Neuromagnetic studies of visual and auditory infrequent target detection tasks

Many electrophysiological studies of infrequent target detection have been performed using ‘oddball’ paradigm in which subjects were required to detect and respond to infrequent target events embedded in a series of repetitive (frequent) events. The oddball tasks have been known to elicit prominent component of event-related potentials (ERPs) called P3, which occurs about 300 ms after the infrequent stimulus onset [1][2]. The P3 is thought to reflect attentional resource allocation when working memory [3] is engaged [4]. These components have been widely used to investigate cognitive brain responses related to novelty, uncertainty, attention, and memory [2], however, the specific brain regions involved in the generation of this component and the modality dependency of these generators were largely unknown. Recent functional magnetic resonance imaging (fMRI) studies have shown that there were multiple brain regions including frontal, parietal and temporal regions, which were either modality specific or modality independent, involved in the detection of infrequent stimuli [5][6][7]. Because these studies have visualized the brain activation in terms of hemodynamic changes during the task execution relative to the control condition, temporal resolution of the results could not exceed a few hundred milliseconds. In the present study, to clarify the spatio-temporal characteristics and the modality specificity of the neural activities in the human brain related to the detection of infrequent target stimuli, which are presented to either visual or auditory sensory modality, neuromagnetic measurements were performed.