Magnetic brain stimulation with a double coil: the importance of coil orientation

The human motor cortex can be excited by currents induced by a transient magnetic field generated in a coil over the scalp. A 9 cm mean diameter circular coil centered at the vertex is optimally placed for exciting the hand area. Anticlockwise current flow in the coil preferentially excites the left hemisphere and vice versa. A double coil has been used to investigate the orientation of inducing currents at which activation of cortical neural elements is maximal. The inducing current flowed in the same direction in the central segment of the coil and followed a monophasic wave form. The coil was rotated through 360 degrees over the motor area in increments of 45 degrees and compound muscle action potentials from the first dorsal interosseous muscle were recorded. The largest responses were obtained with the coil at about 50 degrees to the parasagittal plane with a backward flowing inducing current. The optimal angle did not depend on stimulus intensity or background voluntary contraction. This orientation corresponds to an maximal induced current flowing forwards approximately at right angles to the central sulcus. It is postulated that horizontal neural elements are aligned in this direction and are preferentially excited by these monophasic magnetic stimuli. The results have important implications for mapping the motor areas with magnetic stimulators.