Contextual variations in trisyllabic sequences of Thai tones

It is well known that tones assimilate in much the same way as consonants and vowels do. In tonal assimilation, the height and shape of a given tone is altered by adjacent tones. Earlier studies on tonal assimilation in Thai have used two-tone sequences with an intervening obstruent between the two tones of interest. In the present study, three-tone sequences were used with continuously voiced syllables throughout the utterance. Assimilatory effects were expected to be greater between F0 contours of two successive syllables that are continuous in voicing across the syllable boundary. Stimuli consisted of 125 possible three-tone sequences of the five Thai tones, superimposed on monosyllabic words in a carrier sentence. All syllables were stressed in order to eliminate potentially confounding interactions between stress and tonal assimilation. Acoustic analysis revealed that perseverative effects were assimilatory in nature; anticipatory effects, however, were dissimilatory in nature. Perseverative effects appear to be independent of tonal categories, and restricted to contiguous tones. Perseverative effects extend through more than half the duration of the following tone. Both the height and shape of the tones are affected. Anticipatory effects, on the other hand, are limited to the high and rising tones, and extend through less than half the duration of the preceding tone.