Electromagnetic responses of the human auditory cortex generated by sensory-memory based processing of tone-frequency changes

Event-related brain potentials (ERPs) and magnetoencephalographic (MEG) res ponses to infrequent ('deviant') tones occurring among frequent ('standard' ) tones of different pitch were compared with responses to rare tones prese nted alone. The subjects were to ignore the tones. Deviant tones elicited t he mismatch negativity (MMN) and its MEG counterpart (MMNm), while the rare tones delivered alone elicited a larger N1 and its MEG counterpart (Nlm) t han did standard tones, Source modeling of MEG responses indicated a differ ence in auditory-cortex source locations between:the MMNm to deviant tones and the enhanced Nlm to the rare tones presented alone. Thus, the MMN/MMNm is elicited by infrequent sounds only when they occur among frequent sounds . This supports the idea that a sensory-memory trace formed in the auditory cortex by preceding repetitive sounds is a necessary precondition for MMN/ MMNm elicitation. (C) 1999 Elsevier Science Ireland Ltd. All rights reserve d.