THE EFFECT OF TEMPORAL WAVE-FORM 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 comb inations of component amplitudes and phases [e.g., Horst et al., J. Ac oust. Soc. Am. 88, 2656-2681 (1990)]. The enhancement has been attribu ted to nonlinear processing of high-amplitude peaked waveforms. To det ermine whether such enhancements are observed in psychoacoustic respon ses of human listeners, subjects were asked to discriminate between ha rmonic complexes that differed in the frequency location of three form antlike 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 func tion of waveform shape and overall amplitude. Performance was measured for both normal-hearing and hearing-impaired listeners. Normal-hearin g listeners showed lower thresholds of spectral contrast for peaked wa veforms at high intensities than for any of the other phase-amplitude conditions. Although the effect was small (1 to 2 dB), it is consisten t with an hypothesized enhancement due to nonlinear cochlear processin g. Hearing-impaired listeners did not demonstrate differential perform ance across phase or amplitude conditions, perhaps reflecting more lin ear processing in these damaged ears.