Speech perception and auditory processing in autism

Prior to the process of assigning of words to their meaning, the bases for lexical knowledge involve speech perception and segmentation. Compelling results on the topic were found in several studies relating lexical delays in autism with their enhanced sensitivity to auditory contrasts. Jones et al. (2009) tested 72 adolescents with autism (M age = 15;6) and 48 age- and IQ-matched controls (M age = 15;6) on their ability to discriminate the differences in frequency, intensity, and duration in pairs of sounds. No difference was found between the two groups. However, they found that a subgroup of 20% of the adolescents with ASD performed exceptionally well on frequency discrimination task with the score of 1.65 SDs above the mean of the control group. Additionally, these individuals with ASD had average IQ but a history of delayed first words. The results from this study are consistent with other reported findings that pitch discrimination ability is enhanced in children, adolescents, and

adults with ASD with a history of delayed language onset (Bonnel et al., 2010; Eigsti and Fein, 2013).

While the above studies involve non-speech stimuli such as pure tones in their experi- ments, various studies compared between speech and non-speech stimuli to explore whether auditory sensitivity is limited to non-speech stimuli and whether there is a speech-specific deficit in categorical perception or not. Wang et al. (2017) employed lexical tone linguistic stimuli and harmonic non-speech stimuli in an ERP study with 16 Mandarin-speaking chil- dren with autism (M age = 10.4) and 15 TD controls (M age = 10.3). A 10-step lexical tone continuum was created for monosyllabic speech stimuli. Out of the 10 levels, three tones were chosen at Level 1, 5, and 9 to create a within-category pair (1 and 5) and a between-category pair (5 and 9). The two categories correspond with Tone 2 and Tone 4 in Mandarin. Non-speech stimuli were generated to match in fundamental frequency, am- plitude, and duration to the speech stimuli. A total of 600 stimuli were presented using a passive oddball paradigm. Participants were instructed to watch a muted movie and ignore the presented auditory stimuli. The children with ASD were found to elicit significantly different Mismatch Response amplitudes between within-category and between-category conditions only in non-speech contexts, but not in speech contexts. The TD group, on the other hand, showed significantly different amplitudes between the two conditions in both speech and non-speech contexts. Similar ERP studies also investigated the children with ASD’s neural sensitivity to lexical tones and pure tones. Yu et al. (2015) found that Mandarin-speaking children with autism (6-12 years) have enhanced neural sensitivity in the non-speech (pure tones) condition, but not in the speech (lexical tones) condition. Sim- ilarly, Zhang et al. (2019) compared between the pitch perception of speech and non-speech stimuli in Cantonese-speaking children with ASD (n = 16, M age = 10;5) and their TD controls (n = 16, M age = 9;6). They found that the TD controls showed stronger mis- match negativity responses than the children with ASD in the lexical tone condition. While they did not find enhanced neural sensitivity to non-speech stimuli in the ASD group as in Yu et al. (2015), they found similar mismatch negativity responses between the ASD and

TD groups. The results of these above studies demonstrate that the children with ASD have speech-specific categorical perception deficit, since non-speech stimuli yielded either normal or enhanced responses.

You et al. (2017) extended the research from pure tones and lexical tones to vowel and consonant continuum in French-speaking TD children (n = 19, M age = 10;4), children with ASD with normal language (n = 6, M age = 10;10), and language-impaired children with ASD (n = 10, M age = 10;6). In a categorical identification task, the children were asked to indicate which vowel (/i/ or /y/) or consonant (/b/ or/d/) they heard. In the categorical discrimination task, they were asked to answer whether two sounds they heard belonged to the same category or not. The ASD group performed significantly worse in the categorical identification task, but not in the categorical discrimination task. The authors then concluded that children with ASD struggle with categorical precision, rather than categorical perception, which may have not been teased apart in the earlier ERP studies. Huang et al. (2018) conducted an ERP study on vowel duration, which is not phonological in Mandarin, and pure tones with school-age Mandarin-speaking children with autism and TD children. Contrary to the previous studies, the deficit in the mismatch negativity responses is not restricted to speech signal. In fact, they found diminished mismatch negativity responses in pure tone condition, but not in the vowel length condition. This leads to their conclusion that the speech-specific deficits may be restricted to phonemic contrasts, instead of allophonic contrasts such as vowel length in Mandarin. While these two studies provide interesting remarks on the nature of categorical perception deficits in children with autism, more research is needed to reach a clear conclusion.

As for studies on adolescents and adults with ASD, Bonnel et al. (2003), among others, also found enhanced pitch sensitivity in non-speech stimuli. Chiodo et al. (2019), on the other hand, observed no reduced performance on the perception of speech stimuli in either the group of adults with ASD with or without a history of speech onset delay. Stewart et al. (2017) found categorical perception of the linguistic voice onset time contrast (/g/ vs /k/) to be on par between ASD and TD adults. Additionally, it was found to be correlated

with their ability to read, lexical decision task performance, and verbal IQ.

All in all, a large group of studies observed categorical perception deficits in linguistic stimuli, but not in non-linguistic stimuli. The nature of categorical perception deficits and the improvement in their adulthood remain to be explored by future studies.