Impaired Categorical Perception of Lexical Tones in Chinese Children with Autism: An Event-Related Potential Study

Background:  Previous research has shown enhanced pitch discrimination in individuals with autism in comparison with controls. While the same phenomenon was found for nonspeech stimuli in tonal-language speakers with autism, it did not generalize to the discrimination of lexical tones, which are primarily cued by pitch differences.

Objectives: It was unknown whether the abnormal pitch discrimination could be examined by categorical perception, and whether the distinct pitch processing pattern for speech and nonspeech stimuli in autism was due to a speech-specific deficit in categorical perception of lexical tones. Further it was unknown whether the categorical perception of autism could predict their pitch discrimination ability. The present study was designed to answer the above questions.

Methods:  The stimuli were chosen from a 10-step lexical tone continuum used in previous CP studies (Xi et al., 2010; Zhang et al., 2012), including speech and nonspeech context. Two groups of children participated in this study: 16 children with autism, and 15 typically developing controls matched on age and IQ scores. They all completed both behavioral part and ERP part. In the behavioral part, two classical paradigm of categorical perception—identification task and discrimination task—were used to test two groups of children’s perception of lexical tone in natural condition. We analyzed the accuracies of two tasks in both conditions. A passive oddball paradigm was adopted to examine two groups of Chinese children’s Mismatch Responses (MMRs) to equivalent pitch deviations representing within-category and between-category differences in speech and nonspeech contexts.We analyzed the ERP waveforms in each condition with a traditional approach, measuring the average MMR amplitude.To further examine group-level differences in the MMRs to categorical perception of speech/nonspeech stimuli or lack thereof, neural oscillatory activities at the single trial level were further calculated with the inter-trial phase coherence (ITPC) measure for the theta and beta frequency bands.

Results:  The behavioral results illustrated the categorical perception with autism was impaired in speech context rather than that in nonspeech context.The MMR results showed evidence for the lack of categorical perception in the lexical tone condition with an opposite trend in the harmonic tone condition for children with autism.The frequency data showed that the increased theta was induced in the between-category offering a clear phonetic identify for typically developing children in speech condition while a similar phenomenon was not found for children with autism. In the mean time, the correlation of behavioral data and ERP data was analyzed and it showed that the brain electrical signal could predict individuals’ behavior to some extent.

Conclusions:  All the data from the Chinese children with autism showed evidence for lack of categorical perception in the lexical tone condition. In view of the important role of lexical tones in acquiring a tonal language, the results point to the necessity of early intervention for the individuals with autism who show such a speech-specific categorical perception deficit.