”) in OAE phase slope—the two OAE types disagree about its precise location along the cochlea. Whereas distortion OAEs at frequency 2f1−f2 suggest that the apical-basal transition occurs near the 2.5 kHz place, reflection OAEs locate the transition closer to 1 kHz. To address this discrepancy, distortion and reflection OAEs were measured and analyzed in 20 young human adults from 0.25–8 kHz and at eight primary-frequency ratios f2/f1 in the range 1–1.5. Break frequencies and OAE phase-gradient delays were estimated by fitting segmented linear models to the unwrapped phase. When distortion- and reflection-OAE phase are considered as functions of ln f2—that is, as linear functions of the location of their putative site of generation within the cochlea—the analysis identifies not just two but three main cochlear segments, meeting at transition frequencies of approximately 0.9 and 2.6 kHz, whose locations are largely independent both of primary-frequency ratio and emission type. A simple model incorporating an abrupt transition from wave- to place-fixed behavior near the middle of the cochlea accounts for key features of distortion-OAE phase.

A cause of encephalopathy not to be missed

“…His auditory hallucinations of hearing music is an eloquent manifestation of primary auditory cortex involvement in the posterior superior temporal lobe (Heschl’s gyrus), which was particularly affected on imaging. Note, however, that hearing loss in MELAS is caused by cochlear dysfunction and typically affects high frequencies due to the relatively high energy requirements of the basal cochlea, which is known to transmit higher frequency sounds 4…”

Swept Along: Measuring Otoacoustic Emissions Using Continuously Varying Stimuli

“…Indeed, by helping to interpolate between clicks and tones, swept-tone stimuli facilitate the study of both physiological and behavioral responses to dynamic sounds. As an example of the unexpected insights obtained, swept-tone measurements of reflection- and distortion-OAE phase spanning 5 octaves in frequency and a wide range of primary frequency ratios ( ) suggest that the traditional division of the human cochlea into two broad regions—apical and basal—may be incomplete [ 34 ].…”