What is TES-Transcranial electrical stimulation?

Transcranial electrical (TES) and magnetic (TMS) stimulation for the treatment of neurological and psychiatric diseases has ancient origins. Despite a slow development, the new non-invasive stimulation methods have proved capable of determining consistent clinical effects.

Magnetic stimulation and transcranial electrical stimulation are the main methods of non-invasive stimulation of the nervous system, capable of modulating neuronal excitability, even in a prolonged way. Transcranial electric stimulation (tES) techniques involve applying weak electrical currents (~ 1 - 2 mA) directly to the scalp for several minutes (~ 5 - 30 minutes).

The current is delivered due to the application of two electrodes positioned on the scalp, through a battery-powered current stimulator.

These currents generate an electric field that modulates neural activity based on the mode of application, which can be continuous transcranial direct current stimulation, tDCS), random noise (transcranial random noise stimulation, tRNS), or alternating (transcranial alternating current stimulation), tACS). The tES alters the firing rate of neurons and thus facilitates the activation of the selected cortical areas, favoring the strengthening of the circuits. The tES working at very low intensity turns out to be totally painless and practically free of any contraindication.

Professor Miniussi, to begin with, what is the difference between neurostimulation and neuromodulation?

With neurostimulation, we refer above all to techniques such as Transcranial Magnetic Stimulation - TMS - which involves the induction of depolarization of the neuronal membranes and the initiation of action potentials in the stimulated area by means of electromagnetic induction. Neuromodulation, on the other hand, does not induce action potentials but modulates the neuronal response thresholds, modifying the membrane permeability. Neuromodulation, which is obtained with methods such as direct current stimulation - tDCS -, can substantially modify the excitability of neurons and modulate the response of these neurons to incoming signals.

A tdcs device uses an anode and cathode connected to a battery-powered current stimulator. The current passes through the scalp and allows increasing or decreasing the functionality of the target area. The image shows a subject wearing a cap with electrodes.

What are Transcranial Electrical Stimulation - tES - and Direct Current Transcranial Stimulation - tDCS?

The tDCS is the most used method among the tES techniques. With tDCS, one electrode of interest (cathode or anode) is placed on the area to be studied, while another is placed in an area considered neutral. The use of tDCS involves the application of weak electrical currents (approximately 1-2 mA - milliamps) directly to the scalp, through a pair of electrodes, for 10 - 15 minutes. These currents generate an electric field that modulates neuronal activity. In particular, several studies have shown that the application of anodic tDCS increases the activity of neurons in the stimulated area, while cathodic tDCS has the opposite effect. 

Are there any indications for administering Transcranial Electrical Stimulation?

Yes, there is a whole literature that gives precise indications both on safety aspects and on the technical parameters to be used. Generally, 1 and 2 mA pacing are used most. However, the induced effect depends on the relationship between intensity and duration of use. In fact, stimulation at 1 mA for 15 minutes has a different effect from stimulation at 2 mA for 15 minutes. On the other hand, stimulations of very low intensity and short duration have been shown not to produce significant effects. The suggestion is, therefore, to find a protocol present in the literature, already tested and that works and sticks to it.

What are the effects of the stimulation that we can expect on the cognitive and motor level?

Even if the cognitive and motor sides are studied as different areas, in reality they are complex functions built with the contribution of certain networks of neurons and not of single areas. Neuromodulation can be useful in modifying, enhancing or weakening some of these motor or cognitive circuits. That is, modulate them and increase or reduce their effectiveness (in the event that, for example, you want to reduce hyperactivity). 

There has been a lot of talk in recent years about the enhancement that can be done with these techniques. Are there any contraindications?

We can say that when the induced enhancement is high, it could be at the expense of other functions and therefore have a negative final effect. An overdose of stimulation can cause an effect opposite to that desired and induce excessive plasticity. What we do therefore is to try to obtain an effect that reaches the right measure of arousal that allows a functional balance.

What are the long-term effects of transcranial electrical stimulation?

We have not yet been able to see how long the effect of tDCS lasts. It probably depends on the stability of the activated mechanism. That is, if you use a behavioral learning protocol that manages to build strong synaptic bonds, bonds that are strengthened with neurostimulation, then the effect could be protracted over time. However, we have yet to measure whether this is actually the case. There are data in the literature that demonstrate greater efficacy of VS neurostimulation than non-stimulation. The differences, however, are not so clear-cut. It is therefore not yet clear what the long-term benefits are. One idea that is being worked on is to have follow-up sessions to maintain the strengthening, to prevent the effect from diminishing over time. 

How is it best to place the electrodes? 

Unfortunately, the scientific literature does not present sufficient data to be able to say unambiguously, which is the best editing. By doing so, we are sure not to stimulate the area under the reference electrode. However, cephalic setup - which sees the reference electrode positioned on the scalp - is also adequate. The important thing is that the reference electrode is placed in an area that is considered inactive. When deciding how to place the electrodes, the aspects to consider are the direction of current flow and the distance between the electrodes. These elements determine the final effect. 

The primary motor area in the brain 

For example, if you place the active electrode on the left motor cortex and the reference one on the ipsilateral frontal pole (on the same side) you will have currents going between these two electrodes. If, on the other hand, the reference electrode is positioned on the opposite side, on the right front pole, there will be a flow of current between the two electrodes, but with a completely different path. The neuronal activity that will modulate will be different. Moving the electrodes also moves the stimulated areas. 

Could you give us an example of a rehabilitation protocol using tDCS?

It is difficult to define a standard protocol. However, in most studies, stimulation is applied to the area of ​​interest at an intensity of 1-2 mA. The duration of the daily stimulation session varies from ten to twenty minutes. It is generally repeated five times a week, for a maximum of twenty sessions in total.

Author: Vicki Lezama