ERPs to infrequent auditory stimuli in two- and three-stimulus versions of the inter-modal oddball task

Previous research has shown that auditory targets in inter-modal oddball conditions are processed in two stages, an early modality-specific and a later context-dependent stage. The aim of this study was to investigate how the processing of auditory target stimuli is affected by the inclusion of an additional infrequent auditory non-target stimulus in inter-modal oddball conditions. 17 subjects received 300 stimuli in each of a two- and a three-stimulus version of the inter-modal oddball task. Both tasks included 240 (80%) presentations of a counter-phasing checkerboard randomly interspersed with 60 presentations of an auditory stimulus. In the two-stimulus condition subjects received 60 (20%) target stimuli. In the three-stimulus condition subjects received 30 (10%) target and 30 (10%) non-target stimuli. ERPs to the two target stimuli and ERPs to targets and auditory non-targets in the three-stimulus condition were investigated. Early components N100 and P200 were larger to non-targets than targets. N140, an additional frontal negativity in inter-modal oddball conditions, was larger in the three-stimulus compared with the two-stimulus condition. Targets produced two late positive components (P250/P3a and P300). In the three-stimulus task the P250/P3a was enhanced frontally and P300 was enhanced in right parietal regions. Non-target auditory stimuli produced a broad negativity (N285) at this latency. All stimuli produced a late positivity (P350) that demonstrated task-related topographic differences. The results are interpreted with respect to previous inter-modal oddball findings and research into auditory processing.