Non-invasive brain stimulations (NIBS)
Non-invasive brain stimulations (NIBS) have been widely used as research tool to modulate cortical excitability of motor as well as non-motor areas, such as memory- or language-related areas. With their ability to either inhibit or enhance cortical excitability, NIBS techniques are a promising tool in both research and clinical settings. Various types of NIBS methods are available: transcranial magnetic stimulation (TMS), transcranial direct current stimulation (tDCS), transcranial alternating current stimulation (tACS), which differ in their mode of action.
- TMS uses magnetic fields to induce electrical current in spatially restricted cortical regions. TMS provides a better spatial resolution (2-3 cm) compared with the other techniques. When applied over the motor cortex, TMS also allows the quantification of motor-evoked potentials (MEPs), a measure of the excitability of the motor system. Depending on the frequency of stimulation used, TMS can have inhibitory or excitatory effects on cortical excitability.
- tDCS consists in applying a low-intensity (0.5-2 mA) constant current and two sponge electrodes of opposite polarity: a positive electrode (anode) and a negative electrode (cathode). Since the current used in tDCS is subthreshold, it does not induce action potentials; instead it modulates spontaneous neuronal activity (evoked, ongoing/endogenous activity) in a polarity-dependent fashion. Similarly to TMS, tDCS can have inhibitory or excitatory effects on cortical excitability, but differently from TMS, this feature is not frequency-related but rather polarity-related.
- tACS can be used to modulate oscillatory activity. Whereas in tDCS, a constant current is applied over time, tACS consist in applying a current alternating at a frequency which is believed to be associated with a particular cognitive function. tACS can therefore modulate the power of oscillatory rhythms in the brain in a frequency-dependent manner by synchronizing neural populations across distant brain regions.
Advantages / disadvantages compared to other methods
Advantages of NIBS, especially TMS over neuroimaging studies is that TMS can be used to demonstrate causal brain-behavior relationships. TMS has also a good temporal resolution enabling to study the causal chronometry in brain-behavior relations. NIBS can also be combined other neurophysiological techniques such as electroencephalography (EEG), magnetoencephalography (MEG), functional near infrared spectroscopy (fNIRS) or magnetic resonance imaging (MRI).
Combined NIBS and neuroimaging methods (for example fMRI) provides valuable information about causal interactions between brain areas. MRI enables to position and orient the coil accurately over the stimulation site based on each individual’s anatomy and function. For this purpose, a neuronavigation system is available (Brainsight, Polaris) to guide the TMS coil position over individually based targets. Moreover, neuroimaging can also be used to map TMS-induced effects, this combination provides measures of functional connectivity in the targeted network.
Limitation / Risks
Limitations of TMS include concurrent auditory and somatosensory stimulations that may influence task performance. Appropriate control conditions need to be given careful considerations. Moreover, because of its limited access to structures deeper than a few centimeters from the surface of the scalp, TMS cannot reach subcortical and limbic areas. The development of new coils (for example double cone coil) should enable a deeper stimulation.
The biggest risks and contraindications for NIBS arise from metal implants or metallic objects close to the head. We also discourage participants with history of epilepsy. Further restrictions and details are listed in the volunteer/patient information. A training is required before using this facility. Information will be provided including use of the machines, safety and inclusion criteria.
Dr. Jamila Andoh, Institute of Cognitive and Clinical Neuroscience
Zentralinstitut für Seelische Gesundheit (ZI) - https://www.zi-mannheim.de