From tCS to tDCS: transcranial current stimulation and its variants
- Brain function can be modified by applying a weak electrical current using contact electrodes placed over the scalp (transcranial). This kind of brain stimulation receives the name of transcranial Current Stimulation (tCS, see Ruffini et al. 2013 for a review). To date, the most studied tCS type is transcranial Direct Current Stimulation (tDCS).
- The basic version of this idea is simple: a current is injected using a battery to the brain through one electrode placed over the scalp above the targeted brain cortical area, and recruited by return electrodes positioned over the scalp, or, alternatively, at an extracephalic position.
- Using scalp electrodes, tCS generates weak electrical currents and electric fields (measured in Volts per meter) in the brain. These electric fields modulate neuronal activity.
- The main differentiating features of tCS techniques are a) the delivery of weak currents through the scalp (with electrode current intensity to area ratios of about 0.3-5 A/m2) b) at low frequencies (typically < 1 kHz) resulting in weak electric fields in the brain (with amplitudes of about 0.2-2 V/m).
What is tDCS?
- tDCS (transcranial Direct Current Stimulation) is a kind of tCS where the stimulation currents are held constant (as in DC current), and the most popular and used of tCS techniques.
- In general terms, anodal stimulation (current is injected into the brain) over a cortical region has excitatory effects, and cathodal (current is collected from the brain) has inhibitory effects.
- tDCS stimulation produce short term effects (increase/decrease) on neuronal excitability, and long lasting plastic after-effects involving synaptic modification (see, e.g., Marquez et al., 2012 and references therin). These underlie the clinical utility of tDCS .
What is tACS?
- Like tDCS, tACS is a form of tCS where the transcranial stimulation currents are time dependent with a sinusoidal shape (as in AC current). Amplitude, frequency and relative phases across stimulation electrodes can be controlled. tACS stimulation may provide a powerful way to couple to the oscillatory behavior of the brain, which is at present an active research field in basic and clinical Neuroscience.
What is tRNS?
- tRNS (transcranial Random Noise Stimulation) is a type of tCS where the stimulation current is varied randomly. Unlike tDCS, tRNS has been recently introduced and there is little experience with its use. However, it appears as if its main effects are excitatory.
What is Sham stimulation?
- Sham stimulation is a generic term to indicate an inactive form of stimulation (e.g., a very brief or weak one) that is used in research to control for the placebo effect. The subject believes he/she is being stimulated normally, but there should not be any real effects.
What should a modern tCS device offer? Our considerations in designing StarStim:
- - Current control at each electrode: this ensures that the practitiones is holding constant the Electric field in the brain (e.g., a simple battery system will not ensure this, since the actual electric fields induced in the brain will depend on contact impedance at the electrodes)
- - Multiple channels for better control of the spatial distribution of the electric fields
- - Use of standard sponge electrodes or more modern Ag/AgCl EEG like electrodes
- - Ease of use despite complexity of the technology
- - Multiple waveforms: tDCS, tACS and tRNS
- - Measurement of EEG before, during and after stimulation
- - Safety features including maximal currents and impedance control