The relationship between distortion product otoacoustic emissions and audiometric thresholds in the extended high-frequency range.

Distortion product otoacoustic emissions (DPOAEs) and behavioral audiometry are routinely used for hearing screening and assessment. These measures provide related information about hearing status as both are sensitive to cochlear pathologies. However, DPOAE testing is quicker and does not require a behavioral response. Despite these practical advantages, DPOAE testing is often limited to screening only low and mid- frequencies. Variation in ear canal acoustics across ears and probe placements has resulted in less reliable measurements of DPOAEs near 4 kHz and above where standing waves commonly occur. Stimulus calibration in forward pressure level and responses in emitted pressure level can reduce measurement variability. Using these calibrations, this study assessed the correlation between audiometry and DPOAEs in the extended high frequencies where stimulus calibrations and responses are most susceptible to the effect of standing waves. Behavioral thresholds and DPOAE amplitudes were negatively correlated, and DPOAE amplitudes in emitted pressure level accounted for twice as much variance as amplitudes in sound pressure level. Both measures were correlated with age. These data show that with appropriate calibration methods, extended high-frequency DPOAEs are sensitive to differences in audiometric thresholds and highlight the need to consider calibration techniques in clinical and research applications of DPOAEs.

Effects of noise overexposure on tone detection in noise in nonhuman primates.

This report explores the consequences of acoustic overexposures on hearing in noisy environments for two macaque monkeys trained to perform a reaction time detection task using a Go/No-Go lever release paradigm. Behavioral and non-invasive physiological assessments were obtained before and after narrowband noise exposure. Physiological measurements showed elevated auditory brainstem response (ABR) thresholds and absent distortion product otoacoustic emissions (DPOAEs) post-exposure relative to pre-exposure. Audiograms revealed frequency specific increases in tone detection thresholds, with the greatest increases at the exposure band frequency and higher. Masked detection was affected in a similar frequency specific manner: threshold shift rates (change of masked threshold per dB increase in noise level) were lower than pre-exposure values at frequencies higher than the exposure band. Detection thresholds in sinusoidally amplitude modulated (SAM) noise post-exposure showed no difference from those in unmodulated noise, whereas pre-exposure masked detection thresholds were lower in the presence of SAM noise compared to unmodulated noise. These frequency-dependent results were correlated with cochlear histopathological changes in monkeys that underwent similar noise exposure. These results reveal that behavioral and physiological effects of noise exposure in macaques are similar to those seen in humans and provide preliminary information on the relationship between noise exposure, cochlear pathology and perceptual changes in hearing within individual subjects.