MULTICHANNEL DETECTION OF MAGNETIC COMPOUND ACTION FIELDS WITH STIMULATION OF THE INDEX AND LITTLE FINGERS

Magnetic compound action fields (CAFs) over the right arm were measure d from 63 sensor positions with two 7-channel SQUID gradiometer system s following electrical stimulation of the index and little fingers as well as the ring finger separately. The wave forms of the CAFs were pr imarily biphasic, corresponding to the depolarization and repolarizati on currents of the stimulated nerves. Maximum amplitudes of the CAFs w ere 60-140 fT for the index finger stimulation and 40-90 fT for the li ttle finger stimulation. The field mapping of the CAFs revealed a prop agating quadrupolar pattern with different distributions for the index and little fingers. The results agree with the anatomical location of the median and ulnar nerves for the index and little finger stimulati on respectively. The isofield maps, due to ring finger stimulation, sh owed complex patterns as a result of simultaneous activation of the me dian and ulnar nerves. By comparing the amplitudes of the maxima of th e CAFs due to index finger stimulation with those after median nerve s timulation at the wrist, the numerical ratios of the constituent digit al nerve fibers for the index finger within the median nerve at the wr ist were estimated. The ratios of 0.14-0.41 (mean 0.27), determined wi th measurement of the CAFs, are fairly consistent with those calculate d from the reported histological data.