All neuromodulation techniques work with similar underlying principles. Their clinical effect is based on the fact that they can change functional and effective connectivity in the brain, as well as cross-frequency coupling, in other words, the communication between different points of the brain.
The effect is based on a number of parameters.
Transcranial Magnetic Stimulation or TMS is a technique used to temporarily stimulate a brain region with a magnetic pulse. The effect is not limited to the targeted area but extends to a network that is functionally connected to the targeted area. The effect of a one-time TMS stimulation is transient and may last only for a few seconds. In order to achieve a long-lasting effect, repetitive sessions have to be conducted, termed repetitive TMS (rTMS). A TMS session takes about 20 minutes with some patients experiencing momentary mild head dizziness. Studies on the safety of TMS however report no adverse effects to health (Rossi et al, 2009).
Low frequency stimulation at 1HZ increases the connectivity linked to the target area, whereas high frequency stimulation decreases the stimulation linked to the targeted area.
A special form of TMS is deep brain TMS, one can use double cone coils for this approach or other varieties of coils for this indication.
Transcranial Direct Current Stimulation (TDCS) is a technique used to modulate the sensitivity of brain cells with a weak electric current (1-2 mA). Depending on the position of the electrodes, a specific brain area and the area functionally connected to it may be targeted. TDCS is a painless procedure and takes approximately 20 minutes. The effect of a one-time TDCS stimulation is temporary and can last up to 48 hours. Repetitive stimulation sessions (rTDCS) are conducted to achieve a long-lasting effect.
A newer version of tDCS is HD-tDCS. High Definition (HD)-tDS uses multiple electrodes to stimulate, which permits the applied current to be directed more specifically to a selected target and associated network. But furthermore, current HD-tDCS devices with up to 32 independent channels permit to modulate multiple different areas simultaneously, thereby permitting network modulation or even modulation of multiple networks simultaneously. This generates an unprecedented amount of novel possibilities to disrupt abnormally functioning networks, or possibly even re-normalizing abnormal activity and connectivity within and between networks.
Transcranial Alternating Current Stimulation (tACS) is another form of non-invasive neuromodulation that uses alternating current. This is hypothesized to drive the area and functionally connective network at the selected frequency. tACS is not commonly used in clinical trials.
Transcranial Random Noise Stimulation (TRNS) is a special form of tACS that uses electrical noise to disrupt communication between functionally connected brain regions. Depending on the position of the electrodes, specific brain areas can be targeted. TRNS is painless and takes around 20 minutes per-session. The effect of a one-time TRNS session is temporary and can last up to 48 hours. To achieve a long-lasting effect, repetitive stimulation sessions are conducted, called rTRNS.
There are different forms of tRNS. The most commonly used is white noise stimulation, in which for each frequency that makes up the noise the same power is generated. A special form of tRNS is pink noise stimulation in which the power of low frequency is higher than for higher frequency. It follows the natural structure of the noise which is present in the brain and therefore is a more natural way of stimulation. Both pink and white noise function as stochastic resonance stimulation, i.e. it brings the stimulated area and network closer to firing threshold.
Transcutaneous Electrical Nerve Stimulation (TENS) is the application of low-voltage electric current through the skin targeting peripheral or cranial nerves rather than the brain. A specific form of TENS is vagal nerve stimulation, in which the TENS electrode is targeting the vagal nerve, part of the autonomic nervous system, which controls our internal organs, such as the heart, lungs, intestines, endocrine and immunological system.
sLORETA Neurofeedback with source localisation or Brain Computer Interface training is a therapeutic technique that uses real time display of brain activity, using at least 19 electrode EEG recording. Brain activity in the targeted region and networks that are functionally connected to it, is selected by means of the Cartesian X Y Z coordinates and displayed on a computer screen. This allows the patient to learn how to self-regulate brain activity to more appropriate patterns.
Different forms of sLORETA neurofeedback exist.
One can train the power of a specific frequency band up and/or train it down or use a combination of the two.
One can strengthen or weaken connectivity between areas.
One can train entire networks or one can combine it with NiRS, tDCS, tRNS, tACS or medication.
A Special form of neurofeedback is infraslow neurofeedback which trains frequencies below the 1Hz, typically 0.01-0.1 Hz.