Maturation and Aging of the Human Cochlea: A View through the DPOAE Looking Glass

Abstract

Cochlear function changes throughout the human lifespan. Distortion product otoacoustic emissions (DPOAEs) were recorded in 156 ears to examine these changes and speculate as to their mechanistic underpinnings. DPOAEs were analyzed within the context of current OAE generation theory, which recognizes distinct emission mechanisms. Seven age groups including premature newborns through senescent adults were tested with a swept-tone DPOAE protocol to examine magnitude and phase features of both the mixed DPOAE and individual distortion and reflection components. Results indicate (1) 6-8-month-old infants have the most robust DPOAE and component levels for frequencies >1.5 kHz; (2) older adults show a substantial reduction in DPOAE and distortion-component levels combined with a smaller drop in reflection-component levels; (3) all age groups manifest a violation of distortion phase invariance at frequencies below 1.5 kHz consistent with a secular break in cochlear scaling; the apical phase delay is markedly longer in newborns; and (4) phase slope of reflection emissions is most shallow in the older adults. Combined findings suggest that basilar membrane motion in the apical half of the cochlea is immature at birth and that the cochlea of senescent adults shows reduced nonlinearity and relatively shallow reflection-component phase slope, which can be interpreted to suggest degraded tuning.

Distortion-product otoacoustic emission reflection-component delays and cochlear tuning: Estimates from across the human lifespan

“…(4) To varying degrees, all age groups display a bend in the N R slope located around 1 kHz. This bend is consistent with previous reports of a transition between apical-and basal-like OAE behavior at this frequency in humans (Shera and Guinan, 2003;Shera et al, 2010;Abdala et al, 2011a,b;Dhar et al, 2011;Abdala and Dhar, 2012;Keefe, 2012). (5) As shown by comparison with the dashed line in Fig.…”

“…For example, DPOAE ipsilateral suppression tuning curves are narrower in newborns compared to adults (Abdala, 1998). In addition, previous measurements of the total phase accumulation of the R component are also consistent with sharp cochlear filtering in neonates (Abdala and Dhar, 2012). Phase accumulation provides an average, non-frequency specific metric of tuning-the greater the total phase accumulation, the steeper the mean phase-gradient, which corresponds to a longer delay and narrower filters.…”

“…In newborns, for example, otoacoustic emission (OAE) amplitude and growth functions, suppression tuning curves, and deviations from cochlear scaling all show immaturities that have been attributed to incomplete development of the outer, middle, and inner ears, the latter most notably in the apical half of the cochlea (Prieve, 1992;Smurzynski, 1994;Abdala, 1998;Abdala and Keefe, 2006;Abdala and Dhar, 2012;Prieve et al, 2013). Studying how mechanical filtering in the cochlea changes during development may help disentangle the peripheral and central contributions related to maturation of hearing in humans.…”

“…Conversely, during aging, various aspects of the auditory system become less efficient, showing reduced capability. Among noted aging effects are degradations in audiometric thresholds, cochlear nonlinearity, and speech perception in noise (Dubno et al, 1984;Gordon-Salant, 2005;Abdala and Dhar, 2012). Understanding and isolating the effects of aging on the mechanics of the cochlea in contrast to declines in neural processes would be helpful in developing a comprehensive model of auditory aging and potentially helpful in intervention.…”

Frequency shifts in distortion-product otoacoustic emissions evoked by swept tones

“…(7), fine-structure shifts Df fs in the composite DPOAE depend on the summed frequency shifts, Df d þ Df r , of the distortion and reflection components individually. In practice, the phase gradient of the distortion component is generally considerably smaller than that of the reflection component (e.g., Abdala and Dhar, 2012), so that js d j ( js r j and, as a result, jDf d j ( jDf r j. Thus, our analysis predicts that the frequency shifts observed in composite DPOAE fine-structure are dominated by underlying shifts in the reflection component, and specifically its phase.…”