Technical approaches to hemisphere-selective transcranial magnetic brain stimulation.

Clinical application of transcranial magnetic brain stimulation is mainly used to determine central motor conduction times. With the stimulation coil (Magstim 200, Novametrix) centered conventionally over the midline of the skull convexity and using high stimulus intensities, which are often required in pathological states, the motor cortices of both hemispheres are usually activated simultaneously. Under this condition it is not possible to determine from which hemisphere the descending excitatory volleys to a particular motoneurone pool originate and how the input to the lower motoneurons is organized (uni-/bilateral, ipsi-/contralateral). This limitation can be overcome by two different techniques for selective stimulation of the motor cortex of one hemisphere without coactivation of the other even when using maximal stimulus intensities: 1. A large 12 cm (outer diameter) stimulation coil could be used for selective stimulation when a) the magnetic field radiated over the non-stimulated hemisphere is modified by using a prototype coil shield covering the half of the coil over the nonstimulated hemisphere in combination with b) placing the coil away from the midline towards the preferentially excited hemisphere. The coil shield consists of a sheet of a nickel iron alloy which alters the time course of the induced currents by reducing the initial rate of current intensity change (dI/dt). 2. The use of a smaller 6.5 cm (outer diameter) coil also provided a useful tool for selective stimulation of one hemisphere but was restricted to subjects with low excitation thresholds. In subjects with high excitation thresholds the described use of the large stimulation coil is advisable.