Distortion product otoacoustic emission contralateral suppression functions obtained with ramped stimuli

The purpose of this research was to investigate the changes that occur in human distortion product otoacoustic emission (DPOAE) level functions over continuous frequency bands in response to activation of the medial olivocochlear (MOC) efferent system by contralateral broadband noise. DPOAEs were obtained using continuous upward ramps of the lower frequency tone (f(1)) while the higher frequency tone (f(2)) was fixed. These ramps were designed to change the stimulus frequency ratio f(2)/f(1) over a fixed range for each fixed f(2) value of 2, 3, and 4 kHz. Contralateral noise was presented on alternating ramps and the DPOAEs with and without contralateral noise were averaged separately. Stimulus frequency ratios of 1.10 and 1.22, and noise levels of 60 and 50 dB sound pressure level (SPL) were employed. Changes in DPOAE level were generally suppression (a reduction in DPOAE magnitude), but enhancement was also observed. For most participants, changes were evident for much of the frequency ranges tested. Average absolute changes for 60 dB SPL noise were 0.95, 0.81, and 0.42 dB for the wider stimulus frequency ratios and f(2) of 2, 3, and 4 kHz, respectively. For the narrower ratio and 60 dB SPL noise, the changes were larger with average absolute changes of 1.33, 1.09, and 0.87 dB. For the narrower ratio and 50 dB SPL noise, the changes were 1.08, 0.78, and 0.55 dB with f(2) of 2, 3, and 4 kHz, respectively. DPOAE nulls were observed and a common response pattern was a shift of emission morphology to higher frequencies with contralateral acoustic stimulation. The method appears promising for relatively rapid evaluation of the MOC efferent system in humans and offers information complementary to measurement strategies that explore the effects of stimulus level.