Cerebral neuromagnetic responses evoked by short auditory stimuli

We presented 30 msec sinusoidal tone bursts to the subject's left ear once every 1300 msec. The number of standard tones (1000 Hz) between deviants (1030 Hz) varied randomly from 3 to 15 (even distribution) so that the probability of the standards was 0.9 and that of the deviants 0.1. During stimulation the subject was reading a book. Magnetic responses to the standards and deviants were measured at the estimated ends of the sylvian fissure and electrical responses at Fpz, Fz and Cz. In addition, extensive field maps over the right hemisphere were made from 60 to 75 points in 3 subjects. A least-squares fit was performed to find out the parameters of the equivalent current dipole in a spherically symmetrical head model. Field maps suggested that the source of magnetic response at 100 msec (N100m) can be approximated by a current dipole at the supratemporal plane, possibly at the primary auditory cortex. In two subjects the location and/or the orientation of the equivalent dipole changed during N100m, possibly due to change in the size of the activated cortical area. The deviant stimuli elicited in addition to N100m a second deflection, MMNm, peaking at about 200 msec. This response was regarded as specific to stimulus change. On the basis of field maps it was also concluded that MMNm got a contribution from activity at the supratemporal plane.