Somatically Evoked Magnetic Fields in the Vicinity of the Neck

This paper describes measurements of somatically evoked magnetic fields in the vicinity of the neck using a SQUID with a second-derivative gradiometer. The evoked fields were observed not only at the head, but also at the back of the shoulder and at the back of the neck when the peripheral nerve of the wrist was stimulated. The field at the neck appeared within about 150 ms after the stimulus with an amplitude of about 1 pT. The waveform of the responses at various points over the neck showed a reversal of the polarity across the midline of the neck. The isofield pattern indicated a current dipole in the vicinity of the spinous process C6 as the signal source. It was suggested that the evoked fields originated from the postsynaptic neuronal activity in the spinal cord.