Event-related brain potentials were recorded from the human scalp during selective listening to tone pips differing in location and/or pitch from irrelevant tones. The subjects' task was to discriminate infrequent deviant tones of lower intensity appearing among designated (relevant) tones. A large processing negativity was observed in the event-related potentials to relevant tones differing from the irrelevant tones in location even when both tones randomly varied in pitch. Similarly, a large processing negativity was elicited by the relevant tones differing from the irrelevant tones in pitch even when the location of both tones varied randomly. The results support the theory that the processing negativity to relevant stimuli reflects a match of these stimuli with an “attentional trace,' an actively maintained neuronal representation of the physical feature(s) of relevant stimuli that distinguish these stimuli from the irrelevant stimuli. Furthermore, the infrequent lower-intensity tones appearing among irrelevant tones elicited a mismatch negativity similar to the mismatch negativity elicited by target tones, equivalent lower-intensity tones appearing among relevant tones. This indicates that these infrequent stimulus changes were automatically discriminated by the generator mechanism associated with mismatch negativity.