Mismatch negativity (MMN) is altered by directing attention.

MMN is a negative component resulting from the difference in event-related potential (ERP) waveforms elicited by a standard and a deviant stimulus. It is usually studied in the absence of attentional requirements. We compared this measure of perceptual comparison in a non-task situation (three tones presented) with that obtained in a task requiring focused attention and response to the third tone. MMN (comparison of standard and deviant irrelevant tones) increased with focused attention to the third (target) tone and frontal maxima shifted slightly posteriorly. The succeeding P3 in the difference waveform increased more posteriorly than frontally confirming continued differential processing of irrelevant stimuli under active conditions. This demonstrates that not only attending to stimuli, but the active processing of irrelevant stimuli (vs passive perception) involves small changes in the amount and distribution of neural activity.