What is Transcranial Current Stimulation (tACS)? Benefits Of tACS On Brain Health

Do you know your brain is one of the most complex structures known to man? It's hard to get your head around it, but it's true! After centuries of medical advancements, we still don’t know for sure how our brain works.

However, even the most rudimentary functionality, which has been rigorously investigated for over a century now, can provide us not just a sneak peek but great detail on mental disorders of the brain and how we can fix them. The primary outcome of these investigations promises a new landscape of neuropsychiatric treatments beyond medications and talk therapy.

Non-invasive Brain Stimulation Techniques

A non-invasive brain stimulation technique alters the brain's excitability without needing an operative procedure to gain direct access to brain tissue inside the skull. There are two types of NIBS techniques:

1- Transcranial Current Stimulation (tACS and tDCS)

2- Transcranial magnetic stimulation (TMS)

This article will focus on Transcranial alternating current stimulation (tACS), its mechanism, physiological effects, and evidence-based benefits.

Transcranial alternating current stimulation (tACS)

Transcranial alternating current stimulation (tACS) is a novel technique for transcranial electrical stimulation (tES) of various regions of the brain. With the help of sinusoidal waves generated from one or more electrodes, transcranial alternating current stimulation (tACS) can optimize or improve brain function.

Is tACS invasive?

Transcranial alternating current stimulation (tACS) is a non-invasive brain stimulation (NIBS) technique. It alters

How does tACS work?

A rubber-padded array of electrodes produce weak electric currents of 1-2 mA are applied for 20 minutes, producing an electrical field consisting of complex sinusoidal waveforms. Our brain typically generates electrical activity in 4 types of waves with distinct amplitude and frequencies. These are

1. Delta oscillations (1-3 Hz)
2. Theta oscillations (4-7 Hz)
3. Alpha oscillations (8-12 Hz)
4. Beta oscillations (13-30 Hz)
5. Gamma oscillations (>30 Hz)

When this electric field interacts with brain oscillations (created by neurons generating electrical activity of their own), they affect these brain waves.

It involves training brain activity at a specific stimulation frequency. Low frequencies (0.1-80 Hz) and modulating cortical oscillations (cortical excitability) at higher frequencies in kHz. It does so by generating weak electrical fields from electrodes that modulate neuronal mechanisms and neuroplasticity (the capacity of neural connections to be moldable).

How tACS Stimulation Affects Brain Function

The sinusoidal tACS oscillations synchronize in intensity and frequency with brain oscillations to alter their activity or even desynchronize the activities of abnormally functioning neurons at different regions of the brain.

There are two mechanisms by which transcranial alternating current stimulation (tACS) affects brain functions:

1. Online/Immediate Effect: Change in Polarity of Neurons and Entertainment

In an applied electric field, the redistribution of charges occurs across the cell membrane. This modulates the ability of neurons to be hyperpolarized or depolarized, which changes the ease with which a neuron can fire an impulse.

2. Offline/Late Effect: Neuroplasticity and Neurochemical Modulation

Neuroplasticity is the offline (long-term effects) of transcranial alternating current stimulation (tACS). Neuroplasticity refers to a phenomenon of change in neuronal orientation and neuronal morphology. This effect implies that neural connections called synapses are more easily moldable and flexible, hence “Plastic.” This effect is also seen in the mechanism of action of anti-depressants such as SSRIs. 

Studies done with transcranial direct current stimulation (tDCS) have revealed neurochemical modulation in neurotransmitters like GABA and brain-derived neurotrophic factors (BDNF) and cell receptors like glutamate receptors, NMDA, and AMPA. A similar neurochemical modulation is expected in tACS as well.

Activity-Dependent Plasticity

The neuroplasticity achieved by transcranial alternating current stimulation (tACS) is specifically termed spike-timing-dependent plasticity (STDP). Two of either result can be elicited:

1. Long-term potentiation (LTP) in which the efficacy of synaptic transmission is improved.

2. Long-term depression (LTD), in which the efficacy of synaptic transmission is reduced.

Challenges of tACS And How Scientists Overcame It

tACS is performed by placing a large number of electrodes over an area on the scalp to accurately cover and optimize the effect of applied electrical stimulation over a focus of the brain region.

Brain tissue has been reported to deviate and deflect the electrical fields applied by a single large electrode. Researchers reported that tACS sessions were not producing the expected results since the low-intensity currents were not targeting the focus precisely. So they came up with two methods to optimize the efficacy of transcranial alternating current stimulation (tACS).

High-Definition Electrical Stimulation Method (HD-tACS)

HD-tACS works on the principle of brain stimulation by amplifying the electrical field intensity at a specific frequency to match the frequency of brain activity. This optimization method is achieved by replacing a single large electrode with an array of small electrodes that individually generate a specific electrical field to accurately cover a target over deep brain regions.

Electrodes can be placed in numerous ways, but the typical configuration is a 4x1 configuration, in which a central electrode is placed directly over the focus with 4 return electrodes surrounding it to circumscribe the area of stimulation.

Interference Modulation Method

The electrical field generated by two or more tiny electrodes is modulated by mutual interference for the current to reach a specific intensity, frequency, and phase, intended for the brain stimulation requirements of the focus.

What is tACS used for?

Transcranial alternating current stimulation can be used for multiple

Schizophrenia

Several case reports and clinical trials have been carried out to test the efficacy of tACS for its treatment, safety, and prognostic value. A case report suggested findings that are coherent with previous reports in reducing which states application of 20 min of tACS application on frontal and parietal regions in theta frequency (6 Hz) improved working memory and cognitive deficit over multiple sessions.

In several clinical trials, DLPFC was modulated for alpha oscillations (10 Hz) to improve persistent delusions that were not going away with medications. Patients received 20 minutes of tACS at 2 mA for several days. They showed a reduction in persistent delusions and improvement in positive and negative symptoms.

Depression

A double-blinded randomized clinical trial was conducted in which tACS was administered on both the left and right frontal regions to restore the increased alpha oscillations on the left side and bring synchronicity between both sides. 40 patients underwent tACS administration for 40 minutes sessions for five days.

These patients were divided into 2 groups: the first was given 10 Hz tACS at 4 mA, and the second was given 40 Hz tACS at 2 MA. The dorsolateral prefrontal cortex (DLPFC) was targeted. Researchers found out that the 10 Hz group had more responders and showed improvement in symptoms with no adverse effects than the 40 Hz group.

Obsessive Compulsive Disorder (OCD)

In OCD, gamma waves in the dorsolateral prefrontal cortex (DLPFC) were disturbed. A clinical study included patients with OCD to which 20 minutes of tAC stimulation sessions at 40 Hz were administered for a variable duration of two to seven weeks for different patients.

After doing so, patients were evaluated on Yale-Brown Obsessive Compulsive Scale (YBOCS) and Clinical Global Impression (CGI) scale. All patients showed improvement that lasted throughout the follow-up duration.

Attention-Deficit Hyperactivity Disorder (ADHD)

A clinical trial done on 18 patients of ADHD to test the efficacy of tACS concerning improvement in cognitive processes was carried out. P300 amplitude is disturbed in theta and alpha oscillations. This study was able to augment P300 amplitude with resulting behavioral improvements in cognitive processes and task performance.

Mild Cognitive Impairment (MCI)

In a clinical trial carried out by neuropsychiatric researchers, 25 individuals with MCI were included in the study to test out the prognostic importance of tACS. The gamma band oscillations showed enhancement in EEG (gamma oscillations are affected in MCI) and improvement in mild cognitive impairment according to various neuropsychological tests.

How Long Should tACS Be Used?

Low-intensity currents of 1-2 mA are applied for 20 minutes of stimulation duration according to the available literature. This can vary depending on the experience of clinicians, the device being used and data suggested from further clinical trials.

How long do the effects of tACS last?

A detailed clinical trial has reported the longest-lasting duration of tACS stimulation of alpha band frequency to be 70 minutes. Scientists do not yet know what factors influence this effect. However, the said clinical trial attempted to test the limits up to 90 minutes with promising results.

Why Choose tDCS Over Medications? 

There are two major reasons:

• tACS is supported by clinical literature
• It is a cost-effective procedure.

Is tACS Safe?

No persistent adverse effects have been reported as a result of tACS administration, rendering it safe. Two clinical studies suggested tACS be considered safe to administer in pregnancy. A few mild side effects that can appear include:

• Headache
• Tiny skin lesions resembling burns
• Confusion
• Seizures

However, more data from further clinical studies are needed to design proper safety guidelines for administering tACS to different age groups, body types, genders, co-morbidities, and ethnicities.

Conclusion

In conclusion, tACS has shown promising potential in improving brain health by modulating neural activity. It is a non-invasive and relatively safe method for improving brain function. Thus, tACS holds promise as a new tool for treating various neurological and psychiatric disorders and for cognitive enhancement in healthy individuals. Studies have demonstrated that tACS can enhance cognitive performance, improve mood, and alleviate symptoms of neurological conditions such as depression, Schizophrenia, and OCD.