Pii: S1388-2457(02)00267-5

Objective: To examine the effect of the active task involvement versus the passive knowledge of the structure of the stimulation on eventrelated potentials (ERPs) to violation in number sequences. This should contribute to future development of an ERP test for neurological patients. Methods: ERPs were recorded in subjects who listened to 4-item sequences of numbers incrementing by one. Half of the sequences had an appropriate end item, whereas the other half ended inappropriately. The role of active versus passive condition was investigated in a betweensubject design. Results: Incorrectly ending sequences yielded a large P300 wave in all groups. Subjects who were explicitly informed about the nature of the stimulation but did not have a motor task displayed a slow negativity which followed P300 and slightly suppressed it. In addition, incorrect sequence endings elicited an early (70–200 ms) negativity at frontal sites. This effect was inverted on mastoids. Conclusions: Auditory perception of 3 consecutive numbers constituting a simple incrementing row leads to building a strong expectancy concerning the subsequent number, and unexpected events are detected already after the initial phoneme. q 2002 Elsevier Science Ireland Ltd. All rights reserved.