Previous experimental study of the identification of stop and fricative consonants has shown that some consonant pairs are asymmetrically confused for one another, with listeners’ percepts tending to favor one member of the pair in a conditioning context. Researchers have also suggested that this phenomenon may play a conditioning role in sound change, although the mechanism by which perceptual asymmetry facilitates language change is somewhat unclear. This dissertation uses articulatory, acoustic, and perceptual data to provide insight on why perceptual asymmetry is observed among certain consonants and in specific contexts. It also uses computational modeling to generate initial predictions about the contexts in which perceptual asymmetry could contribute to stability or change in phonetic categories. Six experiments were conducted, each addressing asymmetry in the consonant pairs /k/-/t/ (before /i/), /k/-/p/ (before /i u/), /p/-/t/ (before /i/), and /θ/-/f/ (possibly unconditioned).
In the articulatory experiment, vocal tract spatial parameters were extracted from real-time MRI video of speakers producing VCV disyllables in order to address the role of vocal tract shape in the target consonants’ vowel-dependent spectral similarity. The results suggest that, for consonant pairs involving /k/, CV coarticulation creates—as expected—vocal tract shapes that are most similar to one another in the environment conditioning perceptual asymmetry. However, CV coarticulation was less informative for explaining the vocalic conditioning of the /p/-/t/ asymmetry.
In the second experiment, RF models were trained on acoustic samples of the target consonants from a speech corpus. Their output, which was used to identify frequency components important to the discrimination of consonant pairs, aligned well with these consonants’ spectral characteristics as predicted by acoustic models. A follow-up perception experiment that examined the categorization strategies of participants listening to band-filtered CV syllables generally showed listener sensitivity to these same components, although listeners were also
sensitive to band-filtering outside the predicted frequency bands.
Perceptual asymmetry is observed in CV and isolated C contexts. In the fourth experiment, a Bayesian analysis was performed to help explain why perceptual asymmetry appears when listening to isolated Cs, and a follow-up perception experiment helped to evaluate the relevance of this analysis to human perception. For /k/-/t/, for example, whose confusions favor /t/, this analysis suggested that [t] and [k] both have the highest likelihood of being generated by /t/ (relative to likelihood of /k/ generating each) in the context conditioning asymmetry. The follow-up study suggests listeners are more likely to categorize a [t] and [k] as /t/ if it has higher likelihood of being generated by /t/ (relative to /k/).
The final experiment used agent-based modeling to simulate the intergenerational transmission of phonetic categories. Its results suggest that perceptual asymmetry can affect the acquisition of categories under certain conditions. A lack of reliable access to non-phonetic information about the speaker’s intended category or a tendency not to store tokens with low discriminability can both contribute to the instability of phonetic categories over time, but primarily in the contexts conditioning asymmetry.
This dissertation makes several contributions to research on perceptual asymmetry. The articulatory experiment suggests that confusability can be mirrored by gestural ambiguity. The Bayesian analysis could also be used to build and test predictions about the confusability of other sounds by context. Finally, the model simulations offer predictions of the conditions where perceptual asymmetry could condition sound change.PHDLinguisticsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/155085/1/iccallow_1.pd
