Year

2011

Degree Name

Doctor of Philosophy

Department

School of Psychology

Abstract

This dissertation investigated auditory processing in an inter-modal oddball task. It represents a systematic investigation of ERP activity in inter-modal oddball conditions. The inter-modal oddball task utilises an oddball methodology which includes frequent visual standard stimuli (80%) and infrequent auditory targets (20%). The experiments presented herein showed that auditory targets in this task produce seven ERP components (N100, N130/N140, P200, N220, P250, P300, P350). The N100 was consistent with the traditional N1 and demonstrated refractory and frequency effects. The N130/N140 component had a topography that was consistent with the T-complex, but also showed evidence for activation in frontal regions and was modulated frontally by task requirements. The P200 was a large vertex component that was differentially modulated by response requirements. These early components were shown to be unaffected by the inclusion of the visual standard stimuli in the inter-modal oddball condition. The N220 component was a small negative going wave which was considered to be indicative of the omission of the visual standards at the time of auditory target presentation. The P250 component was consistent with the P3a, was experimentally modulated at frontal sites and showed difference in amplitude and topography as a result of task requirements. The P300 was consistent with the P3b, and was produced by auditory targets presented amidst visual standards. This was somewhat different to the P3b produced when targets are presented amidst auditory standards. The P350 represented a new late positivity that reflected the processing of inter-modal stimuli. It was suggested that it might represent a process of inter-modal feedback from frontal processing regions to the auditory cortex. Overall, these data suggest that ERPs in inter-modal oddball conditions are processed in two stages – an early stimulus dependant stage and a late context dependent stage. They also suggest that there were separate top-down and bottom-up processes evident in the ERP. It was concluded from these experiments that each ERP component was produced in specific cortical regions and represented distinct aspects of auditory target processing. The evidence provided within this dissertation suggests that the inter-modal oddball task provides a new and novel means of understanding the processing of auditory stimuli and that this task might have advantages over traditional techniques that investigate auditory processing as it does not, by in large, suffer from the confounds related to component and process overlap that is evident in other tasks.

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