EEG (electroencephalography) is the measurement of electrical potential differences across points on the scalp using sensitive equipment. These small potential differences are the result of electrical activity within the brain and are associated with brain function. The coherent activity of cortical pyramidal neurons generates ionic currents, and these give rise to electric field and electric potential variations. The measured voltages are in the of uV range (microVolt, or one millionth of a Volt) and are typically recorded at multiple scalp sites simultaneously. Although other important techniques exist to study brain function, EEG offers excellent temporal resolution (millisecond scale) and moderate spatial resolution (cm scale) using modern analysis techniques such as cortical mapping. EEG remains unparalleled in ease of use.
EEG applications typically fall into the following categories: Research, Health/Medical applications and Human Computer Interaction.
tDCS (transcranial Direct Current Stimulation) is a type of tCS where the stimulation currents are held constant (as in DC current), and remains the most popular and used of tCS techniques. Learn more here.
In very general terms, anodal stimulation (where current is injected into the brain) over a cortical region has excitatory effects, while cathodal stimulation (current is collected from the brain) has inhibitory effects (see this wiki entry for more precise information). These effects depend on cortical structure and electrode locations, and they can be modeled using modern tools such as finite element modeling (FEM). You can learn more about modeling the electric fields of tDCS in our wiki.
tDCS produces immediate 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). The mechanism behind this is Hebbs rule: neurons that fire together, wire together. These long-term effects underlie the clinical utility of tDCS.
Multichannel tDCS with small electrodes offers both more focality and versatility than classical bipolar montages using sponge electrodes. If interested, contact us for our StimWeaver service: we can produce the best multichannel tCS (MtCS) montage for your targeted stimulation scenario.
tACS (alternating current) and tRNS (random noise current) are variants of tDCS in which the electric currents are varied in time. tACS is thought to latch onto resonant phenomena in the oscillating brain, whereas tRNS appears to increase neuronal excitability.
tACS is a form of tCS/tES where the stimulation currents have a sinusoidal time dependence (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.
In tRNS the stimulation current is varied randomly. Unlike tDCS, tRNS has only been recently introduced and there is limited experience with its use. However, it appears as if its main effects are excitatory. There are further subtypes of tRNS: 1) Full-band tRNS: tRNS is generated for the entire band of the device (from 0 to 500Hz in Starstim). 2) Band-passed tRNS: tRNS is filtered using a bandpass filter (low pass and high pass included), e.g., from 100-400 Hz.
Yes. For example, you can combine tDCS+tACS in a single channel. The technology in Starstim offers true multichannel tCS/tES: you can configure the current independently at any channel as a linear combination of DC, AC and RN stimulation.
To learn more, visit our wiki.