Perception of nonlinear and linear formant trajectories

Perception of F2 trajectories in synthetic vowels was investigated. Perceptual boundaries (50% response points of identification functions) of 20-step/U-I/continua with various shapes of F2 trajectories were determined and compared with the boundary for vowels with steady-state F2. In experiment 1, the vowels were synthesized in either/j-j/or/w-w/context resulting in parabolic trajectories, then the stimuli were split into halves resulting in quadratic F2 trajectories. All stimuli were 200 ms long. For the/wVw/, and/wV/stimuli, the boundaries were at lower stimulus numbers than for the stimuli with steady-state F2, indicating that the nonlinear F2 trajectories were perceived as having frequencies beyond the extreme values actually synthesized in the stimuli. This type of signal processing has been termed "perceptual compensation." For the /jVj/ stimuli, there was only a trend for perceptual compensation of F2 trajectories. For the/Vw/,/Vj/, and/jV/stimuli, the boundaries were at stimulus numbers corresponding to frequencies of the relatively steady-state vowel-like segments. In experiment 2, the quadratic F2 trajectories of the/wV/stimuli were changed to linear trajectories and F1 and F3 trajectories were either quadratic, linear, or steady state. The results indicated that the shape of F1 and F3 trajectories had no effect on the boundaries. For the linear F2 trajectories, as for the quadratic F2 trajectories in the/wV/ stimuli, the boundaries were at lower stimulus numbers than for the stimulus with steady-state F2, indicating presence of perceptual compensation. In the experiment 3, the F1 and F3 were steady state and the linear F2 trajectories had three different values of frequency difference, delta F, between the initial and final frequencies. The perceptual compensation was found for the 200-ms stimuli with large delta F up to 500 Hz over 200-ms stimulus duration, and perceptual enhancement of final frequencies was found for the stimuli with delta F = 280 Hz.