Auditory processing in an inter-modal oddball task: effects of a combined auditory/visual standard on auditory target ERPs.

Previous research examining auditory target ERPs in an inter-modal oddball task has shown that early components are affected by intra-modal processes, whilst components in the later part of the ERP, around 200-400 ms, are affected by both inter-modal and intra-modal processes. These findings led to the conclusion that there are separate stages of auditory target processing--an early modality-specific stage and a later context-dependent stage. The present study investigated this further by simultaneously presenting a visual standard stimulus with an auditory standard stimulus in an oddball task. The aim was to determine whether the inclusion of the visual standard stimulus in this task affected the ERP to targets. The auditory-visual oddball task consisted of a regularly presented combined auditory and visual standard stimulus (80%) and an infrequent auditory target (20%). The ERPs to targets in the auditory-visual oddball task were compared to those in an auditory oddball task, which had identical auditory stimuli and no visual standards. The results showed that the early components N100, P200 and N200 did not differ between tasks. This was in line with earlier results, and confirmed that activity up to 200 ms is unaffected by visual standard stimuli. The later components P250, P300 and P350 were larger and showed topographic differences in the auditory-visual oddball task. This was interpreted as reflecting separate inter-modal and intra-modal processes at later stages. In particular, the P300 and P350 components were argued to represent separate inter-modal and intra-modal components. Overall, this study provides further evidence of auditory processing occurring in two stages, an early modality-dependent stage and a later context-dependent stage.