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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.

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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) depends
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 influences 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 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 et al 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 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 layer are assumed
homogenous in nature.

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