Objective: To determine the source localization(s) of the midlatency auditory magnetic response M50, the equivalent of the P50 potential, a sleep state-dependent waveform known to habituate to repetitive stimulation. Methods: We used a paired stimulus paradigm at interstimulus intervals of 250, 500 and 1000 ms, and magnetoencephalographic (MEG) recordings were subjected to computational methods for current density reconstruction, blind source separation, time-frequency analysis, and data visualization to characterize evoked dynamics. Results: Each subject showed localization of a source for primary auditory evoked responses in the region of the auditory cortex, usually at a 20-30 ms latency. However, responses at 40-70 ms latency that also decreased following the second stimulus of a pair were not localizable to the auditory cortex, rather showing multiple sources usually including the frontal lobes. Conclusions: The M50 response, which shows habituation to repetitive stimulation, was not localized to the auditory cortex, but showed multiple sources including frontal lobes. Significance: These MEG results suggest that sources for the M50 response may represent non-auditory, perhaps arousal-related, diffuse projections to the cortex. © 2007 International Federation of Clinical Neurophysiology.
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