Neuromagnetic responses of human auditory cortex to interruptions in a steady rhythm.

We have recorded, with a 7-channel SQUID gradiometer, evoked magnetic responses of 6 healthy humans to interruptions of a steady rhythm of 50 ms 'standard' tone bursts repeated once every 610 ms. Ten percent of the tones occurred 'too early', 410 ms after the preceding stimulus. The response to standards peaked, on average, at 90 ms and that to the early tones at 148 ms. Field patterns were dipolar during both responses and the equivalent sources agreed with activation of the supratemporal auditory cortex, at slightly different locations. The dipole moments were more than twice as strong for the early tones as for the standards. The results emphasize the importance of temporal stimulation patterns in activating the human auditory cortex.