An investigation of concurrent segregation abilities in autistic spectrum disorder utilizing the dichotic pitch and mistuned harmonic paradigms
Research using the dichotic pitch and mistuned harmonic paradigms has identified neurophysiological markers for concurrent sound segregation (the Object Related Negativity [ORN], N2 difference and P400). The aim of this thesis was to determine whether these components are elicited, and if they differ in amplitude, for individuals with autistic spectrum disorder (ASDs) and typicals. Experiment 1 found ASDs elicited a diminished ORN compared to typicals for time-shifted dichotic pitch, indicating low-level deficits in ASD. Other stimuli that elicit these components were used in Experiment 2 (mistuned harmonics). The addition of the interaural time difference (ITD) cue to mistuning increased the ORN amplitude in both groups. An ORN was elicited by ITD alone for Experiment 2. Experiment 3 revisited the dichotic pitch stimulus and utilized ITD and interaural amplitude difference (IAD) cues separately. Both groups elicited an ORN for the IAD dichotic pitch. Again ASDs elicited a diminished ORN for the ITD cue. This indicates that Gaussian noise is not the cause of the diminished ORN for ASDs. Experiment 4 explored the effects of separate and combined ITD and IAD cues in dichotic pitch on the ORN amplitude. The ITD cue did not increase the ORN amplitude for ASDs. ASDs only elicited an ORN when the ITD was lateralized to the opposite side of the IAD. This indicates that the ITD interferes with the ability of the ASDs to parse concurrent auditory objects. ASDs also elicited a diminished N2 difference for ITD and IAD dichotic pitch. This might suggest that the ASDs have difficulty with processing spatial information. An N2 difference was not found for mistuned harmonic stimuli. ASDs showed P400s for ITD dichotic pitch and mistuned harmonic stimuli. This indicates that controlled processes in ASD are intact for concurrent segregation. A diminished P400 was elicited by ASDs, compared to typicals, for the ITD and IAD cues in dichotic pitch (Experiment 3). This may indicate controlled processing difficulties with multiple cues for ASDs. Main findings indicate that ASDs have a low-level processing deficit in processing ITD cues but not for other cues for concurrent sound segregation. Controlled processes are intact in ASDs for concurrent sound segregation.
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