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) responses to infrequent (‘deviant’) tones occurring among frequent (‘standard’) tones of different pitch were compared with responses to rare tones presented alone. The subjects were to ignore the tones. Deviant tones elicited the mismatch negativity (MMN) and its MEG counterpart (MMNm), while the rare tones delivered alone elicited a larger N1 and its MEG counterpart (N1m) than did standard tones. Source modeling of MEG responses indicated a difference in auditory-cortex source locations between the MMNm to deviant tones and the enhanced N1m 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.