Perception of Voicing Contrasts in Thai and English

The experiments reported in this dissertation were carried out in order to investigate the perception of voicing features in Thai and English. The Thai language makes phonemic distinctions between voiced and voiceless unaspirated stops at the labial and dental places of articulation. However, Thai does not make a distinction between voiced and voiceless unaspirated stops at the velar place of articulation. English does not make this distinction in word-initial position at any place of articulation. Chapter 2 reviews the literature on selective adaptation relevant to the motivation and interpretation of the experiments reported in Chapter 3. The purpose of the set of experiments reported in Chapter 3 is to investigate differences in the perception of voicing features by speakers of Thai and English using a selective adaptation procedure. These experiments also attempt to specify the levels of psychological processing in which voicing features are implicated. Different patterns of results for the two groups of subjects were obtained in the adaptation experiments. For the Thai-speaking subjects, the -70 msec labial adaptor had no effect on the boundary between voiceless unaspirated and voiceless aspirated stops. The -70 msec velar adaptor did, however, shift this boundary in the velar series. In contrast, both of these stimuli produced boundary shifts on the counterpart boundary between voiced and voiceless stops for the English-speaking subjects. The -70 msec labial and velar adaptors produced cross-series boundary shifts for both groups of subjects. These differences were predicted on the basis of the phonological structures of the two languages. In Chapter 4, the results of a discrimination experiment are reported. The Thai-speaking subjects took part in discrimination tests of both labial and velar stimuli. The English-speaking subjects took part only in a velar discrimination test. Therefore, the Thai velar discrimination functions can be compared both with discrimination functions in which a phonemic distinction is made between voiced and voiceless unaspirated stimuli and also with the English-speaking subjects' discrimination functions, where no such phonemic distinction exists in the language as a whole. The English-speaking subjects' discrimination functions are characterized by a single peak spanning the phoneme boundary. The Thai-speaking subjects' discrimination functions for the labials are characterized by two peaks spanning the two phoneme boundaries. Their velar discrimination functions- are characterized by a large peak spanning the phonemic boundary and a smaller peak spanning the subphonemic but systematically relevant phonetic boundary. The developmental implications of the experiments reported in Chapters 3 and 4 are discussed in Chapter 5.