Otoacoustic emissions and hearing threshold

Abstract

This thesis explores two hypotheses.  Firstly that the moderate correlations between OAE and HTL reported in the literature are a result of a poor choice of OAE measures.  OAE measures that take into account the level dependency of cochlear amplification are expected to have a higher correlation with HTL.  Secondly, that the moderate correlations between OAE and HTL in cross-sectional studies are a result of inter-subject and inter-ear factors unrelated to cochlear amplifier function (e.g. ear canal and middle ear factors).  The correlation between OAE and HTL is expected to increase in longitudinal experiments where these factors are constant within-subjects and within-ears across conditions.  This thesis addresses these issues by investigating the relationship between OAE and HTL in human subjects with normal hearing or mild sensory hearing loss in both cross-sectional and longitudinal studies. A wide range of OAE measures were investigated.  Differnces in OAE between subjects with different HTL status were examined by cross-sectional study, while longitudinal changes in OAE were studied within subjects experiencing a temporary hearing loss due to aspirin consumption.  Both TE and DPOAE were measured and the data were interpreted within the general framework of OAE generation. 

In the cross-sectional study, forty-three subjects with a range of HTL were tested.  TE and DPOAE were measured across a range of frequencies and stimulus levels.  The longitudinal study involved 17 subjects taking aspirin for three days while OAE were measured over the course of seven days (pre-, peri- and post-aspirin).  Correlation and linear regression analyses were performed to investigate the relationship between OAE and HTL in both experiments.  Identical measures were obtained in the cross-sectional and longitudinal, which allowed the two studies to be compared directly.

This research shows that addressing the limitations of previous studies does not, in general, improve the relationship between OAE and HTL.  However for some ears, when measured longitudinally, changes in OAE accurately reflect changes in HTL.  However, this is not true for all ears, and therefore other factors are also important and further work is required to identify those factors.  This research suggests that the lack of high correlation between OAE and HTL in cross-sectional studies is at least partially due to inter-subject and inter-ear factors such as ear canal and  middle ear effects.  This research has direct application to the use of OAE for monitoring changes in hearing in at-risk populations, such as people exposed to noise.

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