Stop voicing in thai after unilateral brain damage

Abstract

Word-initial bilabial stops (/b p ph/), alveolar stops (/d t th/), and velar stops (/k kh/) in Thai were produced by 20 normal adults (10 young, 10 old), 12 non-aphasic patients with unilateral right hemisphere lesions, and 17 aphasic patients with unilateral left hemisphere lesions (nine fluent, eight non-fluent). Voice onset time (VOT) was measured from broad-band spectrograms. Relative frequency distributions of VOT values indicated minimal or no overlap between homorganic stop consonants for normal speakers, only sporadic occurrences of overlap for right hemisphere speakers, slightly more overlap for left fluent speakers, and even more overlap for left non-fluent speakers. By group, mean VOTs of homorganic stop consonants differed for all groups except /d t/ for the left non-fluent. By voicing category, mean VOTs of /b d/ for the left non-fluent group were shorter than those of other groups, and mean VOT of /k/ was longer than those of other groups. Mean VOTs of /ph th kh/ for the old normal group were shorter than those of the young normal and left hemisphere groups. Variability in VOT for the left non-fluent group was larger than that for normal groups only. Though not satistically significant, the left fluent and right hemisphere groups were also more variable in VOT production than normal speakers. Across groups, variability in VOT was smaller for voiceless unaspirated stops, larger for voiced unaspirated, and intermediate for voiceless aspirated stops. Findings are discussed in relation to issues pertaining to phonetic deficits in non-fluent aphasic patients.