Introduction: 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. Method: Here we compare this measure of perceptual comparison in a nontask situation (3 tones presented) with that obtained in a task requiring focussed attention and response to the third tone. Subjects were 9 male and 16 female healthy volunteers aged 18-25 years Results: MMN amplitude (comparison of standard and deviant irrelevant tones) increased with focussed attention to the third (target) tone and frontal maxima shifted slightly posteriorly. The succeding P3 in the difference waveform increased more posteriorly than frontally confirming continued differential processing of irrelevant stimuli under active conditions. Conclusion: This demonstrates that not only attending to stimuli, but the active processing of irrelevant stimuli (vs passive perception) involves small changes of the amount and distribution of neural activity. i.e. active controlled processing in focussed attention can affect the capacity or distribution of resources even for automatic processing of information (the MMN).