An auditory enhancement effect occurs when one component of a harmonic series is omitted for a few hundred milliseconds and then reintroduced: The reintroduced harmonic stands out perceptually. Three experiments are reported that studied a version of this effect in which several components of a harmonic series are enhanced to define the formants of a vowel. Using the accuracy of vowel identification to measure the prominence of the formant peaks in the effective auditory representation, forms of the effect were identified that are qualitatively similar to the incremental and decremental responses seen in primary auditory-nerve fibers. These results are compatible with an origin for the enhancement effect in peripheral auditory adaptation. However, an additional mechanism is required to account for the demonstration [Viemeister and Bacon, J. Acoust. Soc. Am. 71, 1502-1507 (1982)] that enhancement can involve a true gain in the frequency region of the reintroduced component. These effects demonstrate one way in which the auditory system may attenuate the prominence of background noises while preserving the ability to represent changes in spectral amplitude produced by newly arriving signals.

Two steady-state noise-excited vowel sounds of 200-ms duration were synthesized: one similar to a naturally produced whispered vowel, the other with energy removed outside a nominal 50-Hz band centered at each formant peak. The threshold for a brief sinusoidal probe tone was measured at frequencies corresponding to the peaks and troughs in the vowel spectra, in both forward and simultaneous masking. The masking functions for a flat-spectrum masker at each frequency were also measured, so that the vowel masking patterns could be expressed in terms of the equivalent broadband masker level. In the first experiment, masking patterns for the two maskers were measured with three normal-hearing subjects. Increasing spectral contrast was effective in increasing peak–valley differences in the masking patterns up to 2.5 kHz. Contrast was greater in forward than in simultaneous masking, suggesting that the internal representation of a vowel may be enhanced by suppression. In a second experiment, masking patterns and flat-spectrum masking functions were measured with three listeners with moderate sensorineural hearing impairments. The enhancement produced by increasing spectral contrast was less than that obtained with normal-hearing listeners, but still present up to 2.5 kHz. A comparison of forward- and simultaneous-masking patterns again revealed an increase in peak–valley differences, suggesting that suppression mechanisms were still effective in these listeners. The ability to enhance the internal representation of speechlike sounds in hearing-impaired listeners may have implications for the design of signal-processing hearing aids.

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