Magnetic stimulation of the motor cortex-theoretical considerations

An approximation model for the computation of the electric fields produced in the brain tissues by magnetic stimulation is presented. Results are given in terms of induced electric field and current density caused by coils of different radii and locations. Nontraditional coil locations and assemblies are also considered (multicoil arrangements). Model simulations show that a good control of the excitation spread can be achieved by proper positioning of the coil. It is also predicted that one of the major drawbacks of the technique, (i.e. the poor ability to concentrate the current spread into a small brain area) can be partially overcome by more effective coil positioning and/or assembly. Some comparisons are made among the results obtained from electric and magnetic stimulation. This is thought to be helpful in the design of experiments aimed at understanding the relative role of different brain structures responsible for the motor response.