Transcranial Magnetic Stimulation (TMS) is a widely used non-invasive brain stimulation technique. By means of a rapidly changing magnetic field and the associated electrical field TMS can induce post-synaptic potentials in the in the neural tissue under the stimulating coil, leading to alterations in the cortical activity which can be directly detected when primary motor or visual cortex are stimulated (i.e. motor evoked potential or phosphenes) or indirectly by observing the effects on the performance of a wide range of behavioral tasks (Pascual-Leone et al., 2000; Walsh and Cowey, 2000; Walsh and Pascual-Leone, 2003). Being an interference technique, TMS can probe a causal relationship between a cortical area and a cerebral function, thus allowing to effectively map cortical functions whit a good temporal and spatial resolution.
Moreover, TMS is increasingly used for clinical purpose, with the rationale of restoring pathological pattern of cerebral activity underlying a variety of neuropsychological and psychiatric disorders. In 2008 FDA approved TMS use in the treatment of Major Depressive Disorder.
Transcranial direct current stimulation (tDCS) is a painless, non-invasive brain stimulation technique which belongs to the Transcranial Electrical Stimulation techniques (TES) including transcranial alternating current (tACS) and trascranial random noise stimulation (tRNS). tDCS uses constant weak electrical currents to transiently change cortical excitability. At a neuronal level, tDCS exerts a neuromodulatory action on cortical excitability by shifting the resting membrane potential in a polarity-dependent fashion: anodal stimulation increases the spontaneous firing rate, while cathodal decreases it. Notably, the coupling of opposite polarity effects is well documented in the sensory-motor domain, but becomes controversial when higher cognitive functions are targeted (Jacobson et al., 2012).
tDCS has beeb successfully used to affect a wide range of sensorimotor and cognitive functions in healthy and pathological human brains. On top of on-line effects on spontaneous neuronal activity,tDCS induces also long-lasting after-effects, likely mediated by mechanisms of synaptic long-term potentiation and depression (i.e., LTP and Long-term Depression, LTD, respectively), which affect neuroplasticity.This evidence, together with the easiness of use, portability and cheapness of tDCS has fostered the application of this technique in rehabilitation settings (e.g., Brunoni et al., 2011).
Combining TMS with electroencephalography enhances the properties of the two techniques offering new prospects in cognitive neuroscience. The integrated system of TMS-EEG allows indeed a direct measurement of the excitability of any cortical area (beyond primary motor or visual area) by directly perturbing the cortical activity, by means of TMS, and recording with the EEG the cortical response to this perturbation with a high temporal resolution (Taylor et al., 2008), computed as TMS-Evoked Potentials (TEPs). Moreover, continuous high definition EEG recording permits to probe the state of broader cortical networks, unveiling how activation spreads from the stimulated area to interconnected ones, thus allowing a direct measure of effective connectivity. TEPs are considered a reliable measure of brain activation state (Miniussi and Thut, 2010); beside, as long as the same parameters across sessions are maintained, the reliability of the technique has been probed (Casarotto et al., 2011) allowing to link any observed changes in TEPs to the experimental condition (i.e. brain state, task execution, neuromodulation, etc) rather than to others confounding variables.
Recently TMS-EEG has been successfully used to show the breakdown of cortical effective connectivity underlying the loss of consciousness during sleep (Massimini et al., 2005), induced by a pharmacological agents (Ferrarelli et al., 2010) or due to a pathological state such as coma/vegetative state, leading to the use of this technique as an effective approach to detect and track recovery of consciousness in brain-injured patients who are unable to exchange information with the external environment (Rosanova et al., 2012).
Moreover TMS-EEG has been used to trace online and off-line changes in cortical excitability induced by the use of non-invasive brain stimulation techniques such as rTMS (Thut & Pascual-Leone, 2010) and tDCS (Romero Lauro et al., 2014; 2015). Since TEPs analysis provides clear cut information on the speed and the location of cortico-cortico interactions, this technique has also been employed to clarify the temporal dynamics of neural transmission and how different regions of brain networks communicate during the cognitive processing involved by a task performance (Miniussi and Thut, 2010; Mattavelli et al., 2013).