**Comparative approaches to otoacoustic emissions: Towards an understanding of why the ear emits sound**

PhD Thesis by Christopher Bergevin - Massachusetts Institute of Technology (completed June 2007)

Abstract

The ear represents a remarkable achievement in sensory physiology. It is very fast (timescales on the order of 1-100 kHz), has a large bandwidth (~10 octaves in the human), highly sensitive (threshold is ultimately determined by thermal noise), operates over an enormous dynamic range (factor of a trillion in terms of energy input), capable of sharp stimulus selectivity (e.g. frequency and intensity discrimination) and exhibits robust nonlinear behavior. As a sensor designed to detect acoustic sound pressure, surprisingly, the ear also emits sound as well. These otoacoustic emissions (OAEs) have been developed extensively for clinical applications (in general, healthy ears emits while impaired ones do not), though their full potential has yet to be realized. Much of the effort gone into understanding OAEs has been developed within the context of mammals, where specific anatomical and physiological features (e.g. traveling waves and somatic motility) are thought to play an integral role in generation. This thesis approaches OAEs comparatively, systematically characterizing emissions in humans and an array of non-mammals (chickens, geckos and frogs) who lack these mammalian features and exhibit a diverse range of morphologies. First, our results show that for a fixed set of stimulus conditions (employing moderate intensities), emissions are relatively largest in the gecko and frog (the two species with the fewest number of sensory cells) and smallest in the human and chicken for frequencies below ~2 kHz. At higher frequencies (3-5 kHz), emissions descend toward the noise floor for the non-mammals but remain relatively constant in human. Second, OAE phase behavior indicates that emissions are generated by multiple mechanisms in the human and chicken (and possibly gecko in certain stimulus conditions), but not the frog. OAEs in all species exhibit significant delays (~1 ms or longer), those being largest in humans. Tuning can explain these delays in all species except the frog, where some additional source of delay is present. Lastly, non-monotonic growth (relative to stimulus intensity) was found in all species, suggesting the presence of multiple level-dependent sources. We interpret the observed similarities and differences in emission properties across species within the context of anatomical/physiological comparisons.

Key Findings

Both SFOAEs and DPOAEs were observable in all species examined (human, chicken, gecko and frog). Emission magnitudes were largest in gecko and frog (for emission frequencies below ~2-4 kHz) and smallest in the chicken. Given the evidence indicating a lack of somatic motility in non-mammals, OAEs are still prevalent in species lacking outer hair cell motility.

Non-monotonic growth was observed for both SFOAEs and DPOAEs in all species. Properties of the growth functions were inconsistent with predictions from the single-source model (i.e. phase jumps less than 1/2 cycle across a notch, presence of notches is highly frequency dependent and cubic DPOAE growth at low levels is not cubic), even in simpler ears. This finding suggests that two level-dependent emission sources are present and can interfere with one another. Thus, OAEs need to be considered as arising as a spatially summed response over a distributed region.

As revealed by the evoked emission phase gradients, long delays (>1 ms, at least an order of magnitude longer than would be expected from a round trip fluid compressional wave) were observed in all species. These delays are significantly longer in humans compared to the other species. The phase gradients were also clearly level-dependent in both humans and geckos. Given evidence indicating a lack of basilar membrane (BM) traveling wave in some of the non-mammalian species tested, BM waves are not necessary for long delays. Comparison of the frequency dependence with auditory nerve fiber derived Q-values suggest that emission delays can be accounted for by mechanical tuning present in the inner ear.

Comparison of SFOAE and DPOAE (low-side vs. high-side) phase gradients are qualitatively similar between the human and chicken, indicating evidence for multiple generation mechanisms. While similar evidence does not clearly exist for the gecko and frog ears, multiple generation mechanisms can not be ruled out in those species (interpretation hinges upon the presence of scaling-symmetric effects in their ears).

Committee Members

Prof. Christopher A. Shera (Harvard Medical School) [advisor]
Prof. Dennis M. Freeman (Massachusetts Institute of Technology) [advisor]
Prof. John J. Rosowski (Harvard Medical School) [chair]
Prof. John J. Guinan (Harvard Medical School)
Prof. Paul F. Fahey (University of Scranton)
Dr. A.J. Aranyosi (Massachusetts Institute of Technology)

Support

Funding provided by grants T32 DC00038, RO1 DC003687 (CAS), RO1 DC00238 (DMF) and T32 DC00038 (SHBT training grant) from the NIDCD. All experimental protocols were subject to MIT COUHES, MIT DCM/CAC, MEEI Human Studies Committee and UPenn IACUC approval. We also gratefully acknowledge the support provided by Prof. James Saunders at the University of Pennsylvania.