V.E. Amassian

A comparison of corticospinal activation by magnetic coil and electrical stimulation of monkey motor cortex

The effects of different orientations of a Cadwell round magnetic coil (MC) were compared with each other and with surface electrical stimulation of motor cortex in monkeys anesthetized with pentobarbital or urethane. Recordings were made from within the lateral corticospinal tract, either from axonal populations or with a microelectrode from individual axons. A lateral-sagittally orientated MC directly excited corticospinal neurons at lower stimulus intensity than was required for indirect, i.e., transsynaptic excitation via inputs to corticospinal neurons. By contrast, in 2 out of 3 macaques tested, a vertex-tangential orientation could excite corticospinal neurons indirectly at lower intensities than were required for direct excitation; at higher intensities, direct excitation also occurred. The site of direct corticospinal excitation by a lateral-sagittally orientated MC was inferred by comparing the response variability and latency to MC and surface electrical stimuli. Cathodal stimuli elicited more variable corticospinal population responses and later individual axonal responses than were obtained with anodal stimuli. The variability in response is attributed to interaction between nearby, on-going synaptic bombardment and the stimulus, implying that surface cathodal stimuli directly activate corticospinal neurons at the spike trigger zone (presumably the initial segment). By contrast, the consistency and reduced latency of the corticospinal responses to surface anodal stimuli are attributed to the direct excitation of corticospinal fibers within the white matter. When the stimulus intensity is clearly above threshold, surface anodal and cathodal stimuli can activate corticospinal neurons both directly and indirectly. Direct corticospinal excitation by the MC can resemble the effects of either surface anodal or surface cathodal stimuli. We conclude that the MC can activate corticospinal neurons at the spike trigger zone or their fibers deeper in white matter. The findings in the monkey are used to interpret the effects of different MC orientations in the human.

An analysis of peripheral motor nerve stimulation in humans using the magnetic coil

We compared conventional electrical and magnetic coil (MC) stimulation of distal median nerve in 10 normal subjects and 1 patient. Orthogonal (90 degrees to volar forearm)-longitudinal (the plane of the MC aligned with the long axis of nerve or wire), tilted (to 45 degrees) longitudinal, and tangential edge orientations elicited maximal or near maximal compound motor axon potentials (CMAPs) without simultaneous co-activation of ulnar nerve. Transverse and symmetrical tangential orientations were inefficient. A simulation study of an ideal volume conductor confirmed these findings by predicting that the maximum current density was near the outer edge of the MC and not at the center where the magnetic flux intensity is maximal. An orthogonal-longitudinal MC induces a current in the adjacent volume conductor (for example elbow or wrist), which flows in the same circular direction as in the MC. This differs from a tangentially orientated MC which classically elicits current flow in the volume conductor opposite in circular direction to that in the MC. Amplitude and latency of the CMAP were both altered, but not identically, by changing the intensity of MC and cathodal stimuli. Rotating an orthogonal-longitudinal MC through 180 degrees, thus reversing the direction of current flow, elicited single fiber muscle action potentials whose peak latencies differed at most by 100 microseconds. Thus, the (virtual) cathode and anode are significantly closer (i.e., 5-6 mm) with MC than with electrical stimulation where they are at least 20 mm apart. A disadvantage of MC stimulation is the imprecision in defining exactly where the distally propagating nerve impulse originates. In different subjects, using maximum output and orthogonal or tilted (to 45 degrees) longitudinal orientations, the calculated site of excitation in the median nerve varied 2-15 mm distal to the midpoint of the contacting edge of the MC. This limits the usefulness of the MC in its current configuration for determining distal motor latencies. Future advances in MC design may overcome these difficulties.

In vitro evaluation of a 4-leaf coil design for magnetic stimulation of peripheral nerve

The performance of a 4-leaf magnetic coil was evaluated during magnetic stimulation of a peripheral nerve in vitro. The site of stimulation was below the coil center, and a 90 degrees rotation of the coil was equivalent to a change in current polarity. A hyperpolarizing magnetic stimulus failed to slow or block a propagating action potential.

Basic mechanisms of magnetic coil excitation of nervous system in humans and monkeys and their applications

Conclusions from studies utilizing the human peripheral nerve as a model system in developing the optimal magnetic coil (MC) orientation for focal stimulation were applied to the human cerebral cortex. The theoretical predictions were validated by direct recording from corticospinal neurons in the monkey. Appropriate MC orientation over the cerebral cortex elicits: (1) minimal central motor delays; (2) movements of individual digits; (3) a sense of movement in a paralyzed arm; (4) laryngeal muscle excitation from frontal areas related to speech; (5) suppression of visual perception when the MC is over the visual cortex; and (6) transcallosal responses in the opposite hemisphere.<<ETX>>

Studies of 3-dimensional voltage distributions induced in homogeneous media volume conductors by round and butterfly magnetic coils

Magnetic coils (MCs) for physiological stimulation of the human nervous system are discussed. Using a butterfly MC at threshold intensity over the motor cortex, focal movements can be elicited in contralateral digits. These observations can in part be explained by physical models of homogeneous volume conductors which allow measurement of induced voltage gradients in 3-D space. In an infinite-volume conductor, a round MC induces the greatest voltage gradient under the windings. The butterfly MC induces a sharp, central peak separated from smaller lateral peaks.<<ETX>>