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Purpose: We sought to characterize back-ground cortical activity related to sequential sensory stimuli during reaction tasks. It should be considered that the back-ground cortical activity modulate the brain potentials and reaction times. The purpose of the study is to determine whether or not the event related potentials and synchronization I desynchronization (ERS/ERD) show the changes in orienting sensory modality. Method: we recorded electroencephalogram (EEG) from eleven healthy subjects who received visual and auditory stimuli and performed reaction tasks. The conditions of stimuli presentation consist of single, compound and multi modality condition. Visual and Auditory evoked potentials (YEP and AEP) were obtained and Multi band filter analysis (MBFA) was used to calculate EEG spectral power to analyze ERS/ERD; Results and Discussion: We found that, relative to single modality stimuli, the reaction times were increased under the compound stimuli condition, and decreased under the multi modality condition. The ERP obtained by multi modality stimuli showed integrated component and the ERS/ERD showed the different changes in single and multi modality stimuli. By correlating ERS/ERD and reaction times, we identify the back-ground cortical activity that related to orienting to sensory stimuli accompanying reaction tasks. [ABSTRACT FROM AUTHOR]