Transcranial Direct Current Stimulation

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We can think of our brain as a complicated electric circuit consisting of billions of neurons and trillions of synapse (gap between one neuron and the other). Each and every neuron can be considered as a component of this complex circuit. An interesting fact is, every aspect of human behavior, from reflexes to reasoning and emotions, depends on the processing of these circuits. Proper functioning of the healthy excitatory depends on the preciseness by these circuits. And many of the psychiatric disorders like depression can be a result of improper excitability.

Transcranial direct current stimulation, also (tDCS) is a technique which is used to modulate cortical excitability and it has shown an optimistic result. The technique is implemented by placing two electrodes on the scalp and applying a potential difference, which results in an electric field in the brain as shown in figure 1. There are other brain stimulation techniques such as transcranial electrical stimulation (TES) and transcranial magnetic stimulation (TMS) but the difference between these technique and tDCS is that in the latter the range of static field doesn’t create neuronal action potential, but it just modulates the excitability. For this reason, tDCS is also called neuro modulatory intervention.

tDCS is operated by the input of constant DC current (low amplitude) through electrodes which are placed on the scalp. Longer the duration of the stimulation, more will be effect of stimulation. The amplitude of current injected will also increase the effect of stimulation. tDSC can cause two changes in the brain; depolarization and hyper-polarization. The changes (de-polarization and hyperpolarization) depend on the type of stimulation being performed. A positive stimulation (anodal tDCS), which increases neuron excitability, causes depolarization and a negative stimulation (cathodal tDCS), which result in decrease in neuron excitability, causes hyper polarization.

There are many factors that affect the efficiency of tDCS technique such as stimulation duration, amplitude of current injected etc. However, one has to be very careful about the amplitude of the current because there are limits on amount of current in which brain can be operated. From application point of view location and size of the electrode also matters a lot. Other studies imply that electrodes with smaller area results in deep stimulation while larger electrode induce superficial stimulation. Since the effect of the stimulation is a function of current density, it is important to study the factors that influence the distribution inside the brain and many works have been done in the past to study this influence.

To study the distribution of Current density, first we need a mathematical formulation of this electric model. In 1968 Rush & Driscoll [6] formulated an analytical expression for the potential inside a 3-layer head model (spherical). One important inference from this study was the magnitude of current density inside the brain had direct influence on the effect of tDCS. However, magnitude of current density is not the only factor which decides the performance of tDCS. There are other factors such as electrode size, position and inter electrode distance etc.

In 2007 Nitsche studied the effect of electrode size on current distribution, and it resulted in the conclusion that more focal stimulation can be obtained by using smaller electrode. On the contrary increase in electrode area gives less focal stimulation i.e. superficial stimulation.

The choice of the electrode will depend on the application. Smaller electrode can be used for application where deep tissue stimulation is needed. There is already a procedure called Deep brain stimulation used to stimulate deep tissues in the brain, however using implanted electrodes [8] It is also possible to find the optimal electrode size based on the region of interest.

Another factor we mentioned before was inter-electrode distance. It plays an important role in the distribution of current density. We know from the basic electric science that current flows from cathode to anode. Now if cathode and anode are placed very close to each other then most of the current will pass through the scalp, not the brain. Thus we can say that the distance between anode and cathode affects the fraction of injected current that reached brain.

The aim of this work is to validate the results obtained by different researchers on the influence on electrode size and inter-electrode distance on special distribution of current density during tDCS. For this purpose, a spherical head model was modeled and finite element method was used to analyze the behavior of different electrode combinations on the spatial distribution of current density during tDCS. The head model consisted of 4 layers; the scalp, the skull, the CSF and the brain. All the layers are assumed to be homogenous in nature.

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