Visual correlates of Thai lexical tone production: Motion of the head, eyebrows, and larynx?

There is well-established evidence that visual articulatory information in the face and head aids identification and discrimination of lexical tone. However, the nature and locus of this information is only beginning to be specified. In previous work we identified a predominant role of head motion over face motion in both the perception and production of Cantonese lexical tone, the latter using OPTOTRAK motion tracking. We have now extended the set of OPTOTRAK markers to include the eyebrows and the larynx, and collected data from a corpus of Cantonese, Thai and Mandarin speakers. Here we report on a Thai speaker producing the five Thai tones on four Thai syllables in isolated words and sentences and in normal, whispered, and Lombard speech. Principal components (PCs) for the face (eyebrows, lips, jaw), the larynx and for independent head movement were extracted and linear mixed model analyses of range of PC1 scores revealed good differentiation on the basis of syllable identity and context and speech style. Of particular importance, the five Thai tones were best differentiated by head and larynx motion. So, these results add larynx motion as a possible visible cue for tone perception. Studies across speakers and the three languages will follow.