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The localization of effects from magnetic coil stimulation is not immediately obvious. We measured the magnetic fields produced by several different coils and compared the results with theoretical calculations. Magnetic stimuli were delivered from a Cadwell MES-10 magnetic stimulator using 3 circular coils (one 9 cm in diameter; two with an angulated extension, 5 and 9 cm in diameter) and twin oval coils arranged in a butterfly shape (each coil approximately 4 cm in diameter) and from a Novametrix Magstim 200 using two circular flat-spiral coils (6.7 and 14 cm in diameter). Peak-induced strength of the magnetic field was recorded with a measuring loop (1 cm in diameter) at different distances from the center of the coil. When the measuring loop was moved in the same plane laterally from the center of the coil, for all coils except the butterfly-shaped coil, the field was highest in the center and fell off near the circumference of the coil. The field dropped progressively when measurements were made more distant from the plane of the coils. The electric field induced from the magnetic coil could be calculated from the coil geometry. For all coils except the butterfly-shaped coil, the largest electric field was at the circumference of the coils. The 6.7 cm flat-spiral coil induced currents similar to those induced by the larger coils but more focally. The butterfly-shaped coil induced the largest currents under its center, where the circumferences of the two component coils come together. The component of the electric field parallel to the wire in the center of this coil was the largest and most localized.

The time constants of motor and sensory fibers in the human ulnar, median and tibial nerves were determined using the method of latent addition. Two square-wave stimuli were applied: the first one was subthreshold and the second, at various delays relative to the first, was adjusted to achieve threshold activation. Strength-delay curves were obtained, from which the time constant was determined using a mathematical model. Sensory fibers had time constants that were about 3 times the time constant for motor fibers. The strength-delay curves gave similar time constants as those obtained from strength-duration curves.

We studied orthodromic sensory conduction velocity along the distal and proximal segments of the median and ulnar nerves by tactile stimulation of the distal phalanx of the 3rd and 5th digits in 44 healthy subjects divided into 2 age groups: from 16 to 35 years and from 63 to 81 years. In the same nerves, we used selective electrical stimulation of the corresponding digital nerves to obtain sensory potentials. In both groups, responses to tactile stimuli had a longer latency and smaller amplitude than those to electrical stimulation, and they were distributed in a series of 6-7 main deflections, apparently regardless of whether the recording site was distal or proximal. Moreover, irrespective of the nerve and of subject age, conduction velocity along both the digit-wrist and the wrist-elbow nerve segments was significantly slower with tactile stimuli than with electrical stimuli. However, independently of the stimulus used, conduction velocity along the proximal nerve segment was significantly faster than that measured along the digit-wrist nerve segment. In both the median and ulnar nerves, maximum potential amplitude, cumulative area and conduction velocity were significantly reduced in the older age group. This finding could reflect the smaller number of Meissner's corpuscles in older subjects, and the loss of large nerve fibres in individuals over 60.

Magnetic stimulation has not been routinely used for studies of perioheral nerve conduction primarily because of uncertainty about the location of the stimulation site. We performed several experiments to locate the site of nerve stimulation. Uniform latency shifts, similar to those that can be obtained during electrical stimulation, were observed when a magnetic coil was moved along the median nerve in the region of the elbow, thereby ensuring that the properties of the nerve and surrounding volume conductor were uniform. By evoking muscle responses both electrically and magnetically and matching their latencies, amplitudes and shapes, the site of stimulation was determined to be 3.0 ± 0.5 cm from the center of an 8-shaped coil toward the coil handle. When the polarity of the current was reversed by rotating the coil, the latency of the evoked response shifted by 0.65 ± 0.05 msec, which implies that the site of stimulation was displaced 4.1 ± 0.5 cm. Additional evidence of cathode- and anode-like behavior during magnetic stimulation comes from observations of preferential activation of motor responses over H-reflexes with stimulation of a distal site, and of preferential activation of H-reflexes over motor responses with stimulation of a proximal site. Analogous behavior is observed with electrical stimulation. These experiments were motivated by, and are qualitatively consistent with, a mathematical model of magnetic stimulation of an axon.