Tonotopic auditory cortex and the magnetoencephalographic (MEG) equivalent of the mismatch negativity

Two tone stimuli, one frequent (standard) and the other infrequent (a slightly higher, deviant tone), were presented in random order and at short intervals to subjects reading texts they had selected. In different blocks, standards were either 250, 1,000, or 4,000 Hz, with the deviants always being 10% higher in frequency than the standards of the same blocks. Magnetic responses elicited by the standard and deviant tones included N1m, the magnetoencephalographic equivalent of the electrical N1 (its supratemporal component). In addition, deviant stimuli elicited MMNm, the magnetic equivalent of the electrical mismatch negativity, MMN. The equivalent dipole sources of the two responses were located in supratemporal auditory cortex, with the MMNm source being anterior to that of N1m. The dipole orientations of both sources in teh sagittal plane depended on stimulus frequency, suggesting that the responses are generated by tonotopically organized neuronal populations. The tonotopy reflected by the frequency dependence of the MMNm source might be that of the neural trace system underlying frequency representation of auditory stimuli in sensory memory.