Abstract
Somatically Evoked Field (SEF) generated by a peripheral nerve stimulation has been studied using a SQUID with a second-derivative gradiometer in a room without magnetic shield. A three-phase signal is observed at the posterior region of the neck when an ulnar nerve is stimulated by electric current pulses. The signal appears during 30-150ms after the stimulation with a peak-to-peak amplitude of 1-1.5pT. The polarity of the three-phase signal reverses on the right and left sides of the midline of the neck, reaching the maximum at a distance of about 3cm from the midline. The amplitude saturates when the stimulation current exceeds about 8mA.
The SEF signal is also observed at the medial region of the suprascapula. A two-phase signal appears, in addition to the three-phase signal, depending on the measuring point.
As the signal sources for the SEF at the neck minute current elements, i. e., current dipoles, with a magnitude of 20-70nA·m are estimated at about 3.5cm below the surface of the neck in parallel with the midline.
