The Effect of Prestimulus Alpha Activity on the P300

ABSTRACT Trials on which highly discrepant, auditory ‘oddball’ stimuli were presented were sorted into two bins on the basis of prestimulus alpha band RMS magnitude. The trial bins were then separately averaged to produce a ‘high alpha’ auditory ERP (event‐related potential) and a ‘low alpha’ ERP for each subject. Study 1 found that larger amplitude P300s were obtained in the ‘high alpha’ ERP. No effect of alpha was found on the N100. Study 2 employed extra factors of stimulus intensity change (increases and decreases) and alpha measurement period (before and after the ‘oddball’ stimulus). It was found that P300 amplitude enhancement was independent of both stimulus intensity and the amount of alpha poststimulus. The data are discussed in terms of cascaded inhibition from the mesencephalic reticular formation to nucleus reticularis of the thalamus to a thalamo‐cortical system responsible for the generation of both alpha and the P300.