Non-invasive Brain Stimulation

Introduction

Methods of non-invasive brain stimulation used in our group include transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS). TMS and tDCS are used to disrupt cerebral functioning to examine the role of the stimulation target in physiological or pathological conditions, an approach that has been referred to as virtual lesion.

TMS is a noninvasive method of delivering electrical stimuli to the brain through the intact scalp. Depending on the stimulation parameters, TMS is able to excite or inhibit the brain. Repetitive (r)TMS is able to change and modulate activity beyond the stimulation period which is why it sometimes is referred to as offline-TMS. Therefore, rTMS has therapeutic potential in patients with neurological and psychiatric disorders. It is, however, unclear by which mechanism rTMS induces these lasting effects on the brain. Several lines of evidence support the hypothesis that rTMS affects the brain by changes in synaptic plasticity, by long-term potentiation and long-term depression. Plasticity is the ability of the brain to reorganize itself, enabling short- and long-term remodeling of neural communication that outlasts an experimental manipulation or period of training.

An alternative stimulation approach is tDCS. Here, weak direct currents are applied to the brain for several minutes non-invasively. Dependig on the electrode, this leads to shifts in membrane potentials without causing neuronal firing like in TMS. Cathodal tDCS hyperpolarizes whereas anodal tDCS depolarizes the membrane. This mechanisms has been indirectly proven by Nitsche and Paulus from the University of Göttingen, Germany by combining TMS and tDCS and measuring motor evoked potentials in the contralateral limb. They showed that tDCS caused an alteration of the motor evoqued potentials of plus minus 20 percent. In our own research we study tDCS as a method to modulate and potentially treat auditory hallucinations in schizophrenia (Figure 1).

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