Event probability has been traditionally regarded as the major determinant of P3b amplitudes, with amplitudes increasing when stimuli are less likely. Here we show in a simple variant of the continuous performance task that this "oddball effect" does not universally apply. Stimuli were a continuous series of (A or B) -> (X or Y) pairs, with the letter X requiring a key-press response and occurring in 80% of trials after A and in 20% after B (vice versa the Y). P3b amplitudes were equally large with probable and improbable occurrence of X. This was in contrast to visual Mismatch Negativity which was consistently larger with less probable stimuli, and also in contrast to no-go P3 amplitudes, which were larger with improbable than probable Y. The only effect on P3b amplitude was due to stimulus onset asynchronies (SOA): P3b was larger with SOAs of 2000 ms compared to 1500 ms. This result dovetails with previous evidence in the oddball task that the main determinant of the oddball effect is not event probability but rather time interval between stimuli. The absence of probability effects on P3b was in sharp contrast to the presence of these effects on no-go P3. Implications are discussed for theories about the psychological meaning of the P3b component.
Keywords: EEG; Event probability; P3b; Visual MMN.
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