A form of neurostimulation that uses constant, low current delivered to the brain area of interest via electrodes on the scalp.

1. Transcranial Direct Current Stimulation (tdcs)
It is a form of neurostimulation (also known
as neuromodulation) where very low levels of constant
current are delivered to specifically targeted areas of
the brain, often producing profound results. It was
originally developed to help patients with brain injuries
such as strokes.

2. History
The basic design of tDCS, using direct current (DC) to stimulate the area of interest, has existed for over 100
years. There were a number of rudimentary experiments completed before the 19th century using this
technique that tested animal and human electricity. Luigi Galvani and Alessandro Volta were two such
researchers that utilized the technology of tDCS in their explorations of the source of animal cell electricity. It
was due to these initial studies that tDCS was first brought into the clinical scene. In 1801, Giovanni Aldini
(Galvani's nephew) started a study in which he successfully used the technique of direct current stimulation
to improve the mood of melancholy patients. Aldini gave a detailed account of his treatments of melancholy
patient Luigi Lanzarini and also described the stunning result when he first tried the treatment on his own
head:
First, the fluid took over a large part of my brain, which felt a strong shock, a sort of jolt against the inner
surface of my skull. The effect increased further as I moved the electric arcs from one ear to the other. I felt a
strong head stroke and I became insomniac for several days.

3. Understanding tDCS
The brain is the most complex organ in
the human body, comprised of an
intricate network of BILLIONS of nerve
cells, called neurons. These special cells
control and react to everything that
happens in our bodies. The neurons in
your brain communicate to each other
using tiny electrical, and chemical
impulses called synapses. Electrical
synapses, unlike chemical synapses,
conduct nerve impulses faster
(approximately 10 times faster), causing
vital information to pass from neuron to
neuron more quickly.

4. How long do effects from tDCS last?
The immediate effects usually last anywhere from 5 to 90 minutes after the
end of the stimulation session. Some research has shown that repetitive
stimulation, for example on a daily basis for one week, can educe longer
lasting, and more “ingrained” effects. Additionally, a study on depression
found that the beneficial effects of tDCS stimulation were shown to be
apparent an entire month after the treatment. Furthermore, one study
(considered anecdotal, not yet confirmed) recorded a lasting cognitive
enhancement effect resulting from Transcranial Direct Current Stimulation
ONE YEAR after the initial stimulation study.

5. Mechanism of action
One of the aspects of tDCS is its ability to achieve
cortical changes even after the stimulation is ended.
The duration of this change depends on the length
of stimulation as well as the intensity of stimulation.
The effects of stimulation increase as the duration of
stimulation increases or the strength of the current
increases. The way that the stimulation changes brain
function is either by causing the neuron’s resting
membrane potential to depolarize or hyperpolarize.
When positive stimulation (anodal tDCS) is delivered,
the current causes a depolarization of the resting
membrane potential, which increases neuronal
excitability and allows for more
spontaneous cell firing. When negative stimulation
(cathodal tDCS) is delivered, the current causes a
hyperpolarization of the resting membrane potential.
This decreases neuron excitability due to the
decreased spontaneous cell firing.
Neuroplasticity refers to the ability of the brain to change
throughout life based on experiences. The way that
transcranial direct current stimulation functions could be
due to the plasticity concepts of long term potentiation
(LTP) and long term depression (LTD) since the two share
some basic similarities. Long term potentiation is
strengthening of the connection between two neurons
while long term depression is weakening of the
connection between two neurons. These effects are
achieved mainly through an alteration of synaptic
transmission ability. LTP enhances transmission and LTD
hinders transmission. Likewise, tDCS stimulation involves
the alteration of synaptic transmission ability through
modifications of intracellular cAMP and calcium levels, as
well as glial activation. Also, both LTP, LTD, and the effects
of tDCS are protein synthesis dependent. It is for these
reasons that LTP and LTD are proposed mechanisms of
the function of tDCS.

6. Side effects of stimulation
There are a few minor side effects including
skin irritation, a phosphine at the start of
stimulation, nausea, headache, dizziness, and
itching under the electrode. Nausea most
commonly occurs when the electrodes are
placed above the mastoid for stimulation of
the vestibular system. A phosphene is a brief
flash of light that can occur if an electrode is
placed near the eye. A recent study of over
500 subjects using the currently accepted
protocol reported only a slight skin irritation
and a phosphene as side effects.
There are several ways to reduce the skin
irritation felt during stimulation. Electrodes
may be prepared with saline solution and the
skin prepared with electrode cream. Also,
ramping up (slowly increasing) the current can
reduce the irritation.
It is not advised to administer this stimulation to
people susceptible to seizures, such as people
with epilepsy. However, seizures do not seem to
be a risk for healthy individuals.

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