Transcranial Brain Stimulation: Clinical Applications and Future Directions
Neurosurgery clinics of North America
Keywords Transcranial brain stimulation; Brain mapping; Transcranial magnetic stimulation; Functional neuroimaging In human brain mapping, 2 basic strategies are commonly used to obtain information about cortical functional representation: (1) recording brain activity during task performance (the passive approach) and (2) observing the effects of eliciting/extinguishing brain activity (the active approach). 1 Techniques using the passive approach include magnetoencephalography (MEG) and
... ncephalography (EEG), which provide direct measures of neuronal activity, and positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), which capture brain hemodynamic and metabolic responses as indirect measures of neuronal activation. For the most part, such approaches fail to provide information about causal relationships between certain cortical regions and behavior or cognition. Furthermore, all these techniques are generally based on changes in brain activity that occur during task performance, and therefore depend on collaboration of the individual and careful behavioral assessments. Resting-state fMRI or EEG measures 2 are valuable, novel approaches to studying brain connectivity and network activity, but their usefulness in cortical output mapping is unclear. On the other hand, the active approach minimizes dependency on the individual's cooperation, because external stimuli are used to elicit or extinguish brain activity, although state-dependent influences on the effects of and responses to brain stimulation need to be considered and controlled for. 3 The active approach investigates whether a specific region of the brain is critical for implementing particular cognitive or behavioral functions and therefore is able to answer questions about causal relationships between brain and function. Noninvasive techniques using such an approach include transcranial electric stimulation (TES) and transcranial magnetic stimulation (TMS).