Integral perception, but separate processing: The perceptual normalization of lexical tones and vowels

In tonal languages, speech variability arises in both lexical tone (i.e., suprasegmentally) and vowel quality (segmentally). Listeners can use surrounding speech context to overcome variability in both speech cues, a process known as extrinsic normalization. Although vowels are the main carriers of tones, it is still unknown whether the combined percept (lexical tone and vowel quality) is normalized integrally or in partly separate processes. Here we used electroencephalography (EEG) to investigate the time course of lexical tone normalization and vowel normalization to answer this question. Cantonese adults listened to synthesized three-syllable stimuli in which the identity of a target syllable — ambiguous between high vs. mid-tone (Tone condition) or between /o/ vs. /u/ (Vowel condition) — was dependent on either the tone range (Tone condition) or the formant range (Vowel condition) of the first two syllables. It was observed that the ambiguous tone was more often interpreted as a high-level tone when the context had a relatively low pitch than when it had a high pitch (Tone condition). Similarly, the ambiguous vowel was more often interpreted as /o/ when the context had a relatively low formant range than when it had a relatively high formant range (Vowel condition). These findings show the typical pattern of extrinsic tone and vowel normalization. Importantly, the EEG results of participants showing the contrastive normalization effect demonstrated that the effects of vowel normalization could already be observed within the N2 time window (190–350 ms), while the first reliable effect of lexical tone normalization on cortical processing was observable only from the P3 time window (220–500 ms) onwards. The ERP patterns demonstrate that the contrastive perceptual normalization of lexical tones and that of vowels occur at least in partially separate time windows. This suggests that the extrinsic normalization can operate at the level of phonemes and tonemes separately instead of operating on the whole syllable at once.