The Role of the Motor System in Speech and Language in Autism

Abstract

Communication challenges in autism are often attributed to social and cognitive factors, but emerging research suggests that motor processes also play a critical role. Despite growing interest in this area, the brain and behavioral underpinnings of speech-motor differences in autism, and their relationship to language, remain poorly understood. The present dissertation examined how motor system differences in the brain contribute to speech and language challenges in autistic individuals who use speech to communicate. This work integrated studies that bridge brain and behavior, while also incorporating the perspectives of autistic people. The first study examined how three key language-relevant brain networks – speech perception, working memory, and speech production networks – related to language challenges in autistic children. FMRI results revealed that autistic children showed atypical and reduced engagement of the speech production network, but not more traditional language processing regions, during nonword repetition. This atypical motor engagement correlated with poorer task performance, higher autism traits, and lower social communication skills. The second study built on this finding to explore how motor system differences impact speech and language in autism beyond nonword repetition. Research in nonautistic individuals, including lesion-based work, has suggested the importance of key motor regions in both the production and perception of prosody. Thus, we turned to prosody as an area of language that may be impacted by brain-based motor differences. Prosody – the use of pitch, duration, and loudness in speech – plays a key role in communication, and atypical prosody is commonly noted in autism. Study two revealed that autistic individuals reported greater challenges with prosody, and that these challenges were strongly correlated with autism traits in both autistic and nonautistic adults. Acoustic analyses showed that the autistic participants produced speech with atypical timing, with longer pauses and a slower speech rate, during a spontaneous picture description task. Notably, diadochokinetic and articulation rate did not differ between groups, suggesting that these prosody differences were unlikely due to an underlying speech-motor disorder (e.g., apraxia of speech), but may stem from broader motor system differences. The third study further examined the motor-based brain differences in autism, by first precisely localizing each participants’ speech production regions, and then examining how these functionally-defined regions were engaged during prosody perception. Results from this fMRI study revealed that nonautistic individuals engaged several of the same functional regions for speech production as prosody perception (i.e., listening to sentences with dynamic versus flat pitch contours). In contrast, autistic individuals showed a reduced activation in these regions. Significant group differences emerged across several motor regions, including the supplementary motor area. Moreover, activation in motor regions during prosody perception was positively correlated with participants’ accuracy on an out-of-scanner prosody perception task, highlighting the behavioral relevance of this motor activation. Taken together, these studies provide evidence that atypical engagement of the motor system contributes to language differences in autism. By reframing language differences in autism through the lens of speech motor system function, this dissertation offers new insights into the neurobiological mechanisms underlying language challenges. These findings lay a foundation for future research aimed at supporting autistic communication.