Transcranial Magnetic Stimulation (TMS) is delivered to the brain by passing a strong brief electrical current through an insulated wire coil placed on the skull. This rapid phasic current flow generates a transient magnetic field, which propagates in space and in turn induces a secondary current in the brain that is capable of depolarising neurons if the coil is held over the subject’s head (Pascual-Leone et al., 2002). Single-pulse TMS of the primary motor cortex (M1) allows recording the amplitude of Motor Evoked Potentials (MEP). MEP amplitude is a measure of the cortico-spinal reactivity and is known to reflect the internal covert sensorimotor simulation of the observation actions (Fadiga et al., 1995) and somatic states (Avenanti et al., 2005). Crucially, this index has been proven to be modulated by group membership (Avenanti et al., 2010; Molnar-Szakacs et al., 2007).
Depending on the frequency, duration of the stimulation and the strength of the magnetic field, TMS can activate or suppress activity in cortical regions. rTMS is a methods of inducing electrical currents into the brain that may be used to produce long-lasting plastic changes in neural systems (facilitatory or inhibitory, according to the frequency and intensity of the stimulation).
tDCS uses homogenous DC field delivered at intensities of around 1 mA via two electrodes placed on the scalp. An active electrode is placed on the site overlying the cortical target, and a reference electrode is usually placed over the contralateral supraorbital area or in a non-cephalic region. tDCS induces long-lasting changes in the brain and it can be used to manipulate brain excitability via membrane polarisation: cathodal stimulation hyperpolarises, while anodal stimulation depolarises the resting membrane potential, whereby the induced after-effects depend on polarity, duration and intensity of the stimulation (Paulus, 2011).
Differences between tDCS and TMS include presumed mechanisms of action, with TMS acting as neuro-stimulator and tDCS as neuro-modulator. Moreover, TMS has better spatial and temporal resolution, TMS protocols are better established, but tDCS has the advantage to be easier to use in double-blind or sham-controlled studies and easier to apply concurrently with behavioural tasks. Despite their differences, both TMS and tDCS can induce long-term after-effects on cortical excitability that may translate into behavioural impacts that can last for months (Fregni etl al., 2006; Triggs et al., 1999). These long-term after-effects are believed to engage mechanisms of neural plasticity.