- Abstract A violation of a rule or expectation is known to evoke a phasic negative potential over the medial frontal cortex. This electrophysiological effect has been shown for incorrect mathematical equations and incongruent words at the end of sentences. The cognitive processes elicited in rule violation seem to involve violation of expectation, error detection, and conflict between competing cognitions. Consistent with the conceptual relation between rule violation and error/conflict detection, rule violation conditions should involve a power increase in the theta frequency band involving the anterior cingulate cortex (ACC). The present study verifies the connection between rule violation and theta activity using a wavelet analysis. Moreover, low resolution brain electromagnetic tomography (LORETA) source localization connects this theta activity to the ACC. Furthermore, the results show that theta activity is sensitive to the salience of the violation, that is, the degree of deviation of the conflicting/erroneous stimulus from the correct (expected) one.