Novel distracter stimuli in a 3-stimulus oddball task evoked a novelty P3 potential with a more frontocentral topography than the parietal-maximum P3b potential to target stimuli (Friedman et al., 2001; Polich, 2007; Spencer et al., 1999). While depressed patients showed overall reduced P3 amplitude when compared to healthy controls, the size of this reduction was larger for novel than target stimuli. The novelty P3 was significantly reduced in depressed patients, whereas the parietal P3b to target stimuli was only marginally reduced in these patients. The effect size at the midline parietal site was relatively large for novelty P3 (Cohen's d = 0.85), but smaller for target P3b (0.60). The marginal size of the group difference in P3b to targets may account for why some studies have found reduced P3 amplitude in depressed patients, while others have not. Prior studies failed to differentiate P3 subcomponents and most used a two-stimulus oddball task, which yields a weaker frontal P3a than the novelty oddball task (Polich and Criado, 2006). We have previously argued that simple oddball tasks are not challenging enough to elicit
