Mechanism of TMS

For magnetic stimulation a brief, high-current pulse is produced in a coil of wire, called the magnetic coil, which is placed above the scalp. Figure 2 illustrates how a current is generated beneath the scalp and skull noninvasively by a coil placed outside the head. As current runs through the coil, a magnetic field is produced with lines of flux passing perpendicularly to the plane of the coil. The magnetic field is not impeded by the skin and skull, and therefore induces an oppositely directed current in the brain that flows tangentially with respect to skull. This electric field lies perpendicularly to the magnetic field and causes current to flow in loops parallel to the plane of the coil, assuming a homogeneous medium. The strongest current travels in loops near the circumference of the coil, while current loops are weaker near the centre of the coil. Current is nonexistent at the center of the coil. This, however, is a simplified model because of the homogeneous medium assumption. The brain is a non-uniform conductor with an irregular shape, so the path of the current is complex to model. It is however possible to approximate the target of TMS within a few millimeters using MRI-based control, as is discussed in greater detail in the section covering modeling of TMS.

Figure 2

Although the exact extent to which neurons are activated remains unknown, observations have been made that link variations in neuron activation to the intensity of stimulation. Ordinarily, corticospinal neurons are not activated directly by TMS. Instead, corticospinal neurons are activated indirectly via synaptic inputs. This has been determined by the observation that TMS produces a corticospinal volley with indirect waves (I-waves) rather than with an early direct wave (D-wave).1