Early N1P2 and late N2P3 responses generated in an auditory oddball paradigm are topographically compared in three psychiatric patient groups. In schizophrenia N1 and N2 amplitude is comparable with dementia and significantly decreased with respect to affective disorder. In contrast, P3 amplitude does not allow discriminating schizophrenia from affective disorder but is significantly diminished in dementia. The late N2P3 response shows a topographic effect along the fronto-occipital axis. Schizophrenia is characterized by an iCNV and N2 maximum over the frontal planes and a compound P3 lacking distinct frontal and parietal components. The findings are discussed in reference to literature data and current hypotheses/theories concerning information processing. Our findings favour an important dysfunction of automatic processing including early selection in schizophrenia.