Early deflections of cerebral magnetic responses to median nerve stimulation.

We have recorded early components of somatosensory evoked magnetic fields with a sensitive 7-channel first-order gradiometer using a wide recording passband (0.05-2000 Hz) and high sampling frequency (8000 Hz). The left median nerve was stimulated at the wrist and responses were recorded over the right hemisphere. The responses typically consisted of a N20m peaking at 18-20 msec, a small P22m peaking at 21-23 msec and a P27m peaking at 29-31 msec. The topography of N20m could be explained by a tangential current dipole in the posterior wall of the central sulcus (probably in area 3b). The equivalent dipoles of P27m were located on average 10 mm antero-medially to the sources of N20m. This suggests that P27m may get a contribution from the anterior wall of the central sulcus. An increase of stimulus repetition rate from 2 to 5 Hz decreased the amplitude of P27m more than that of N20m, which implies that these two deflections are generated by different neural networks.