Types of TES

Types

There are different TES types and they differ by how the current inside them works:

• tDCS (transcranial direct current stimulation) :  involves a constant flow of electrical charge, where the stimulating electrodes are always either anodes or cathodes. One of the two electrodes will be used as the site of interest, and the other electrode as the reference site. 

If one chooses to use cathodal stimulation, as it's charged negatively, it acts more like a virtual lesion approach. This disrupts performance, decreases neuron firing rates, and modulates the glutamate system (an excitatory neurotransmitter).

On the other hand, anodal stimulation will work by the opposite mechanism and increase neural firing rate, therefore enhancing performance.

• tACS (transcranial alternating current stimulation): this uses alternating currents, meaning that anode and cathode are constantly switched and take turns in being the stimulated site. This  technique trains the brain to oscillate at specific rhythms or train the neurons to fire at a certain rhythmic rate (such as in alpha and beta bands). 

• tRNS (transcranial random noise stimulation): is similar to tACS, but uses random current alternation instead of a regular pattern. This lets us manipulate the brain in different ways.

More details 

Two conductive rubber electrodes are placed on saline-soaked sponges and at least one of the two is attached to the head with nonconductive elastic straps.

tDCS is the most commonly used technique, employing a direct and constant current, usually of 0.5-2 mA for 2-20 minutes. This current is propagated through the skull and returned to the reference electrode. 

Anodal tDCS depolarizes the neuron, which leads to an increased activation (inside the cell it becomes more positive than the resting potential, see here for more). Depending on the area of the brain, this increases certain transmitter concentrations like glutamine and reduces others like GABA (Gamma-aminobutyric acid, which is an inhibitory molecule). Anodal tDCS also enhances the release of the brain-derived neurotrophic factor (BDNF), which supports synaptic plasticity. 

On the other hand, cathodal tDCS usually decreases neural firing, this is because an increase in negative current hyperpolarizes the neuron (see part on Action potentials: if a negative charge is applied, the voltage drops below the resting potential, preventing the neuron from firing). [1]

Reference electrodes

The location of the reference electrode (can be the cathode or anode) will determine the flow of current. It’s usually either on top of a cortical region, which isn’t considered relevant for the experimental questions, or a location like the shoulder. It can also be placed to analyse competition between two brain regions. 

In tDCS, the reference electrode's placement determines current flow. For tACS and tRNS, alternating currents eliminate the need for a static reference electrode.

Current quantity

The maximum safe current is 2 mA, as higher levels can cause discomfort, including redness and skin sensations. Severe side effects, like headaches, are rare.

The contact area of the electrodes is relatively large, often around 25cm^2, this reduces spatial precision, as the current spreads beyond the target area. Smaller electrodes can improve precision, but require lower currents - as a smaller radius increases the voltage (I = V/R). 

As we are dealing with currents, longer stimulation results in prolonged after effects, reducing temporal resolution. 

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6326965/