Magnetic stimulation of the human brain is performed in clinical and research settings, but the site of activation has not been clearly localized in humans or other species. We used a set of magnetic stimulus coils with different field profiles to isolate movement of single digits at motor threshold and to calculate corresponding electric field strengths at various distances beneath the scalp. Two coils could produce the same electric field intensity at only 1 point. Thus, we could estimate the depth of stimulation by finding the intersection of the electric field plots, which were then superimposed on MRIs of the underlying brain. In each of 3 subjects the field plots intersected at the crown of a gyrus, in the region of the central sulcus, an near the level of the gray-white junction. This position and the electric field orientation support localization to layer VI of cerebral cortex.

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Transcranial magnetic stimulation requires a great deal of power, which mandates bulky power supplies and produces rapid coil heating. The authors describe the construction, modeling, and testing of an iron-core TMS coil that reduces power requirements and heat generation substantially, while improving the penetration of the magnetic field. Experimental measurements and numeric boundary element analysis show that the iron-core stimulation coil induces much stronger electrical fields, allows greater charge recovery, and generates less heat than air-core counterparts when excited on a constant-energy basis. These advantages are magnified in constant-effect comparisons. Examples are given in which the iron-core coil allows more effective operation in research and clinical applications.

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Abstract-A detailed analysis of the membrane voltage rise commensurate with the electrical charging circuit of a typical magnetic stimulator is presented. The analysis shows how the membrane voltage is linked to the energy, reluctance, and resonant frequency of the electrical charging circuit. There is an optimum resonant frequency for any nerve membrane depending on its capacitive time constant. The analysis also shows why a larger membrane voltage will be registered on the second phase of a biphasic pulse excitation [1]. Typical constraints on three key quantities voltage, current, and silicone controlled rectifier (SCR) switching time dictate key components such as capacitance, inductance, and choice of turns.

Abstract-Transcranial Magnetic Stimulation (TMS) is used commonly as both a diagnostic tool and as an alternative to electric shock therapy for the treatment of clinical depression. Among the clinical issues encountered in its use is the mitigation of accompanying pain. The objective becomes one of minimizing the induced surface field while still achieving the target field objective. Three techniques discussed for realizing this end are (1) placing a conducting shield over a portion of the central target region, (2) using supplementary coils of opposite polarity in tandem with the primary field, and (3) opening the core angle to distribute the field. Option (3) shows the greatest promise for reducing the ratio of the maximum surface field to induced target field Index Terms-electric field, iron core, magnetic, stimulation, suppression