The N1 and N2 components did not differ between depressed patients and controls, which indicates that their reduced novelty P3 is unlikely due to an earlier deficit in auditory processing or detection of deviant novel stimuli. The novelty P3 reduction may indicate a deficit in automatic orienting of attention and evaluation of novel environmental sounds. Both ERP and fMRI studies suggest that frontal mechanisms are involved in orienting of attention to novel stimuli (for review, see Polich and Criado, 2006). Frontal cortex, and particularly the anterior cingulate, is of key importance for attentional processing, and has been found to be dysfunctional in depressed patients during a variety of cognitive tasks (Bremner et al., 2004; Drevets et al., 1997; Siegle et al., 2004). The P3a potential to novel stimuli is markedly reduced by prefrontal lesions (Knight et al., 1998). Intracranial recordings have found evidence for generators of novelty P3 in multiple areas, including frontal and posterior association cortex, hippocampus and anterior cingulate cortex (Halgren et al., 1995). ERP studies using source localization techniques have localized generators of novelty P3 to anterior
