Neural Underpinnings of Prosody Processing in Autism

Background: Prosody, defined as changes in the pitch (fundamental frequency), intensity (amplitude), and duration of speech, is central to the deficits in social communication that characterize individuals with autism spectrum disorders (ASD). Despite its clinical significance, the comprehension of prosody has not been extensively investigated using structured empirical designs. Furthermore, little is known about the neural mechanisms that underlie this important communicative cue. Objectives: This study aims to illuminate the mechanisms that underlie the perception of prosodic cues to both grammatical and emotional qualities in speech. Methods: Adolescents with ASD (n = 4 to date, ages 9 -17 years) and age-, gender-, FSIQ-, and CELF core language-matched typically developing controls (TD; n = 4) completed a implicit prosody perception task while their brain activity was recorded using functional magnetic resonance imaging (fMRI). Prosodic stimuli included emotional (angry, neutral) and grammatical (question, statement) versions of semantically simple sentences (e.g., It is five oclock; She is typing fast). Participants gave a yes/no response, judging whether each sentence was about a living creature; thus, their explicit task was to make a semantic judgment about the sentence. Prosodic sentences were recorded using natural speech, matched on stimulus duration across affective and grammatical manipulations. Statements and questions differed in pitch contour (rising versus falling); angry and neutral stimuli differed in fundamental frequency (high versus medium) and pitch range (large versus medium). Stimuli were presented in 6 runs in a block design, where each run included two blocks of each of the four conditions (emotional prosody: angry X neutral; grammatical prosody: sentences X questions) as well as an auditory attention control task (detecting a beep) and a rest condition. fMRI data were collected at 3.0 Tesla and image processing was performed with BrainVoyager. Results: Behavioral response data indicated that adolescents with ASD were as accurate as controls overall in judging the animacy of stimuli (72% vs. 86%, p = .32). There was a trend towards a group difference in effects of emotional prosody, with sentences in neutral conditions judged less accurately than those in angry conditions by the ASD group (.77 vs. .68, group by condition interaction, p = .09), possibly reflecting a speed/accuracy tradeoff, as the neutral condition response times tended to be shorter. FMRI data suggest a more diffuse and bilateral pattern of activation in response to prosodic contrasts in participants with ASD. In addition, the contrast between emotional prosodic conditions elicited a smaller difference in brain activation for ASD relative to TD participants. Conclusions: This implicit prosody perception task provides a measure of whether participants with ASD engage typical brain networks when passively hearing prosodic cues. While behavioral data indicated that participants were processing the language stimuli with similar attentional engagement, patterns of brain activation suggested a less lateralized pattern of activity in response to acoustic cues to prosody. This difference in neural activity is consistent with previous findings for other language-related tasks, and suggests that brain organization for processes related to language in ASD may be less specialized.