Perceiving vowels from uniform spectra: phonetic exploration of an auditory aftereffect.

Abstract

A carefully spoken vowel can generally be identified from the pattern of peaks and valleys in the envelope of its short-term power spectrum, and such patterning is usually necessary for the identification of the vowel. The present experiments demonstrate that segments of sound with uniform spectra, devoid of peaks and valleys, can be identified reliably as vowels under certain circumstances. In Experiment 1, 1,000 msec of a segment whose spectrum contained peaks in place of valleys and vice versa (i.e., the complement of a vowel) preceded a 25-msec spectral amplitude transition, during which the valleys became filled, leading into a 250-msec segment with a uniform spectrum. The segment with the uniform spectrum was identified as the vowel whose complement had preceded it. Experiment 2 showed that this effect was eliminated if the duration of the complement was less than 150 msec, if more than 500 msec of silence separated the uniform spectrum from the complement, or if the uniform spectrum and the complement were presented to different ears. This third result and comparisons with parameters of auditory aftereffects obtained by others with nonspeech stimuli suggest that the effect is rooted in peripheral adaptation processes and that central processes responsible for selective attention and perceptual grouping play only a minor role at most. Experiment 3 demonstrated that valleys in the spectral structure of a complement need be only 2 dB deep to generate the effect. The effect should therefore serve to enhance changes in spectral structure in natural speech and to alleviate the consequences of uneven frequency responses in communication channels.