The influence of disappearing and reappearing spontaneous otoacoustic emissions on one subject's threshold microstructure

Abstract

The effects of a consistently disappearing and reappearing spontaneous otoacoustic emission (SOAE) at around 2280 Hz on microstructure for pure tones of varying durations in a 33 year-old woman with normal hearing was studied. The SOAE began to appear after 10–15 min in a quiet test room and increased in level by up to 22 dB over a 30–40-min period. The SOAE was measured every 12 to 15 min. Between measurements, the subject performed a signal detection task for pure tones with total durations varying from 20 to 320 ms. The signal frequencies were within a ±30-Hz range relative to the SOAE frequency. For signal durations of 40–320 ms, there was a local dip at the target SOAE frequency when it was either not detectable or its level was lower than −14 dB SPL. Subjective threshold levels were as much as 12 dB better than those obtained when the SOAE was −6 dB SPL or greater. The results suggest that a region of the cochlea with high sensitivity and instability can be put into self-oscillation producing an SOAE, possibly by a change of efferent activity. Hearing threshold is affected possibly due to adaptation or masking.

Introduction

Spontaneous otoacoustic emissions (SOAEs) are measured in the outer ear canal when no external acoustic stimulation is presented. Early studies of normal-hearing humans have reported a prevalence of SOAEs on the order of 50% (see review by Probst et al., 1991). More recent papers (e.g. Talmadge et al., 1993; Penner and Zhang, 1997) reported results from off-line methods of data analysis that identified an SOAE prevalence of over 80% for women and over 60% for men. The frequency and amplitude of SOAEs can exhibit both within- and between-session instability. It has been found that: (a) the majority of SOAEs is larger at the end of recording sessions than at the beginning (Whitehead, 1991); (b) the fluctuations of SOAEs may be influenced by cardiovascular changes (Bell, 1992), fever (O'Brien, 1994), and, for women, menstrual cycles (e.g. Penner et al., 1994); and (c) some SOAEs in normal-hearing subjects exhibit rapid fluctuations, or the emissions are present only intermittently (e.g. Penner et al., 1993; Burns et al., 1994; Penner, 1996).

Spontaneous OAEs are related to microstructure of the auditory threshold. Threshold minima have been found in the vicinity of SOAEs (e.g. Long, 1984; Long and Tubis, 1988) when pure-tone or narrowband-noise signals are used to measure the threshold of audibility. An SOAE may either beat with or be entrained by external signals close in frequency to the SOAE. Thus, additional clues for signal detection might be available to the subject, resulting in a dip of the microstructure. McFadden and Mishra (1993)reported that subjects with SOAEs had better hearing (by approximately 3 dB) than those without SOAEs. Interestingly, this behavior occurred across the 1–6-kHz region, not just in the frequency regions of SOAEs.

Typically, low-level SOAEs that are relatively stable in frequency and amplitude are not audible. However, unstable SOAEs are often audible and subjects are able to make reliable pitch and loudness matches to them (Burns, 1996). Powers et al. (1995)reported that high-level SOAEs elevated neural thresholds of chinchillas in neurons with characteristic frequencies near the SOAE frequency. They postulated that the SOAEs created a `line busy' signal that substantially reduced the neuron's ability to respond to sound and thus resulted in an elevated threshold. High-level SOAEs (>30 dB SPL) have also been reported in humans (e.g. Wilson and Sutton, 1983; Probst et al., 1991; Mathis et al., 1991). Masking by high-level SOAEs was considered as a possible explanation of the hearing loss that has been reported in about half of the subjects with high-level SOAEs. However, it is more likely that such SOAEs are associated with cochlear damage (Probst et al., 1991).

During the collection of data for a study on temporal integration in ears with SOAEs, we found a peculiar behavior of SOAEs in a single subject. The subject exhibited no or low-level SOAEs at the beginning of each of 25 test sessions. The SOAEs started to appear after 10–15 min, and their levels increased over a 30–40-min period. This type of SOAE behavior was observed consistently over a period of several months. Most of the data were recorded from the right ear, which exhibited up to seven SOAEs. Similar effects of reappearing SOAEs were recorded in the left ear. One SOAE, with a frequency of approximately 2280 Hz and level of up to +3 dB SPL, from the right ear was targeted for testing. The purpose was to determine the effect of the changes in this SOAE on psychoacoustic threshold microstructure for pure-tone signals of varying duration obtained in the vicinity of the SOAE. It was believed that data acquired from extensive measurements of this subject could lead to further information about cochlear micromechanics.