Stimulus deviance and evoked potentials.

In many studies on the effect of selective attention and stimulus significance on evoked potentials, the target, or otherwise significant, stimuli were also infrequent stimuli. Repetitive homogeneous auditory stimuli were presented at short constant intervals. One of two deviant stimuli, one slightly lower and the other slightly higher in pitch than the repetitive stimulus ('standard'), randomly replaced it at the same low probability. One at a time of these two physically equally deviant and equally infrequent stimuli was designated as a target and the subject's task was to count its number in a run and to ignore the other deviant stimulus. The two deviant stimuli elicited very similar potentials; hence, no target effect was obtained. On the other hand, both potentials were much larger and more complex than those elicited by standard stimuli. Additionally, a 'probe' stimulus, a widely deviant auditory stimulus randomly replaced, with a very small probability, a standard stimulus in these conditions. Even this irrelevant, physically widely deviant, stimulus elicited a wave complex basically similar to that elicited by slightly deviant stimuli, but of much larger amplitude. The comparison of these brain-wave sequences to those elicited by the same stimuli in reading subjects led to the conclusion that in detection conditions, deviant stimuli elicit two overlapping sequences of brain events: exogeneous and endogeneous. The former sequence, mainly including the processes producing the N1 and the N2 (neuronal mismatch) components, is an automatic, inflexible set of brain processes which appears as if providing a central-level stimulus to the endogeneous sequence. The latter seems to include a triphasic frontocentral complex overlapping the mismatch N2 and preceding the parietal late positive component and frontal late negative component. This endogeneous set of brain waves was regarded as a sign of detection of stimulus deviance. Consequently, it did not occur in response to the slightly deviant stimuli in reading, but the widely deviant stimuli which were also (involuntarily) perceived by the subject tended to elicit it in attenuated and delayed form.