Transcranial Alternating Current Stimulation (tACS) Devices: How They Differ from tDCS

If you’re researching brain stimulation techniques, we have no doubts that a transcranial alternating current stimulation device might suddenly pop out. However, besides tACS devices, an increasing number of transcranial direct current stimulation (tDCS) machines appear too.

It’s easy to get confused, and hard to immediately answer: so what is the difference?

In this article, we will clarify the differences between these two technologies: tACS and tDCS. We will also dive deeper into the clinical profiles of both, and you will soon be able to understand how each differs in terms of mechanics, brain targets, and application.

We will also focus on the evidence base of these devices, and discuss where tACS is mostly being used. And if by any chance you haven’t read about tACS yet, let’s start there.

What Is tACS? Understanding Alternating Current Stimulation

The Technical Definition

Similar to how tDCS works, tACS uses electrical current delivered through scalp electrodes to stimulate brain activity. The key difference between the two is that tACS uses alternating currents, which oscillate, gently moving back and forth in time with neural rhythms, at specific frequencies rather than constant direct current.

Such frequencies typically range from 1-100 Hz depending on the target area. Like tDCS, tACS treatment is non-invasive and uses a similar setup.

What It Targets

tACS treatment influences brain activity by targeting specific neural oscillations and aims to train and affect the natural rhythms of the brain.

You might have heard about brain waves, and we’re talking exactly about that. For example, alpha waves (which are reflective of a restful or creative brain state) are characterized by 8-12 Hz frequencies, and gamma waves (representing multi-processing and peak experiences state) by 30+ Hz.

This differs from tDCS: the latter modulates the overall excitability of neurons rather than rhythmic activity.

The Analogy

To understand the differences between tACS vs tDCS, think of it like a radio: if tDCS is like adjusting the baseline volume of brain activity, then tACS treatment is more like tuning the channels.

Rather than changing overall activity levels, tACS aims to align brain activity with particular rhythmic frequencies, strengthening or synchronizing patterns that are already present.

tACS vs tDCS: What's Actually Different?

• Current Pattern: tDCS applies a constant, unidirectional, direct current between electrodes placed on the scalp. This steady flow changes the electrical environment of the targeted brain region. tACS, in the meanwhile, uses an alternating current that oscillates back and forth at specific frequencies (e.g., 10 Hz, 40 Hz, or others), allowing the stimulation to interact with the brain’s rhythmic activity.

• Mechanism of Action: Each technique influences the brain in different ways. As slightly mentioned earlier, tDCS primarily modulates overall neural excitability, making neurons more or less active (or likely to “fire”). This mechanism underlies many clinical protocols, including stimulation of the dorsolateral prefrontal cortex (DLPFC) in depression. In comparison, tACS aims to train brain oscillations to influence the coordination of neural activity.

• Primary Applications: tDCS has been studied for many conditions and is increasingly used in routine care settings. To name a few, we’re talking about depression, chronic pain, stroke rehabilitation, cognitive enhancement, and others.
  
  tACS is most commonly applied in research contexts, where it is used to investigate where it will best fit, if proven to be of use, in clinical settings. So far, attention is focused on cognitive functions, motor control, sleep, and some psychiatric conditions.

• Clinical Evidence Base: tDCS has two decades of research, hundreds of clinical trials, and multiple regulatory approvals for medical devices and evidence to back it up. On the contrary, tACS research is only now growing, but has fewer large-scale clinical trials and comparatively narrow regulatory clearance.

• Availability: Availability of these treatments varies as well, primarily due to the reasons above.

tDCS is already accessible in clinical environments, and in some cases (such as Flow Neuroscience) through regulated home-use devices. tACS devices are largely confined to research laboratories and experimental settings.

Where tACS Is Being Used (Mostly in Research)

Cognitive Research

At the moment, tACS is catching up with tDCS in terms of a clinical backing in the form of studies on working memory, attention, learning, and perception.

It is, however, present in investigations on how brain rhythms relate to cognitive function as well. An example of this can be seen in how alpha frequency stimulation (8-12 Hz) relates to visual processing.

Despite that, we want to emphasise that this technique is still largely experimental and is not fully ready for clinical applications.

Motor Function Research

If we had to highlight one area, we should mention that there has been research on the use of tACS in movement disorders and motor learning in which frequency-specific stimulation is important.

Different motor functions are associated with distinct neural rhythms. For example, beta-frequency stimulation (~13–30 Hz) is commonly used in motor studies because beta rhythms are closely linked to motor control.

By interacting with these rhythms, tACS provides researchers with a tool to probe how motor-related brain oscillations contribute to movement and learning.

Sleep Research

Another area is sleep research. It is known that low-frequency tACS, with stimulation at around 1 Hz, is designed to interact with slow-wave activity associated with deep sleep.

Early studies suggest that this may enhance aspects of the mentioned sleep stage and potentially improve some processes such as memory consolidation. Despite interesting primary findings, it is not yet ready for routine clinical use, too.

Mental Health Research

Finally, studies in depression, schizophrenia, and obsessive–compulsive disorder have too been a focus in tACS research. Much of this focuses on how disrupted brain oscillations and neural rhythms may contribute to psychiatric symptoms, and whether frequency-specific tACS brain stimulation can help modulate these patterns.

While some pilot studies using a transcranial alternating current stimulation device have reported promising effects, the evidence base remains far smaller than that for direct current stimulation.

Why tDCS Has More Clinical Evidence (And Why That Matters)

The Evidence Base for tDCS

tDCS has been studied for over 25 years in clinical populations, meaning it has more evidence and research to back up its uses for treatment. This is the result of hundreds of trials researching tDCS as a device for depression, involving thousands of participants across all applications.

For example, large-scale trials like the one led by the University of East London in collaboration with King's College London using a Flow device found that 10 weeks of tDCS delivered remotely improved depressive symptoms in adults compared with placebo stimulation. Following research in Nature Medicine, this technology is already used in routine clinical care in some countries, including several NHS clinics in the UK.

From a regulatory perspective, tDCS devices like Flow’s have already received all major regulatory clearances and certifications including CE marking, FDA approval, UKCA & MDR certification, and TGA listing, all based on extensive safety and efficacy data.

The Evidence Base for tACS

Every transcranial alternating current stimulation device, and tACS as a technology, is a relatively new approach in clinical research, with studies emerging over the past 15 years.

Compared with more established neuromodulation techniques, tACS has not been evaluated in large-scale clinical trials, but emerges from small pilot studies often focusing on proof-of-concept rather than clinical efficacy.

tACS, therefore, has limited regulatory approval as a medical device, reflecting the need for larger, better-controlled studies to establish safety and standardised protocols.

Why This Matters for Treatment Decisions

When taking a treatment decision, you want something that has been extensively studied in clinical settings, with a wide range of demographics. Many times, preliminary laboratory findings do not translate into clinical benefit. In the context of depression, tDCS has evidence of such benefit, whereas tACS has only preliminary findings.

Always remember that regulatory approval serves as an important indicator that a treatment has undergone independent evaluation for both safety and efficacy. Clinicians and patients should consider these factors when considering treatment options.

Flow's Position: Why We Use tDCS

Following the Evidence

Flow uses tDCS because the clinical evidence for depression treatment is strongest, as simple as that. With 55,000+ users across the world with real-world data, 77% of whom report improvement after 3 weeks of consistent use, this backs up the already compelling evidence in previous studies, such as the largest tDCS depression trial to date (conducted by King's College London, with 174 participants in the study.)

Regulatory Confidence

Regulatory approvals and certifications (CE, MDR, UKCA, FDA, TGA) are all based on tDCS protocols and not tACS. This is because such approvals require rigorous safety and efficacy, which no transcranial alternating current stimulation device has, at least yet.

With these approvals, users can have confidence in the independently verified effectiveness of tDCS.

Monitoring Emerging Research

Since Flow follows emerging research, tACS definitely falls under our radar. It is, indeed, an active research field.

If compelling clinical evidence emerges, this will trigger a reassessment as Flow always seeks to provide the best level of care to users. For now, we prioritise proven approaches, and tDCS is the proven method that has a strong evidence base behind it.

Could tACS Become More Common in the Future?

The Research Trajectory

tACS research is steadily growing among increasing scientific interest, both from patients and clinicians, as well as researchers. While some applications remain exploratory, certain areas, such as sleep enhancement and cognitive function, are showing promise. Researchers are building a foundation for potential clinical use as we speak. Continued innovation in protocols and a better understanding could expand its relevance.

Path to Clinical Use

Like all clinical tools, tACS will require rigorous and sizable trials to demonstrate safety and efficacy. As with tDCS, regulatory approvals are essential to ensure independent verification. Some applications may move from lab research to clinical settings faster than others in areas where success is delivered, but only with repeatable and reliable results.

Timeline Realistic Expectations

Now, tACS is primarily a research tool with clinical applications still years (definitely not months) away. While early studies are encouraging, widespread medical use will require careful validation and integration into standard care practices.

All scientists and clinicians, such as those working in Flow, monitor emerging data with interest, remaining adaptable and critical to evaluate tACS alongside other interventions in the future. In doing so, we ensure that patient care remains both innovative and evidence-based.

Choosing a Neuromodulation Approach: Questions to Consider

Like any treatment, it is important to ask the right questions to make an informed choice. With your doctor, you may try to answer these questions first:

• What condition are you trying to treat?
• What's the strength of clinical evidence for that specific condition?
• Is the device you're considering clinically validated in trials?
• What regulatory approvals does it have?
• Are you comfortable using something experimental (tACS) or do you want established evidence (tDCS)?
• Is the device available for home use or only in research/clinical settings?
• What support and monitoring comes with it?

Discuss whether the treatment is correct for the illness. If you’re eligible for something more experimental, ask if you are comfortable with that, or if you prefer treatments with established evidence.

It is only by asking professionals such questions that you can come to better understand which treatment option is better for you.

Conclusion

Both tACS and tDCS use electrical brain stimulation through scalp electrodes, but they work through different mechanisms. For depression treatment, tDCS has the required clinical evidence to achieve regulatory approval, while at this time tACS remains primarily in research.

Given tDCS has the strongest evidence base for depression, validated in the largest tDCS depression trial to date with over 55,000+ real-world users, it becomes a clear choice for treatment.

As the neuromodulation field evolves, Flow will continue monitoring emerging tACS research while prioritizing proven, evidence-based approaches.