tDCS essentially consists in delivering for a few minutes a weak direct current (below the percep- tual threshold) over the scalp: the resulting con- stant electric field penetrates the skull and influ- ences neuronal function.
tDCS does not directly induce neuronal firing, because the current densities that it produces in the cortex – between 0.77 and 2.00 mA/cm2 according to a computer modelare well below the action potential threshold for cortical neurons.
Whereas stimulation at these current densities may not directly produce action potentials empirical evidence shows that even small voltage gradients of this magnitude can change neuronal firing rates.
Several studies show that anodal DCs increase, wheras cathodal DCS decrease, spontaneous neuronal firing in vivo in animal studies. The changes were shown to be long-lasting: continuous DC stimulation for 5 to 10 min elicited after-effects lasting for up to 5 hours.
Whereas anodal stimulation shifts the neuronal resting membrane potential towards depolarisation, cathodal stimulation shifts it towards hyperpolarisation. Intracellular recordings from animal studies have shown that anodal stimulation (30–400 μA/mm2, 5–40 s) causes neuronal depolarisation, whereas cathodal stimulation causes hyperpolarisation. These non-synaptic mechanisms receive further support from human motor cortex studies.
The effect of tDCS on motor cortex excitability can be measured by assessing motor evoked potentials (MEP) recorded from peripheral muscles. Anodal tDCS to the motor cortex (0.2–5 mA, 4 s–5 min) increases MEP size, whereas cathodal tDCS reduces MEP size.
Similarly, tDCS to the visual cortex produces changes in visual evoked potentials, contrast sensitivity and motion detection threshold, also in a stimulation polarity-independent direction.
tDCS induces changes by altering the strength of synaptic transmission.
Anodal tDCS induces long-term potentiation (LTP) through increased pre-synaptic activity coupled with postsynaptic depolarisation; conversely, cathodal tDCS induces long-term depression (LTD) by reducing presynaptic discharges and postsynaptic hyperpolarisation.
Complelling evidence therefore shows that synaptic neuroplasticity contributes to the effects of tDCS.
These tDCS-induced changes include correlations between the neuronal activity patterns known to underlie LTP/LTD, and those observed after tDCS, protein synthesis during stimulation, and especially the emerging NMDA receptor role in regulating the aftereffects of stimulation.
No evidence currently exists of severe adverse reactions after tDCS in humans. Common adverse effects include mild headache and an itching sensation at the electrode site. Skin lesions (burns) have been reported after tDCS and considerable care is needed in examining and preparing the skin-electrode interface.
No reports describe tDCS inducing seizures.
The literature advises strongly against exceeding the threshold of 0.05 mA/cm2 because ths intensity could cause a painful sensation and tissue damage.
In this way, the DC stimulation is far from the threshold for tissue damage (14.3 mA/cm2) recently indicated for tDCS in an animal model.
Those intending to prescribe DC treatment should first read and understand the scientific literature related to transcranial and trans-spinal DC stimulation.
Electrodes are usually positioned on the scalp using the 10-20 EEG international system to identify the correct stimulation site. tDCS dedicated cap is available in order to reduce the time of preparation of the patient and guarantee the correct reproduction of the montage also at home.
SIZE OF ELECTRODES
The electrodes used for tDCS are thick (0.3 cm), rectangular saline-soaked synthetic sponges and typically have an area ranging between 25 and 35 cm2.
Newronika also products a new generation of electrodes for direct current stimulation characterized by an ellipsoidal geometry in order to reduce undesiderable concentration of the electrical current on the edge (“Edge Effects”).
The amplitude used in recent clinical trials is between 1 and 2mA.
Stimulation duration: In the literature, tDCS is delivered for 10–20 min/session. The total duration of a treatment can vary in length from one day up to 10 days, with daily sessions.