Abstract: In order to reason about the nature and directionality of constraints on tonal variation and change, researchers have typically focused on the comparison of acoustic F0 trajectories in real (or apparent) time, as well as the analysis of patterns of contextual tonal variation. However, like segmental sound change, tone change must ultimately involve perceptual realignment as well as changes in production norms. If there exist regions of greater or lesser perceptual equivalence within the tonal perceptual space, we could then ask whether these correlate with higher or lower regions of acoustic variability in tone pitch trajectories. Establishing this relationship between production and perception is crucial to formulating testable hypotheses about more and less likely patterns of tone change.
In this talk, I present our ongoing work on this topic, focusing on two case studies of Central Thai and Southern Vietnamese. First, we assess the variability of tonal production patterns using kernel density estimates of F0 trajectories extracted from spoken language corpora. In Southern Vietnamese, tonal onsets are found to be the region of greatest variability, while in Thai, the onset is generally a more stable region. Next, using data from a similarity judgment task, we show that the two languages also partition the perceptual tonal contour space in different ways. In Thai, the primary dimension of perceptual similarity is rises vs. falls, while in Vietnamese, contours are clustered according to the register of the midpoint (high, mid, or low).
Comparing the production and perception results reveals that the portions of the F0 trajectories with the highest stability across talkers do not correlate straightforwardly with the regions of maximal perceptual salience. The regions of lowest variability in production are almost invariably located from the onset to midpoints of tonal contours, while in perception, listeners of both languages seem to prioritize information in the second half of tone contours, consistent with previous work establishing the relatively greater perceptual salience of F0 offsets. We conclude with a discussion of the implications of our findings for theories of tonal variation and change.