Impaired perceptual normalization of lexical tones in Cantonese-speaking congenital amusics.

Human listeners perceive speech sounds relative to acoustic cues in context. In this study the authors examined how congenital amusia, a pitch-processing disorder, affects perceptual normalization of lexical tones according to the distribution of 0 cues in context. Sixteen Cantonese-speaking amusics and 16 controls were tested on the effects of shifting 0 level in four types of contexts on tone perception: nonspeech, reversed speech, semantically anomalous speech, and meaningful speech contexts. Performance of controls replicated previous studies, showing contrastive changes of tone perception according to the shifted 0 level of anomalous and meaningful contexts, which were native speech contexts with phonological cues to estimate a talker's tone space. Effects of nonspeech and reversed contexts were small and inconsistent, and tone perception performance varied depending on the typicality of a talker's 0 range. In contrast to controls, amusics showed reduced context effects in anomalous and meaningful contexts, but largely comparable context effects in nonspeech and reversed contexts, indicating a deficit of amusics in tone normalization through phonological cues in native speech contexts. These findings suggest that the ability to perceive speech sounds relative to acoustic cues in context is not a universal endowment, and that this ability is impaired substantially in amusics.