Abstract: Three modifications of the paradigm of visual stimulus-to-sample comparison were used to study mental operations under the conditions of sensory repetition and mismatch. Ninety-seven healthy adult subjects took part in the studies, where event-related potentials (ERPs) were recorded and analyzed in 31 channels in the current source density (CSD) montage. It is has been shown that the repetition of stimuli in an active, response-demanding test construction induces the N250 component localized in posterior temporal derivations (T5, T6) and associated presumably with synaptic habituation. On the contrary, a mismatch in physical characteristics of a stimulus induces the positive component P2 localized in the same derivations and presumably associated with a recurrent income of information from higher to lower cortical areas. Similar correlates of repetition/mismatch are also observed in a passive, not response-requiring test construction, but the amplitude of the components in the passive construction was lower.