Eliminating the P300 rebound in short oddball paradigms

In short oddball paradigms, the P3 (or P300) amplitude often shows a curvilinear "rebound" pattern, such that an initial drop during the first few trials is followed by an increase in later trials. While the initial attenuation has been attributed to habituation, no explanation has been offered for the later increase. Our hypothesis was that this curvilinear pattern is due to anticipation of the end of each fixed-length block. In study 1, a series of 16 short, fixed-length auditory oddball tests (with six targets each) replicated the rebound effect. In study 2, a further set of 16 variable-length blocks (7-10 targets) eliminated the rebound, and the P300 amplitude decreased linearly as expected. Results are interpreted in light of various constructs related to the P3: habituation, attentional salience, and updating in working memory.