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Reference

 tDCS informaion

Transcranial direct current stimulation (tDCS) is a simple, non-invasive technique for brain stimulation that induces prolonged functional changes in the cerebral cortex (Priori, 1998; 2003). tDCS essentially consists in delivering for a few minutes a weak direct current (below the perceptual threshold) over the scalp: the resulting constant electric field penetrates the skull and influences neuronal function.

MECHANISMS OF ACTION

A leading view is that changes in spontaneous neuronal firing rates contribute to intra-stimulation effects and synaptic neuroplasticity to post-stimulation effects.

Spontaneous neuronal firing: 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 model –are 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.

Non-synaptic mechanisms: 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.

Synaptic mechanisms: 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.

INDICATION FOR USE IN HUMANS

Electrode montage: 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”).

Current strength: 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.

Protocols Used in Scientific Literature:

Reference

Simulation Electrode Position

Reference Electrode Position

Polarity

Electrode Area

(cm²)

Intensity (mA)

Current Density

(mA/cm²)

Duration (min)

Effect and Side Effects

Healthy Subjects Or Patients

Number of Subjects

Boggio et al. (2008)

Anodal tDCS of the left dorsolateral prefrontal cortex (active group – ‘DLPFC’); anodal tDCS of the occipital cortex with the active electrode placed on the midline and 2 cm above the Inion (active control group – ‘occipital’).

Contralateral supraorbital area

A/S

35 cm²

2

0.057

20min (10days)

EFFECTS: Anodal tDCS leads to a significant decrease in depression scores that lasts for at least 30 days after the end of treatment.

SIDE EFFECTS: Mild adverse events equally distributed across the 3 groups (headache, itching, redness of skin).

Patients with Major Depression

40