Level, spectral and pre-exposure effects of contralateral acoustic stimulation on transient evoked otoacoustic emissions

Otoacoustic emissions (OAEs) are a non-invasive method of measuring outer hair cell (OHC) function. OAEs are elicited by introducing a stimulus to the ear and recording the resulting response in the ear canal. In a properly functioning system, the stimulus travels through the middle ear into the cochlea. A response is generated, presumably by OHC motion that travels back through the system to be recorded by a microphone in the ear canal. Transient evoked otoacoustic emissions (TEOAEs), utilized in this research, are elicited by a click stimulus. A strong response is indicative of normal cochlear function, suggesting normal hearing.; Suppression is defined as a reduction in amplitude of the resulting TEOAE, and is mediated via the efferent auditory system. The purpose of this study was to determine whether variation of contralateral acoustic stimuli (CAS) intensity, spectral content or pre-exposure to the stimulus has any effect on the resulting TEOAE.; Various CAS were utilized (250Hz pure tone, 4000Hz pure tone and white noise) for all eleven normal-hearing subjects. Each stimulus was delivered to the subject's contralateral ear while OAEs were recorded from the ipsilateral ear. Two intensity levels, 35 and 50dB HL, were utilized as the CAS, with a 60 second pre-exposure of 80dB HL for certain conditions.; Overall waveform amplitude reduction and frequency specific suppression were analyzed using the Kresge EchoMaster 4.0 system. All subjects demonstrated suppression in multiple conditions. The CAS that produced the greatest suppression across all subjects and conditions was white noise, followed by 4000Hz, and then 250Hz. A level effect was evidenced, showing greater intensity CAS causing a larger suppressive effect. White noise was the only stimulus that consistently demonstrated a reduction in amplitude during pre-exposure trials.; These findings suggest that the human auditory system may use suppression in an effort to assist with figure ground. Due to its broad spectral content, white noise often characterizes background noise. In addition, the greater effect shown with a higher intensity signal suggests that the efferent auditory system is able to serve as a gain control and reduce background noise, allowing the listener to focus on critical communication.