Articles

Positioning of tDCS Electrodes

Brain stimulation techniques using electrical current have been used for a long time some of which were very invasive with many adverse effects. Today, neuroscience is moving its focus towards less invasive brain stimulation procedures, includingTranscranial Direct Current Stimulation (tDCS)(Brunoni et al., 2012). This approach can have positive effects on many neurological and psychiatric conditions, including chronic pain, and depression.

Types of tDCS
In essence, this method allows the electrical impulses to travel through the brain tissue between electrodes placed on the surface of the scalp. Effects of stimulation are different and they depend on polarization of the current. Cathodal stimulation lowers the excitability of neurons in the cortex of the brain, while anodal stimulation induces higher excitability of cortical neurons(Nitsche & Paulus, 2000).

Positioning of tDCS Electrodes
Depending on the underlying medical condition, the electrodes are placed differently on the surface of the scalp in order to optimally target certain regions of the brain. Size of the sponges which are placed between electrodes and the skin is also variable for different conditions. Electrodes are usually made of metal and incorporated in sponge pockets. The sponges are soaked with fluid rich in electrolytes in order to better conduct electrical impulses(Merrill, Bikson, & Jefferys, 2005). If rubber electrodes are used, conductive gel can replace soaked sponges. Direct contact between electrodes and the skin is avoided because it can produce local changes and irritation of the skin. Sometimes, local anesthetic creams can be used to avoid unwanted sensations on the surface of the skin, mostly itching.

Before Placing The Electrodes
Before placing the electrodes, it is necessary to perform measurements for each individual patient, giving that people have different sizes and shapes of the skull. This is needed in order to accurately locate parts of the cerebral cortex which will be targeted by tDCS. After that, the skin should be examined for any lesions. Electrodes should not be placed on damaged skin. Hair should also be moved away from the surface where electrodes will be placed.

Placing The Electrodes
After the electrodes are incorporated in soaked sponge pockets, and the areas for electrode placement are identified, non-conductive elastic straps are fixed in order to prevent electrode movements. Then the electrodes are placed and the stimulation session can be started. Initially, most of the patients report itching sensations in the place of electrodes, but they usually disappear during the session(Dundas, Thickbroom, & Mastaglia, 2007). It is important to monitor the device, the position of the electrodes, as well as hydration of the sponges during the session. The current should be gradually decreased before finishing the treatment. After the session, the patient will be asked to fill the questionnaire containing questions about side effects of tDCS.

Different Electrode Positions for Different Conditions
As mentioned above, different parts of the brain need to be targeted by tDCS in patients with different disorders. For example, anodal tDCS in the left prefrontal cortex is usually performed in patients with depression(Vanderhasselt et al., 2013). Improvement of motor abilities in some neurological diseases is accomplished by applying anodal tDCS on the affected brain hemisphere and cathodaltDCS on the opposite brain hemisphere (Lindenberg, Renga, Zhu, Nair, & Schlaug, 2010). For chronic pain management, anodal tDCS is applied to the motor cortex of the brain(DaSilva, Volz, Bikson, & Fregni, 2011). Several different positions are recommended for psychiatric conditions which can help with concentration, improving learning abilities, socialization, memory, insightfulness, and other aspets.

Although tDCS is a relatively new technique of non-invasive brain stimulation, it has promising future in the treatment of many disorders. Further research is needed to discover all the potentials of this novel approach.

References
Brunoni, A. R., Nitsche, M. A., Bolognini, N., Bikson, M., Wagner, T., Merabet, L., . . . Fregni, F. (2012). Clinical research with transcranial direct current stimulation (tDCS): challenges and future directions. Brain Stimul, 5(3), 175-195. doi: 10.1016/j.brs.2011.03.002

DaSilva, A. F., Volz, M. S., Bikson, M., & Fregni, F. (2011). Electrode positioning and montage in transcranial direct current stimulation. J Vis Exp(51). doi: 10.3791/2744

Dundas, J. E., Thickbroom, G. W., & Mastaglia, F. L. (2007). Perception of comfort during transcranial DC stimulation: effect of NaCl solution concentration applied to sponge electrodes. Clin Neurophysiol, 118(5), 1166-1170. doi: 10.1016/j.clinph.2007.01.010

Lindenberg, R., Renga, V., Zhu, L. L., Nair, D., & Schlaug, G. (2010). Bihemispheric brain stimulation facilitates motor recovery in chronic stroke patients. Neurology, 75(24), 2176-2184. doi: 10.1212/WNL.0b013e318202013a

Merrill, D. R., Bikson, M., & Jefferys, J. G. (2005). Electrical stimulation of excitable tissue: design of efficacious and safe protocols. J Neurosci Methods, 141(2), 171-198. doi: 10.1016/j.jneumeth.2004.10.020

Nitsche, M. A., & Paulus, W. (2000). Excitability changes induced in the human motor cortex by weak transcranial direct current stimulation. J Physiol, 527 Pt 3, 633-639.

Vanderhasselt, M.-A., De Raedt, R., Brunoni, A. R., Campanhã, C., Baeken, C., Remue, J., & Boggio, P. S. (2013). tDCS over the Left Prefrontal Cortex Enhances Cognitive Control for Positive Affective Stimuli. PLoS ONE, 8(5), e62219. doi: 10.1371/journal.pone.0062219

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