Top-down and Bottom-up Processing in Auditory Stream Segregation for Individuals with Autistic Spectrum Disorder

Abstract

The Weak Central Coherence Theory of Autism argues that bottom-up processing is intact and top-down processing is compromised in autistic individuals. Conversely, the Enhanced Perceptual Functioning Theory argues that bottom-up processing is enhanced in Autism and that top-down processing is intact. The primary aim of this research was to investigate top-down and bottom-up processing with regards to auditory stream segregation in Autism. For the first experiment, autistic individuals were matched to healthy, normal hearing participants according to age, gender and handedness. Lateralized and centralized dichotic pitch were used as stimuli and an electroencephalogram was used to measure auditory event-related potentials. Bottom-up processing was indexed by the ORN whereas top-down processing was indexed by the P400 effect. The results showed that autistic participants had a limited ability for detecting dichotic pitch stimuli, indicating that auditory stream segregation ability is compromised for autistics. The ORN was absent for autistic participants indicating that bottom-up processing is impaired in autism. Similarly, the P400 effect was reduced for autistic participants, suggesting that top-down processing is compromised. The analysis also revealed a diminished capacity for location-specific processing for autistic participants. A second experiment, designed to investigate if there were differences in ERP components of autistic individuals who could and who could not hear dichotic pitch, found that both groups displayed similar ERPs. These results do not support the Weak Central Coherence Theory or the Enhanced Perceptual Functioning Theory. Both theories are primarily based on visual processing and this research highlights the importance of conducting auditory studies to investigate deficits in top-down and bottom-up processing in Autism.