Perception of complete and incomplete formant transitions in vowels.

In everyday speech, formant transitions rarely reach the canonical frequencies of a target vowel. The perceptual system often compensates for such production undershoots, called vowel reduction (VR), by a perceptual overshoot of the final transition frequencies. The present investigation explored the perceptual parameters and existence region of VR. In a series of experiments a 100-ms steady-state vowel V(1) was followed by a formant transition toward a target vowel V(2). By manipulating both its duration and velocity, in most stimuli the transition was truncated and only seldom reached the target. After being presented with the vowel V(2) before each block of trials, listeners were asked to rate their confidence that the transition actually reached the V(2) target. Transitions along six trajectories connecting the three cardinal vowels /a/, /i/, and /u/ in both directions as well as the transition /ie/ (halfway along the trajectory /ia/) were examined in experiments in which either the duration of the transition was fixed and its velocity was varied or vice-versa. Results confirmed the existence of perceptual overshoot and showed that, at the point a transition short of reaching the vowel V(2) was just perceived as if it had reached the target, transition duration and transition velocity were inversely related. The amount of overshoot was found to be larger for larger V(1)-V(2) distances and shorter trajectory durations. The overshoot could be reliably predicted by a linear model based on three parameters--the extent of V(1)-V(2) distance, transition velocity, and transition acceleration. These findings suggest that the perceptual dynamics of speech relies on mechanisms that estimate the rate of change in the resonant characteristics of the vocal tract.