Transcranial alternating current stimulator (tACS) is a non-invasive brain stimulation method in which electrical current is applied to the scalp to alter brain activity. With origins dating back to the early 20th century, researchers began using electrical stimulation brain activity and studied the role of brain waves and neural communication. However, these first methods are labor intensive and involve the insertion of electrodes directly into the brain, preventing their use and application.
The development of tACS device technology
It was not until the 1960s that non-invasive electrical stimulation methods were developed, such as transcranial direct current stimulation (tDCS) and transcranial magnetic stimulation (TMS). These methods have provided a way to change brain activity without invasive procedures and have made great opportunities and safety in research and clinical settings.
In the 1990s, researchers began to explore the use of alternating current stimulation as a non-invasive way to change brain activity. The first study using tACS devices involved simultaneously applying electrical current to the shoe and measuring changes in brain activity using electroencephalography (EEG).
Since then, research into tACS has grown rapidly, with studies investigating its ability to improve cognitive function, treat neurological disorders, and even improve sleep quality. Researchers have identified certain frequency bands, such as alpha, beta, gamma and delta, which correspond to different neural processes and can be targeted with tACS to change brain activity.
This article will review the current state of knowledge about tACS, its mechanism of action, and potential applications, while comparing it to another commonly used brain stimulation technique, transcranial direct current stimulation (tDCS). For an in-depth review of all the pros and cons of both technologies visit our tDCS vs tACS page.
How tACS devices work
In tACS, low-frequency electrical currents are applied to the scalp at specific intervals, reactivating neuronal activity in targeted brain regions. Electric currents can interfere with the brain’s natural oscillatory rhythm, promoting or inhibiting the activity of specific brain networks. For example, the application of alpha-frequency (8-12 Hz) tACS to the occipital cortex has been shown to cause an increase in visual evoked potentials, suggesting visual processing.
Despite the fact that the mechanism by which tACS modulates brain activity is not fully understood, studies show that electrical currents can affect neuron firing rates and neural network interactions. One proposed mechanism is that tACS can drive the endogenous oscillatory activity of the brain by applying an external time to continuous oscillations. This relaxation can strengthen the connectivity of the neural network and improve the efficiency of information processing.
tACS has shown promise in various neuroscience applications, particularly in the treatment of neurological disorders such as epilepsy, depression, and chronic pain. For example, a recent study investigated the effects of alpha-frequency tACS in patients suffering from chronic neuropathic pain and found that the stimulation reduced pain and improved quality of life. Another study applied delta frequency (1 Hz) tACS to patients with major depressive disorder and found that stimulation improved symptoms of depression compared to sham stimulation.
tACS is also used in cognitive neuroscience research to study the neural mechanisms underlying various cognitive functions, such as attention, memory, and decision making. For example, one study examined the effects of gamma-frequency (40 Hz) tACS on working memory performance and found that stimulation improved the accuracy and speed of working memory tasks. Another study applied beta-frequency (20 Hz) tACS to the prefrontal cortex and found that the stimulation improved decision-making performance.
Compared to tDCS, tACS has key differences in mechanism and effect. While tDCS applies directly to the head, tACS applies a variable, which can cause ongoing oscillatory activity of the brain. tACS can also alter the interactions of neural networks, while tDCS has a more localized effect on neural excitability. In addition, tDCS has been shown to have more lasting effects than tACS, with some studies reporting effects that last longer than the duration of stimulation.
Using tACS has both pros and cons
Despite the potential benefits of using transcranial alternating current stimulation devices, there are still limitations and challenges associated with this technique. One limitation is the lack of standardized stimulation methods, since there is currently no consensus on the optimal timing, intensity and duration of tACS. This variability can make it difficult to compare results between studies and limit the reproducibility of results. Another challenge is the difficulty in targeting certain brain regions with tACS. Although the electric current can change the activity in the targeted area of the brain, it can also spread to the adjacent areas, which can cause unexpected effects. This issue can be solved by using self-stimulation based on neuroimaging data, which can identify the best targets for stimulation and ensure high accuracy.
In addition, tACS is not suitable for everyone, some people may experience side effects such as headache, dizziness, or physical irritation. It is important to consider individual factors such as age, drug use, and medical history when administering tACS and to monitor participants for any adverse effects.
Despite these challenges, tACS has great potential in the field of neuroscience and may lead to new treatments for various neurological diseases and cognitive deficits. For example, tACS can be used together with neurofeedback to improve self-regulation of brain activity and improve cognitive function in people with attention deficit hyperactivity disorder (ADHD) or autism spectrum disorder (ASD).
Recent developments in tACS technology have led to the creation of wearable devices that can be used outside the laboratory. These devices can make tACS easier and better, allowing long-term or even daily stimulation to improve cognitive function or treat neurological problems.
Advances on tACS devices: HD-tACS
The main difference between High Definition Transcranial Alternating Current Stimulation (HD-tACS) and tACS lies in the spatial precision of the stimulation. HD-tACS utilizes an array of smaller electrodes, typically arranged in a grid or pattern, which allows for more targeted and precise stimulation of specific brain regions. This spatial precision is achieved by directing the current flow between the different electrodes, thereby enabling the targeting of specific neural circuits or areas of the brain with higher accuracy.
In contrast, traditional tACS typically uses larger electrodes, which distribute the current more broadly over the scalp. While tACS can also target specific brain regions to some extent, it lacks the same level of spatial resolution as HD-tACS. The broader current distribution of tACS may result in less precise stimulation and a wider spread of effects across the brain.HD-tACS has been developed to overcome some of the limitations of traditional tACS by offering more focal and specific stimulation, which is particularly valuable for studies or applications that require precise targeting of particular brain areas or circuits. However, it is important to note that both HD-tACS and tACS are still evolving techniques, and further research is needed to fully understand their mechanisms of action and potential therapeutic applications.
Finally, transcranial alternating current stimulation devices are a promising technique in the field of neuroscience that has the potential to modulate brain activity and improve cognitive function. Although there are still limitations and challenges associated with this process, research continues to expand our understanding of the underlying mechanisms and discover new applications. By developing standardized protocols and personalized stimulation methods, tACS can lead to new treatments for many neurological diseases and cognitive deficits, and ultimately improve the quality of life for many people.