Chapter Three - Processing of Non-Speech Auditory Stimuli in Individuals with Autism Spectrum Disorders: The Impact of Stimulus Characteristics

Several theoretical models propose a dissociation in the processing of simple versus complex sensory stimuli in autism spectrum disorder (ASD), with simple stimuli resulting in intact (or enhanced) processing and complex stimuli resulting in processing impairments. In the auditory domain, atypical neural and behavioral responses to complex speech stimuli in ASD are reported relatively consistently. However, whether processing of more simple, nonspeech auditory stimuli is, in fact, intact in ASD remains less clear. Thus, this review sought to clarify how well the hypothesized simple-complex stimulus processing dissociation fits existing data in the auditory domain. To do this, the review focused exclusively on the processing of nonspeech stimuli. Specifically, we examined whether specific acoustic properties are associated with atypical processing in ASD. The review highlights that, whereas pitch processing has been studied often in ASD, there is a relative paucity of literature examining the impact of intensity, timing, and filtering demands on the integrity of auditory processing. Though reviewed studies offer substantial evidence for intact behavioral performance on pitch-related tasks, neurophysiological studies reveal atypicalities in the neural correlates of pitch processing. Moreover, this review found limited support for the oft-cited pitch processing enhancement in ASD. In contrast, responses to increasing stimulus intensity appear abnormal in ASD, as do responses indexing encoding and perception of timing aspects of auditory stimuli. Individuals with ASD also appear to have marked difficulty in filtering competing auditory stimuli. Taken together, findings reviewed here yield substantial evidence indicating that nonspeech auditory stimuli often elicit aberrant neural and behavioral responses in individuals with ASD. Further, these deficits encompass both pure tone and more spectrally complex stimuli. Additional research regarding auditory processing outside of the speech domain is clearly needed.