Workshop 2: Transcranial Magnetic Stimulation (TMS) WS2.1. Introduction to TMS

Abstract

Transcranial magnetic stimulation (TMS) transports a short (about 100 µs) high intensity (∼ 2 Tesla) magnetic field painless through the skull into the brain and induces an electric current flow. This in turn excites neurons or fiber tracts. TMS can only induce tangential current flow. Its intensity declines exponentially; thus mainly cortical areas closed to the surface are targeted. The best available data are those obtained at the motor cortex (M1). M1 stimulation allows easy quantification of effects by recording electromyographic muscle activity. When stimulating the visual cortex phosphenes can be induced. Computational models allocate the lowest threshold elements into terminals of the axon collaterals. The dependence of threshold and polarization on spatial and temporal stimulation parameters are strongly affected by morphological features of the axonal arbor, including myelination, diameter, and branching. The thresholds for current flow are directionally sensitive, with lower threshold from the back to the front as vice versa. Intensity dependent a D-wave and several I-waves with 1.5 ms intervals are induced, which can be recorded in man invasively at the cervical spinal cord. In psychology it allows to induce short virtual lesions which provide data on cortical functions arising in the area stimulated. TMS is clinically used to quantify central motor conduction time in particular in patients with multiple sclerosis. Reliable information on amplitude changes e.g. in cervical myelopathy can be obtained by the triple stimulation technique.