Combination of Transcranial Magnetic Stimulation with Electromyography and Electroencephalography: Application in Diagnosis of Neuropsychiatric Disorders

Transcranial magnetic stimulation (TMS) is a non-invasive brain stimulation technique that allows for in vivo examination of cortical processes. In 1985, Barker et al. introduced TMS as a tool for investigating the functional state of the motor pathways in patients with neurological disorders as well as in healthy human subjects (Barker et al., 1985). It was shown that a single TMS pulse applied to the motor cortex could activate cortical tissues associated with the hand or leg muscles and this activation could elicit a motor evoked potential (MEP) at the periphery captured through electromyography (EMG) recordings. The amplitude, area, latency and duration of this TMS induced MEP could then be used to investigate the integrity of the corticospinal pathways and the activation threshold of human motor cortex. Since this discovery, the combination of single and multi-pulse TMS with peripheral EMG recordings has allowed for examining various other processes in the human motor cortex such as excitability, plasticity, cortico-cortical connectivity, as well as the interaction between excitatory and inhibitory cortical processes. The integration of EMG with TMS has, therefore, offered a valuable tool for assessment of pathological processes that underlie neurological and psychiatric disorders such as Parkinson’s disease, dystonia, stroke, Alzheimer’s disease, schizophrenia, and depression. The combination of paired pulse TMS with EMG recordings, for example, permits altering the excitability of the motor cortex and observing the effect of this alteration on subsequent stimulation. In paired pulse TMS, the first TMS pulse (conditioning stimulus) inhibits or facilitates the MEP response to the second TMS pulse (test stimulus). The nature and the strength of this modulatory effect depend on the intensity of the conditioning stimulus and the latency (i.e. interstimulus interval (ISI)) at which it is delivered with respect to the test stimulus. It has been demonstrated that the TMS induced modulation of MEPs are