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Detection, direction discrimination, and off-frequency interference of center-frequency modulations and glides for vowel formants
Journal of the Acoustical Society of America, 117(5), 3042-3053
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Vowels are mainly classified by the positions of peaks in their frequency spectra; the formants. For normal-hearing subjects, we measured change detection and direction discrimination for linear glides in the center frequency of formant-like sounds. A center frequency rove was used to prevent subjects from using either the start or end points of the glides as cues. In addition, change detection and starting-phase (start-direction) discrimination were measured for similar stimuli with a sinusoidal 5-Hz formant-frequency modulation. The stimuli were single-formant ├Čvowels├« generated using a number of different stimulus parameters including fundamental frequency, spectral slope, frequency region, and position of the formant relative to the harmonic spectrum. The change detection thresholds were in good agreement with the predictions of a model which combined the processing of place-of-excitation and temporal cues . For most stimuli, thresholds were approximately equal for change detection and start-direction discrimination. Exceptions were found for stimuli that consisted of only one or two harmonics. In a separate experiment, we showed that change detection and start-direction discrimination of linear and sinusoidal formant-frequency modulations were impaired by off-frequency frequency-modulated interferers. This frequency modulation detection interference (FMDI) was larger for formants with shallow than for those with steep spectral slopes.