Investigation of Frequency Characteristics of DPOAEs Using Suppressors of Varying Bandwidth and Center Frequency Presented in a Forward Masking Paradigm
McAlister, Erin Christine
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This study was designed to investigate the effect of the bandwidth and center frequency of narrowband noise suppressors presented in a forward masking paradigm on distortion product otoacoustic emission (DPOAE) level. Young adult female listeners with normal hearing participated. DPOAEs were recorded for two different pairs of primary frequencies (f1 = 1666 Hz, f2 = 2000 Hz and f1 = 3333 Hz, f2 = 4000 Hz) in an unsuppressed condition and three suppressed conditions for each of two experiments. In Experiment 1, the three noise suppressors were centered at the f2 frequency and had bandwidths selected to be equal to, narrower than, and wider than the estimated equivalent rectangular bandwidth (ERB) at that frequency. It was hypothesized that increasing the suppressor bandwidth from less than the estimated ERB to equal to the ERB would provide a significant increase in magnitude of suppression, but that a further increase in suppressor bandwidth beyond the estimated ERB would provide little if any additional suppression. In Experiment 2, the three noise suppressors had a constant bandwidth and were centered at the f2 frequency, ½ octave below the f2 frequency, and ½ octave above the f2 frequency. It was hypothesized that suppressors centered at the f2 frequency would cause greater suppression than suppressors centered above and below the f2 frequency. Results of Experiment 1 revealed a significant effect of the suppressors compared to the unsuppressed condition, but there were no significant differences between the suppressor bandwidths. Results of Experiment 2 support the hypothesis that a narrowband suppressor centered at the f2 frequency would have a greater suppressive effect than suppressors centered above or below the f2 frequency, at least for f2 = 2000 Hz. These results demonstrate that significant DPOAE suppression can be recorded using noise suppressors presented in a forward masking paradigm. Implications of these results for advancing understanding of the frequency tuning of the cochlea and the medial olivocochlear system are discussed.