Altered mapping of sound frequency to cochlear place in ears with endolymphatic hydrops provide insight into the pitch anomaly of diplacusis

Abstract

A fundamental property of mammalian hearing is the conversion of sound pressure into a frequency-specific place of maximum vibration along the cochlear length, thereby creating a tonotopic map. The tonotopic map makes possible systematic frequency tuning across auditory-nerve fibers, which enables the brain to use pitch to separate sounds from different environmental sources and process the speech and music that connects us to people and the world. Sometimes a tone has a different pitch in the left and right ears, a perceptual anomaly known as diplacusis. Diplacusis has been attributed to a change in the cochlear frequency-place map, but the hypothesized abnormal cochlear map has never been demonstrated. Here we assess cochlear frequency-place maps in guinea-pig ears with experimentally-induced endolymphatic hydrops, a hallmark of Ménière’s disease. Our findings are consistent with the hypothesis that diplacusis is due to an altered cochlear map. Map changes can lead to altered pitch, but the size of the pitch change is also affected by neural synchrony. Our data show that the cochlear frequency-place map is not fixed but can be altered by endolymphatic hydrops. Map changes should be considered in assessing hearing pathologies and treatments.

A Guinea Pig Model Suggests That Objective Assessment of Acoustic Hearing Preservation in Human Ears With Cochlear Implants Is Confounded by Shifts in the Spatial Origin of Acoustically Evoked Potential Measurements Along the Cochlear Length

“…ANOW thresholds in Figure 2B are consistent with those made previously showing that ANOW thresholds relate to low-characteristic frequency auditory nerve fiber firing thresholds as well as CAPs relate to high-characteristic frequency auditory nerve fiber firing threshold (Lichtenhan et al 2013). Moreover, the before-implantation audiometric configuration is similar to what we have reported previously (Lee et al 2020;Valenzuela et al 2020a, b;Guinan et al 2021;Lefler et al 2021).…”

“…CAP tone-bursts audiograms of naive (i.e., non-implanted) guinea pigs are most sensitive (i.e., had the lowest thresholds) at 8 to 11 kHz and are less sensitive elsewhere (Fig. 1B; Lee et al 2019, 2020; Valenzuela et al 2020a, b; Guinan et al 2021; Lefler et al 2021). While the threshold of audibility is essentially “U-shaped,” the initial cochlear implant insertion elevated 3 to 11 kHz thresholds and the audiograms became “W-shaped” and were indicative of dysfunction in mid-frequency tonotopic cochlear regions where the implant was in place (Fig.…”

Characterizing a Joint Reflection-Distortion OAE Profile in Humans With Endolymphatic Hydrops

“…Finally, our observation of shortened SFOAE delays (Fig. 7; 40 dB FPL) may appear to conflict with the notion—recently supported by measurements of the cochlear frequency map in guinea pigs with induced hydrops (Guinan et al 2021)—that a stiffened cochlear partition in hydropic ears will shift the peak of the traveling wave toward the apex. If SFOAE delays were determined primarily by the round-trip distance between the stapes and the peak of the traveling wave, then an apically shifted peak would produce longer delays.…”

Impact of endolymphatic hydrops on DPOAE in subjects with normal to mild hearing loss

“…In the EH ear, excessive fluid in the scala media can press on the basilar membrane thereby stiffening it and causing an altered frequency‐position map of the cochlea and the operating point of OHC transfer function. 13 , 14 , 28 , 29 , 30 , 31 In the apical turn of the cochlea, where compliance of the basilar membrane is highest, source deflection might occur, resulting in a phase change in DPOAEs. 32 It has previously been reported that unstable operating points likely affect the phase rather than the amplitude of DPOAEs.…”

“…In contrast, there were frequency ranges where the EH ear showed improved SNR and also significant DPOAE responses. In the EH ear, excessive fluid in the scala media can press on the basilar membrane thereby stiffening it and causing an altered frequency‐position map of the cochlea and the operating point of OHC transfer function 13,14,28–31 . In the apical turn of the cochlea, where compliance of the basilar membrane is highest, source deflection might occur, resulting in a phase change in DPOAEs 32 .…”