In the Cognitive NeuroLab we use the Stimulator Magstim Super Rapid2 (Magstim Company Ltd., Whitland, UK)
Transcranial Magnetic Stimulation is a recently technique in the field of neurophysiology. It involves the application of a strong magnetic field (similar to that generated by an MRI scanner) on the scalp in order to produce an electrical current in the underlying brain tissue. Th magnetic field is generated by an electric current flowing through a coil of wire copper encapsulated in a plastic (stimulation coil).
The nervous response to transcranial magnetic stimulation depends on a combination of different parameters such as the frequency and intensity of the magnetic pulses, as well as the location of the stimulation.
In the therapeutic field, in order to obtain lasting changes in neural activity, the magnetic pulses are applied repetitively. The frequency of applied pulses have differential effects on cortical excitability; applying high frequency pulses (> 1Hz) has an excitatory effect on the cortex while applying low frequency (≤ 1 Hz) has inhibitory effects.
Recently new stimulation protocols have been developed that produce more lasting effects with much shorter application times. Specifically, the theta burst stimulation (TBS) protoco, is a special type of repetitive TMS consisting of bursts of 3 pulses applied at a frequency of 50 Hz with a total of 600 pulses. From this basic pattern there are two paradigms with contrary effects on the nervous system
Another parameter to consider when applying TMS is the intensity of the magnetic field. To adjust the level of stimulation intensity, the motor threshold must be determined. The motor threshold is defined as the lowest intensity needed to produce a minimal motor evoked response in the first dorsal interosseous muscle in 50% of trials, applying a single pulse of TMS to the motor cortex.
Finally, the location of the area stimulated also determines the effects of TMS on brain functioning. The transcranial magnetic pulses affect both the underlying brain tissue and distant areas functionally and structurally connected. To date, different strategies have been used to locate the target area for stimulation based on the location of relevant scalp points or based on an MRI cortical targets are previously established and the stimulation guided by a stereotactic neuronavigation system. This technique allows the administration of highly specific stimulation based on the brain function of each individual.
The duration of the effects of TMS depends on the type of protocol applied and the number of sessions conducted. After a single application of repetitive TMS, effects on brain activity last beyond the stimulation for at least the same time as the stimulation duration and after TBS, between 20 minutes and an hour. However, plastic changes promoted by a treatment of 10 sessions with TMS can remain long term.