Laura E. Dreisbach

Distortion-product otoacoustic emissions measured at high frequencies in humans

Distortion-product otoacoustic emissions (DPOAEs) elicited with stimulus frequencies less than or equal to 8 kHz have been used in hearing clinics to assess whether the middle ear and cochlea are normal, but high-frequency hearing (>4 kHz) is most vulnerable to cochlear pathology. It might prove useful to measure DPOAEs with even higher frequency stimuli (>8 kHz), but there have been few reports of such studies in humans. DPOAEs have been measured in other mammals to the upper range of hearing sensitivity. The purpose of this study was to compare some characteristics of DPOAEs in human subjects elicited with high-frequency stimuli with those that have been extensively measured with lower-frequency stimuli. The primary goal was to establish if the same phenomenon responsible for the behavior of low-frequency DPOAEs is responsible for the behavior of high-frequency DPOAEs. Specifically, the DPOAE level with stimuli varied from 2 to 20 kHz, growth functions of DPOAEs, effects of varying the primary frequency ratio (f2/f1) on the DPOAE level, and DPOAE group delay were determined. Because the behaviors appeared to vary smoothly with stimulus frequency, the study suggests that emissions measured from 2 to 20 kHz were the product of the same biological process.

Racial and gender effects on pure-tone thresholds and distortion-product otoacoustic emissions (DPOAEs) in normal-hearing young adults

This study examined racial and gender effects on behavioral thresholds and distortion-product otoacoustic emissions (DPOAEs) in the same subjects. Pure-tone behavioral thresholds and DPOAEs were measured in 60 young normal-hearing adult subjects (20 Caucasian, 20 Asian, 20 African-American, with ten females and ten males in each group). Behavioral thresholds were measured from 1000 through 16 000 Hz using Békèsy tracking. A DPOAE frequency sweep was measured with primary stimulus levels of L1/L2=60/45 dB SPL, and an f2/f1 of 1.2 at discrete f2 frequencies between 2000 through 12 000 Hz for each subject. Significant racial and gender differences in behavioral thresholds were found at 14 000 and 16 000 Hz, with the African Americans and females having the best hearing sensitivity. Based on the current results, similar findings for DPOAE frequency sweeps can be expected amongst different racial groups given that no significant differences were identified between the groups. To further define the effects of race and gender on auditory measures, future studies should include larger numbers of subjects, measurement of body size and middle ear reflectance, and examine emission generators.

Upward spread of Schroeder‐phase maskers

Harmonic complexes with phases selected according to the Schroeder algorithm can produce large differences in masking, depending on whether phases increase (positive Schroeder) or decrease (negative Schroeder) with frequency. This finding has been attributed to an interaction between the phase characteristics of auditory filters and the stimulus. The current study investigates effects of filter asymmetry on masking by harmonic complexes using an upward spread of masking paradigm. Maskers were Schroeder phase maskers with frequencies from 200 to 2000 Hz, and signal frequencies ranged from 1000 to 4000 Hz. When the signal fell within the masker bandwidth, it was added in‐phase with the identical masker component. At signals below 2000 Hz, negative Schroeder maskers produced more masking than the positive maskers. When the signal frequency was above the masker, there was a rapid decrease in masking for both maskers, but for most subjects, the positive masker became more effective than the negative. This shift in the Schroeder masking effect may be related to phase changes occurring in the lower skirts of the auditory filters. [Work supported by NIH.]

Growth of spectral contrast enhancement in Schroeder‐phase harmonic complexes

Discrimination of spectral differences in harmonic complexes is better for waveforms that are highly modulated and presented at high levels relative to lower‐level stimuli with flatter temporal envelopes [M. R. Leek and V. Summers, J. Acoust. Soc. Am. 94, 2074–2082 (1993)]. This may reflect an enhancement of contrast between spectral peaks and valleys related to nonlinear cochlear processing. To further explore this relationship, discrimination of harmonic complexes with slightly different peak frequencies was measured at several stimulus levels. Peak‐to‐background differences required for discrimination were taken as a measure of the spectral contrast preserved in the auditory system. Phases were selected to generate either flat or peaked internal waveforms (negative or positive Schroeder phase), and peak frequency regions were near 2, 3, or 4 kHz. In normal‐hearing listeners, the positive Schroeder waveforms produced lower spectral contrast thresholds than the negative Schroeder stimuli at all tested in...

Low‐ and high‐frequency threshold reliability

The purpose of this study was done to determine intrasubject low‐ and high‐frequency hearing threshold reliability. Low‐frequency (1, 4, 8 kHz) and high‐frequency (10, 12, 14, 16, 18 kHz) thresholds were obtained on each ear of 30 normal hearing young adult subjects over four trials separated by i but no more than 2 weeks using a Beltone 2000 audiometer. At each frequency, the thresholds were not significantly different for the first versus second ear tested or for the four trials. Between trial threshold differences for each possible trial minus trial threshold combination (N = 6) were determined for each ear of each subject at each frequency. For the possible 2880 between trial threshold comparisons (6 trial combinations = 8 frequencies = 30 subjects = 2 ears), only 23 resulted in threshold differences ⩾ ± 11 dB. It was concluded that repeated intrasubject high‐frequency thresholds were as reliable as for the lower frequencies. Clinical implications regarding high‐frequency serial monitoring of hearing ...