Unequal Effects of Speech and Nonspeech Contexts on the Perceptual Normalization of Cantonese Level Tones

Context is important for recovering language information from talker-induced variability in acoustic signals. In tone perception, previous studies reported similar effects of speech and nonspeech contexts in Mandarin, supporting a general perceptual mechanism underlying tone normalization. However, no supportive evidence was obtained in Cantonese, also a tone language. Moreover, no study has compared speech and nonspeech contexts in the multi-talker condition, which is essential for exploring the normalization mechanism of inter-talker variability in speaking F0. The other question is whether a talker's full F0 range and mean F0 equally facilitate normalization. To answer these questions, this study examines the effects of four context conditions (speech/nonspeech × F0 contour/mean F0) in the multi-talker condition in Cantonese. Results show that raising and lowering the F0 of speech contexts change the perception of identical stimuli from mid level tone to low and high level tone, whereas nonspeech contexts only mildly increase the identification preference. It supports the speech-specific mechanism of tone normalization. Moreover, speech context with flattened F0 trajectory, which neutralizes cues of a talker's full F0 range, fails to facilitate normalization in some conditions, implying that a talker's mean F0 is less efficient for minimizing talker-induced lexical ambiguity in tone perception.