Transcranial stimulation and cognition.

Noninvasive brain stimulation (NIBS) is a unique method for studying cognitive function. For the study of cognition, NIBS has gained popularity as a complementary method to functional neuroimaging. By bypassing the correlative approaches of standard imaging techniques, it is possible to establish a putative relationship between brain cognition. In fact, functional neuroimaging data cannot demonstrate the actual role of a particular cortical activation in a specific function because an activated area may simply be correlated with task performance, rather than being responsible for it. NIBS can induce a temporary modification of performance only if the stimulated area is causally engaged in the task. In analogy with lesion studies, NIBS can provide information about where and when a particular process occurs. Based on this assumption, NIBS has been used in many different cognitive domains. However, one of the most interesting questions in neuroscience may not be where and when, but how cognitive activity occurs. Beyond localization approaches, NIBS can be employed to study brain mechanisms. NIBS techniques have the potential to influence behavior transiently by altering neuronal activity, which may have facilitatory or inhibitory behavioral effects. NIBS techniques include transcranial magnetic stimulation (TMS) and transcranial electrical stimulation (tES). TMS has been shown transiently to modulate neural excitability in a manner that is dependent mainly on the timing and frequency of stimulation (high versus low). The mechanism underlying tES is a change in neuronal membrane potentials that appears to be dependent mainly on the direction of current flow (anodal versus cathodal). Nevertheless, the final effects induced by TMS or tES depend on many technical parameters used during stimulation, such as the intensity of stimulation, coil orientation, site of the reference electrode, and time of application. Moreover, an important factor is the possible interactions between these factors and the physiological and cognitive state of the subject. To use NIBS in cognition, it is important to understand not only how NIBS functions but also the brain mechanisms being studied and the features of the area of interest. To describe better the advanced knowledge provided by NIBS in cognition, we will treat each NIBS technique separately and underline the related hypotheses beyond applications.