Electrophysiological responses to transcranial magnetic stimulation in depression and schizophrenia

Abstract

Transcranial magnetic stimulation (TMS) is generally employed as a diagnostic electrophysiological tool, for the investigation of the corticospinal pathways (Hallet and Cohen 1989). The application of a magnetic stimulus across the skull causes electrical changes in the brain that may be measured as an evoked motor response. These changes may be registered, and it is then possible to obtain a measure of the threshold of brain excitability and of conduction time along the corticospinal pathways. Magnetic fields, unlike electric fields, are not absorbed by intervening tissue. Thus, magnetic stimuli can depolarize neurons with no pain at the skin or skull. For instance, magnetic stimulation over the motor cortex can cause an arm or leg to move in a conscious, unanesthetized patient. The magnetic stimulation can be localized easily to right arm or right leg and, with somewhat more difficulty, to specific arm or leg muscles (Hallet and Cohen 1989). Concurrent electromyography can quantitate the conduction time from motor cortex stimulation to appearance of a muscle potential in a specific arm or leg muscle; the amplitude of the muscle potential in the arm or leg muscle; and the threshold of magnetic stimulation before appearance of a detectable muscle potential. The total conduction time from magnetic stimulation of motor cortex to potentials in the arm or leg is composed of central and peripheral components. The peripheral conduction