Spatiotemporal Electrophysiologic Responses to Linguistic Stimuli in Children with Low Language and Cognitive Ability
Autism Spectrum Disorder (ASD) is a heterogeneous neurodevelopmental disorder often including impairment to language ability. Previous work investigating auditory speech processing with magnetoencephalography (MEG) has found event related desynchronization (ERD) in auditory cortex between 5-20Hz sensitive to a variety of linguistic factors such as semantic congruity, word frequency and word repetition. Recent work suggests that, in higher functioning children with ASD, ERD following linguistic stimuli is correlated with language ability.
In order to better understand the neural responses to language stimuli in the approximately 30% of the ASD population categorized as minimally-verbal/non-verbal (MVNV), we used MEG to measure ERD of the auditory cortex in response to linguistic stimuli (Words, NonWords, ReversedWords) in two cohorts: 1) MVNV children with ASD, 2) lower Intelligence Quotient (IQ) Intellectual Disability/Developmental Disability (ID/DD, non-ASD) clinical controls.
Nineteen participants (aged 8-12yrs) were included in the final analysis (MVNV children with ASD, n=12, 9.6±1.1yrs; lower IQ ID/DD (non-ASD) clinical controls, n=7, 10.7±1.4yrs). MEG data were obtained using a 275-channel whole-cortex CTF magnetometer (CTF MEG, Coquitlam, Canada). Subjects listened to three types monosyllabic stimuli with different linguistic properties (100 each type): words (low age of acquisition), pronounceable nonwords, and acoustically reversed words. Bilateral ERD of the auditory cortex was measured between 5-20Hz and 200-1650ms. Cognitive ability was assessed using the Nonverbal IQ (NVIQ) of the Leiter-3. Language ability was assessed using the Peabody Picture Vocabulary Test 4th edition (PPVT).
No significant group differences in age or NVIQ were observed (p>0.05), while significantly lower PPVT scores (p<0.01) were observed in MVNV children with ASD (33.0±11.0) compared with the ID/DD group (60.4±10.3). Higher NVIQ was significantly correlated with higher PPVT scores (R2=0.27; p<0.05). Linear mixed models with subject entered as a random effect found significantly less ERD in MVNV+ASD children compared to ID/DD (p<0.01) and a significant interaction between hemisphere and group. No main effects of age, hemisphere, stimulus type, or other interactions (ps>0.1) were observed. Post-hoc analysis of each hemisphere found significant (p<0.01) bilateral decreases in ERD in MVNV+ASD children with a larger magnitude difference in the Right Hemisphere (Left: ID/DD 8.2±3.3; MVNV+ASD 5.7±1.9 Right: ID/DD 9.5±3.3; MVNV+ASD 5.2±1.6). Hierarchical regression was used in an attempt to disentangle the effects of group and PPVT. When entered first, both variables explained significant amounts of variance (group 42%; PPVT 40.8%; ps<0.01). When entered second neither variable explained significant amounts of variance (group 5.7%, PPVT 4.1%; ps>0.1).
These findings suggest a correlation between the ERD and the behavioral classification of MVNV in children with ASD. These children exhibited little to no expressive linguistic ability, and their brain responses show little of the ERD previously linked to processing linguistic input. As suggested by hierarchical regression, the observed decreases in ERD in this study could be due to either ASD or poor language ability. Future work will potentially disambiguate these effects. Finally, the fact that no differences were observed between stimuli types in any group suggests that semantic differences between word types are processed outside of early auditory cortex.