Hemispheric Differences in Auditory Complexity Processing in ASD

Background:  Autism Spectrum Disorder (ASD) can be accompanied by delays in speech acquisition and a puzzling combination of auditory abilities, with enhanced pitch processing abilities often coexisting with atypical spoken language processing, particularly involving prosody.   Speech has complex auditory characteristics and there is evidence for hemispheric specialization of auditory processing, with temporal processing more lateralized to the left hemisphere and spectral processing to the right.  It is possible that some aspects of autistic individuals’ atypical auditory behavior results from lateralized differences in complex sound processing in both primary and non-primary auditory cortex.

Objectives:  N/A

Methods:  We employed fMRI to explore the neural basis of complex sound processing in 12 ASD and 15 typical participants, ages 16-30.  Participants performed a simple visual discrimination task while hearing 7 sec trials of parametrically varying complex auditory stimuli with a random spectrogram that had 5 levels each of spectral and temporal complexity.  These stimuli are noise-like and do not resemble speech sounds.  We used an accelerated multiband (MB) echo planar imaging (EPI) pulse protocol providing both high temporal and spatial resolution, thus enabling more accurate measurement of functional responses.  Single subject fixed effect regression models were used to compute parameter estimates for each of the 25 spectral/temporal sound complexity combinations.  Head motion covariates and motion outlier exclusion were used to minimize the effects of participant head motion. Group mixed effects models were used to identify group and groupXtask interaction effects.

Results:  Both groups showed strong bilateral activity modulations with increasing spectral complexity in the planum temporale (PT; secondary auditory cortex), and transverse temporal gyri (TTG; primary auditory cortex).  Increasing temporal complexity had a weaker modulatory effect and was only seen in the left PT.  Averaging across all levels of spectral and temporal complexity, ASD participants exhibited higher activity in the PT and TTG, with the effect larger on the left than the right. 

Conclusions:  While we observed similar spatial patterns of hierarchical functional organization for auditory processing in both groups, the ASD participants exhibited higher task-related activity in left primary and non-primary auditory cortex when listening to complex sounds.  This lateralized difference was largest in auditory cortex lateral to TTG. Greater auditory stimulus complexity effects in regions sensitive to acoustic temporal features could represent atypical left hemisphere processing of complex auditory stimuli in ASD, possible related to atypicalities in spoken language processing.