The role of formant transitions in the perception of concurrent vowels

When two voices compete for the attention of the listener, the spectral peaks that define the formants of one voice can be intermittently obscured or distorted by formants of the other voice. However, formant peaks vary slowly and continuously in frequency and time, providing a basis for tracking through regions of overlap. Three experiments investigated the ability of listeners to exploit formant-pattern continuity to segregate pairs of synthesized vowels that were presented simultaneously and monaurally. Experiment 1 and 2 examined the effects of introducing one member of the pair with formant-frequency transitions that specified syllable-initial glides /w/ or /j/. Identification accuracy was generally higher in conditions where glides were present. Gliding formants provided smaller benefits than a two-semitone difference in fundamental frequency between the vowels. Experiment 3 found larger effects of formant transitions specifying initial or final /l/. Overall, formant transitions did not make it easier to identify the vowel to which they were linked; instead, they helped by making the competing vowel more identifiable. One explanation for improvement in the glide conditions is a formant-tracking process which groups together the formants of each voice using the Gestalt principle of good continuation. However, this account predicts improvement for both vowels which was generally not observed. An alternative explanation is suggested by models that apply a brief, sliding temporal window to determine which region of the signal provides the strongest evidence of each vowel constituent. The formant transition region may provide a time interval during which the competing steady-state vowel is perceptually more prominent.