The effect of temporal waveform shape on spectral discrimination by normal-hearing and hearing-impaired listeners.

A listener's ability to discriminate differences in amplitude spectra is generally thought to be independent of the temporal waveform shapes of the stimuli. However, there have been reports of enhanced contrast between spectral peaks and valleys of harmonic complexes represented in the temporal firing patterns of auditory-nerve fibers for some combinations of component amplitudes and phases [e.g., Horst et al., J. Acoust. Soc. Am. 88, 2656-2681 (1990)]. The enhancement has been attributed to nonlinear processing of high-amplitude peaked waveforms. To determine whether such enhancements are observed in psychoacoustic responses of human listeners, subjects were asked to discriminate between harmonic complexes that differed in the frequency location of three formantlike increments in harmonic amplitude. The stimuli were constructed to have either a very flat or a very peaked temporal envelope. Over a range of 1- to 10-dB spectral contrast between elevated harmonics and background harmonics, discrimination ability was determined as a function of waveform shape and overall amplitude. Performance was measured for both normal-hearing and hearing-impaired listeners. Normal-hearing listeners showed lower thresholds of spectral contrast for peaked waveforms at high intensities than for any of the other phase-amplitude conditions. Although the effect was small (1 to 2 dB), it is consistent with an hypothesized enhancement due to nonlinear cochlear processing. Hearing-impaired listeners did not demonstrate differential performance across phase or amplitude conditions, perhaps reflecting more linear processing in these damaged ears.