What is tES?

Transcranial electrical stimulation - commonly abbreviated to tES - is a non-invasive method of stimulating the brain by means of applying a weak electrical current to an individual's scalp.

In its simplest form, tES involves the placement of an anode and a cathode on a subject's head, allowing the electrical current to flow between the electrodes with the aim of stimulating the underlying brain regions.

tES has been used extensively in both research and clinical environments and is generally considered to be a safe and painless method of non-invasive brain stimulation.

Transcranial Direct Current Stimulation (tDCS)

The idea of tDCS was based on work showing that the delivery of polarising current along a neuron for 5 - 20 minutes produces a lasting change in the number of spikes a second within that neuron (Bindman et al., 1964). A similar principle was used for non-invasive stimulation in humans when a weak electric current (up to 2mA) was applied to a human scalp (Nitsche & Paulus, 2003). When anodal current (left M1 —> right orbita) was applied, the amplitude of motor evoked potentials (MEPs) as evoked by transcranial magnetic stimulation (TMS) increased. In contrast, when cathodal (right supraorbita —> left M1) stimulation was applied, the amplitude of MEPs decreased. However, considerable variability has been revealed when attempting to use tDCS to change cortical excitability, particularly when the effects are probed using TMS (e.g. Tremblay et al., 2016). Recently, current flow models and comparisons with experimental data have highlighted the importance of applying current in a direction that is orthogonal to a gyrus, which reduces variability when stimulating the motor cortex at 1mA (Rawji et al., 2019).

Initially, tDCS was applied with one anode and one cathode, usually with 5 x 7cm electrodes which often has led to peak induced electrical field at a site in between the electrodes as opposed to directly underneath the anode (Datta et al., 2009). However, the development of ‘high definition-tDCS (HD-tDCS) involves a ‘4 x 1 ring’ with four cathodes surrounding one anode in a circular arrangement of disc-shaped electrodes (Datta et al., 2009). An HD-tDCS montage around the motor cortex led to a peak in the electric field immediately beneath the anode (in M1) but peaks in the electric field elsewhere were absent (Datta et al., 2009), suggesting that HD-tDCS can be used to selectively depolarise regions beneath the anode with a reasonable degree of spatial resolution.

Transcranial Alternating Current Stimulation (tACS)

tACS, on the other hand, enables changes in cortical excitability to occur at a particular frequency. This usually involves sinusoidal changes in the amplitude of tES to occur at a certain frequency, based on intracranial work showing that changes in local field potential were synchronised with an externally applied field (Frohlich & McCormick, 2010). In one of the first instances, tACS was applied at the individual alpha frequency (as measured in the occipital cortex) and revealed an increase in alpha power during electroencephalography (EEG) measurement after tACS compared to before (Zaehle et al., 2010), which some reports of an after-effect up to 30 minutes after stimulation (Neuling et al., 2013). Moreover, when applying tACS at a range of frequencies (4 - 16 Hz), an increase in alpha power was observed at 10 H, with a decrease in power that eventually fades into non-significance as frequencies outside a band of 8 - 12 Hz were used (Merlet et al., 2013). Taken together, these experiments suggest that tACS could be a promising means of providing causal evidence for oscillatory changes in voltage underlying basic neurophysiological processes. 

Possible Side-Effects of tES

While tES is generally regarded to be a safe and painless method of non-invasive brain stimulation, there are naturally a number of side-effects that subjects may experience, including:
   • An itching/tingling sensation on the scalp
   • Fatigue
   • Nausea
   • Headache
   • Insomnia

A full overview of the safety of tES techniques can be found in this journal reference.

Applications of Transcranial Electrical Stimulation

Transcranial electrical stimulation has been used extensively in neuroscience research for many years, with tDCS, tACS, and tRNS showing promise for the treatment of a number of neurological and mental conditions and disorders, including:
   • Depression
   • Memory Disorders
   • Aphasia
   • Chronic Pain
   • Parkinson's Disease
   • Schizophrenia
   • Addiction Disorders

Further Explanation

In the below video, taken from the Brainbox Initiative webinar series, Professor Charlotte Stagg (University of Oxford) further details how and why neuroscientists may combine MRI and tES techniques, and provides insights into how to achieve this.