Neuromagnetic correlates of temporal gap detection in human auditory cortex

In this study, spatiotemporal profiles of brain responses to gaps in tones were investigated using magnetoencephalography (MEG). Participants listened passively to diotically presented stimuli consisting of leading and trailing markers with gaps of 0, 30, or 80 ms in duration that were inserted between the two markers. The leading and trailing markers were 300-ms pure tones of either 800 or 3200 Hz in frequency. For the withinfrequency condition, the leading and trailing markers were identical in frequency, whereas for the between-frequency condition, the leading marker and the trailing marker were presented at different frequencies from each other. Neuromagnetic brain activities were recorded from 6 participants. Auditory evoked fields (AEFs) were obtained from temporal areas in both hemispheres. Auditory MEG responses (N100m) appeared in response to the onset of the trailing markers as well as the leading markers. In the between-frequency conditions, N100m appeared for all trailing marker onsets regardless of the gap duration. In within-frequency conditions, the stimulus did not produce a clear N100m response when there was no gap. Even though N100m appeared with gap durations of 30 ms and 80 ms, the amplitudes were smaller than those observed in the between-frequency conditions. Minimum norm estimate (MNE) was applied to localize the activated areas for these conditions. The results indicated that the areas activated by an 800-Hz tone were distributed more posteriorly than those for a 3200-Hz tone.