Transcranial magnetic stimulation (TMS) is a non-invasive means of stimulating the human brain via Faraday’s law of electromagnetic induction (Hallet, 2007). TMS was first used to non-invasively stimulate the motor cortex (M1) whilst measurements from muscle tissue were made using electromyography (EMG) (Barker et al., 1987). As a consequence, TMS became one of the most popular techniques to non-invasively probe basic physiological and cognitive processes in conscious humans.
Applications of TMS
TMS can be used to probe intra-cortical connections (within the stimulated region) by delivered paired pulses of TMS (e.g. Kujirai et al., 1991). For example, delivering a low intensity TMS pulse prior to a higher intensity TMS pulse can reduce MEP amplitude by altering the activating inhibitory neurons within M1 (Kujirai et al., 1991). The use of two TMS coils can be used investigate inter-cortical connections (between two different brain regions). For example, interactions between the motor cortex in the left and right hemisphere (Chen et al., 2003) or hierarchical connections between the visual system, such as V5 and V1, V2 and V3 (Walsh & Pascual-Leone, 2005). TMS can also be used to probe the chronometry of basic processes. In an influential study, Amassian et al. (1989) delivered single pulses of TMS to early visual cortex at certain intervals after a visual target appeared. Such a technique revealed that early visual cortex is critical in reporting target identity from 80ms to 120ms after a visual target appears (Amassian et al., 1989). All of these approaches can generalised from motor cortex or visual cortex to study the causal relevance of a brain region to a cognitive or basic physiological process.
TMS has also proven to be very popular when applied at a rhythmic frequency for prolonged period of time, which has become known as repetitive TMS (rTMS). For example, the application of rTMS for 15 minutes at 0.9 Hz to the left motor cortex leads to a decrease in the amplitude of motor evoked potentials (MEPs) (Chen et al., 1997). MEPs were examined after the application of rTMS and the reduction lasted for at last 15 minutes. A different way to approach rTMS to to apply ‘bursts’ or ‘trains’ of TMS pulses at fixed intervals separated by inter-burst or inter-train interval. One incredibly popular rTMS protocol that relied on bursts of TMS pulses is theta-burst stimulation (Huang et al., 2005), which involved administering 3 pulses at 50 Hz every 200ms (5 Hz). Theta burst stimulation can be applied continuously (cTBS) or with 8 second intervals (with no TMS pulses) between the the application of TMS pulses (iTBS) (Huang et al., 2005). Theta burst stimulation has successfully affected the amplitude of MEPs (Huang et al.,2005).
The response to rTMS can be very variable (see Hamada et al., 2013 for a theta burst example). One relatively new rTMS protocol is quadrapulse (QPS) rTMS, which involves delivering a burst of 4 TMS pulses with either 5ms or 50ms between each pulse. Regardless of whether 5ms or 50ms separates each pulse, there is always a 5 second interval between bursts (Hamada et al., 2008). QPS appears to be less variable than other rTMS protocols, and employs a monophasic pulse shape, which may be superior to the commonly employed biphasic pulse shape (Hamada et al., 2013).
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