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 differ ent blocks, standards were either 250, 1,000, or 4,000 Hz, with the de viants always being 10% higher in frequency than the standards of the same blocks. Magnetic responses elicited by the standard and deviant t ones included N1m, the magnetoencephalographic equivalent of the elect rical N1 (its supratemporal component). In addition, deviant stimuli e licited MMNm, the magnetic equivalent of the electrical mismatch negat ivity, MMN. The equivalent dipole sources of the two responses were lo cated in supratemporal auditory cortex, with the MMNm source being ant erior to that of N1m. The dipole orientations of both sources in the s agittal plane depended on stimulus frequency, suggesting that the resp onses are generated by tonotopically organized neuronal populations. T he tonotopy reflected by the frequency dependence of the MMNm source m ight be that of the neural trace system underlying frequency represent ation of auditory stimuli in sensory memory.