STIMULUS CONTEXT DETERMINES P3A AND P3B

P300 differences for target (.10), nontarget (.10), and standard tones (.80) were assessed using a three-stimulus oddball paradigm in which participants responded only to the target (n = 12). Target/standard (e asy or difficult) and nontarget/standard (large or small) pitch differ ences were manipulated orthogonally. In all conditions, target tones e licited a parietal P300, which was affected only by the target/standar d discrimination ease. Nontarget in the easy/large and difficult/small conditions elicited a parietal but smaller P300 than the target but i n the easy/small condition elicited similar ERPs to the standard. Howe ver, nontarget stimuli in the difficult/large condition elicited an an terior maximum and earlier P300 (P3a) component. The findings suggest that target P300s are not influenced by the nontarget stimulus configu ration, whereas the nontarget P300 outcomes are determined directly by the stimulus context. The theoretical implications are discussed.