Comparative approaches to otoacoustic emissions : towards and understanding of why the ear emits sound

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 (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.

(cont.) 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.