Cochlear compression estimates from measurements of distortion-product otoacoustic emissions

Abstract

Evidence of the compressive growth of basilar-membrane displacement can be seen in distortion-product otoacoustic emission (DPOAE) levels measured as a function of stimulus level. When the levels of the two stimulus tones (f1 and f2) are related by the formula L1 = 39 dB + 0.4 x L2 [Kummer et al., J. Acoust. Soc. Am. 103, 3431-3444 (1998)] the shape of the function relating DPOAE level to L2 is similar (up to an L2 of 70 dB SPL) to the classic Fletcher and Munson [J. Acoust. Soc. Am. 9, 1-10 (1933)] loudness function when plotted on a logarithmic scale. Explicit estimates of compression have been derived based on recent DPOAE measurements from the laboratory. If DPOAE growth rate is defined as the slope of the DPOAE I/O function (in dB/dB), then a cogent definition of compression is the reciprocal of the growth rate. In humans with normal hearing, compression varies from about 1 at threshold to about 4 at 70 dB SPL. With hearing loss, compression is still about 1 at threshold, but grows more slowly above threshold. Median DPOAE I/O data from ears with normal hearing, mild loss, and moderate loss are each well fit by log functions. When the I/O function is logarithmic, then the corresponding compression is a linear function of stimulus level. Evidence of cochlear compression also exists in DPOAE suppression tuning curves, which indicate the level of a third stimulus tone (f3) that reduces DPOAE level by 3 dB. All three stimulus tones generate compressive growth within the cochlea; however, only the relative compression (RC) of the primary and suppressor responses is observable in DPOAE suppression data. An RC value of 1 indicates that the cochlear responses to the primary and suppressor components grow at the same rate. In normal ears, RC rises to 4, when f3 is an octave below f2. The similarities between DPOAE and loudness compression estimates suggest the possibility of predicting loudness growth from DPOAEs; however, intersubject variability makes such predictions difficult at this time.

Distortion-product otoacoustic emission input/output characteristics in normal-hearing and hearing-impaired human ears

“…Dorn et al ͑2001͒ fitted an ad-hoc equation to median I/O functions from 77 subjects and observed some of the same trends with frequency, HL, and stimulus level as reported in the present study. Neely et al ͑2003͒ noted the similarity in the shape of the average DPOAE I/O functions for normal-hearing ears with the shape of the loudness function described by Fletcher and Munson ͑1933͒. However, the logarithmic function that successfully characterized both DPOAE growth and loudness growth in that study failed to fit individual DPOAE I/O functions.…”

Relation of distortion-product otoacoustic emission input-output functions to loudness

“…This reduction in DPOAE level is usually accompanied by elevation of audiometric thresholds (e.g., Boege and Janssen, 2002;Gorga et al, 2003) and loss of compression. In a healthy cochlea, DPOAEs grow approximately linearly with level at low levels, but growth becomes more compressive as levels increase (e.g., Neely et al, 2003). This normal compressive characteristic manifests as a DPOAE I/O function with slope that is steeper at low levels compared to high levels, when the primary levels are selected using the paradigm of Kummer et al (1998).…”

“…Peripheral measures of cochlear compression based on OAE have been shown to be correlated with growth of loudness Muller and Janssen, 2004;Epstein and Florentine, 2005;Epstein and Silva, 2009;Thorson et al, 2012). Neely et al (2003) showed that the Fletcher and Munson (1933) log-loudness function and DPOAE I/O can both be described by similar log functions. Muller and Janssen (2004) demonstrated the similarity of estimates of hearing-aid gain based on DPOAE I/O functions to gain based on CLS functions.…”

“…Otoacoustic emission (OAE) input/output (I/O) functions and categorical loudness scaling (CLS) functions both typically become steeper when hearing loss is present (e.g., Neely et al, 2003;Thorson et al, 2012). The common mechanism for the steepening of both functions presumably is outer hair cell (OHC) dysfunction, despite the fact that more central factors may influence loudness judgments.…”

Low-frequency and high-frequency cochlear nonlinearity in humans

“…At its maximum, the compression ratio estimated from DPOAE I/O functions is approximately 4:1, at least for mid and high frequencies. In previous work, we have estimated the range of input levels over which compression takes place and the amount of compression in both normal and impaired ears (Dorn et al, 2001;Neely et al, 2003). Both the amount of compression and the range of input levels over which compression was observed decreased as hearing loss increased.…”