Transcranial Direct Current Stimulation (tDCS) and its potential in improving executive function

By Dr Martine Stoffels, consultant neuropsychiatrist and clinical director, Phoenix Mental Health Services 

Transcranial Direct Current Stimulation (tDCS) has emerged as a promising non-invasive neuromodulation technique that modulates brain activity by delivering a low electrical current to targeted regions of the brain.

tDCS is used in people with depression and is used in people who are either treatment resistant or are unable to tolerate antidepressants.

Among its various applications, tDCS has been increasingly explored for its potential to improve executive function.

Executive functions encompass a range of cognitive processes including working memory, attention, cognitive flexibility, planning, and problem-solving.

These functions are crucial for everyday tasks, decision-making, and goal-directed behaviour.

This article will explore the evidence behind the use of tDCS to improve executive function and its potential in both healthy individuals and those with neurological impairments.

What is tDCS?

tDCS involves the application of a weak electrical current, typically between 1 to 2 milliamps, through two electrodes placed on the scalp.

The anode electrode (positive) enhances neural activity in the brain region it is placed over, while the cathode electrode (negative) typically inhibits activity in the region beneath it.

The electrical current applied during tDCS is low in intensity and is usually applied for 10-30 minutes, making it a safe and well-tolerated intervention.

The mechanism of action of tDCS relies on its ability to alter the membrane potential of neurons, which affects their excitability.

This means that tDCS influences the likelihood that neurons will fire in response to a given stimulus.

Anodal stimulation tends to depolarize neurons, making them more likely to fire, while cathodal stimulation hyperpolarizes neurons, making them less likely to fire.

This modulation can influence cognitive processes by either enhancing or inhibiting specific neural circuits involved in tasks such as working memory, attention, and decision-making.

The Schedule of Treatment

A typical tDCS treatment involves several sessions, often ranging from five to 20 sessions, depending on the condition being treated and the individual’s response.

The treatment is generally administered on a daily basis or every other day, with each session lasting between 10 and 30 minutes.

In some cases, individuals may experience cumulative benefits, meaning that the effects of tDCS build up over time with repeated stimulation.

A treatment schedule for depression typically incudes 5 sessions a week for 3 weeks, followed by 2 weeks of 3 sessions a week. Some individuals however befit from sessions 5 days a week on a long term basis.

tDCS is often combined with cognitive training exercises or rehabilitation programs to enhance its effects. In these cases, individuals perform cognitive tasks during or shortly after the stimulation to capitalise on the increased neural plasticity induced by tDCS.

Practicalities of treatment

Transcranial direct current stimulation (tDCS) can be delivered at home, and it is increasingly being used in home settings, especially with the rise of commercially available, user-friendly devices.

These devices are designed to be safe and easy to use, typically involving a small battery-powered unit that delivers a low electrical current through electrodes placed on the scalp. Users can administer tDCS to themselves by following the instructions provided by the device manufacturer.

One example is the Flow device, which runs off an app on a smart phone. Stimulation happens at home,; whilst the treatment occurs, participants can complete other tasks if they wish to, such reading emails.

There are also encouraged to complete exercises between sessions to maximise treatment efficacy. Exercises include mindfulness activities, behavioural activation. Mood is monitored through regular questionnaires.

How tDCS Enhances Treatment: Modulating Brain Electrical Conductivity

tDCS is particularly effective because it influences the electrochemical properties of brain cells.

Neurons communicate with one another through electrochemical signals—an essential process for all cognitive functions.

By modulating the electrical conductivity of neurons, tDCS alters the brain’s overall excitability, thus influencing the ease with which neural signals are transmitted across synapses.

This makes tDCS an effective tool in both enhancing cognitive function and promoting neuroplasticity, which is the brain’s ability to adapt by forming new connections.

Since tDCS is capable of influencing the brain’s electrical activity, it can help facilitate the transmission of signals between neurons in specific brain regions.

This ability to modulate the synaptic activity has implications for improving functions like memory, decision-making, and problem-solving.

By promoting enhanced communication within neural circuits, tDCS can support the rehabilitation of cognitive functions that have been impaired due to conditions like stroke, traumatic brain injury, or neurodegenerative diseases.

The Brain as an Electrochemical Organ

To understand the impact of tDCS, it is crucial to see the brain not only as a complex network of cells but also as an electrochemical organ.

The brain’s electrical activity is the result of ions moving across the membrane of neurons, generating electrical signals that facilitate communication between cells.

These electrical signals, combined with chemical neurotransmitters, enable the brain to process information and regulate behavior.

Neurons communicate across synapses by releasing neurotransmitters that bind to receptors on neighboring cells, triggering electrical changes that propagate the signal.

The electrochemical nature of the brain is foundational to its function. Without the precise balance of electrical and chemical activity, neurons would be unable to transmit signals efficiently, and cognitive functions would be impaired.

This is why tDCS, which directly influences the electrical properties of neurons, is able to modulate brain activity and improve executive functions.

By optimising the brain’s natural electrical processes, tDCS facilitates enhanced signal transmission and neuronal communication, allowing for better cognitive performance.

Electrochemical Activity in the Brain: Passing Signals Between Neurons

The brain’s electrochemical activity is necessary for neurons to pass signals from one to the next.

Neurons are equipped with an electrical membrane potential that fluctuates depending on the ion concentration inside and outside the cell.

When a neuron is activated, ion channels open, allowing charged particles (such as sodium and potassium ions) to flow in and out of the cell.

This creates a small electrical impulse, known as an action potential, which travels down the length of the neuron to the synapse, where neurotransmitters are released to transmit the signal to the next neuron.

This process is critical for all cognitive functions, from basic motor control to complex tasks like decision-making.

The efficiency of signal transmission depends on the neuron’s membrane potential and the excitability of the synapses.

By applying tDCS, the membrane potential of neurons can be altered, making them more or less likely to transmit signals.

This modulation of neural excitability is one of the reasons why tDCS has the potential to improve cognitive performance, particularly in tasks that rely on executive functions such as attention, working memory, and problem-solving.

tDCS and Executive Function: The Evidence

A growing body of research has suggested that tDCS can have beneficial effects on executive function, particularly in the prefrontal cortex (PFC), a brain region that plays a pivotal role in these higher cognitive processes.

Several studies have demonstrated that tDCS can improve performance in tasks related to working memory, decision-making, and cognitive flexibility, which are central components of executive function.

1. Enhancing Working Memory

Working memory is one of the most studied areas of executive function in relation to tDCS. Several studies have shown that anodal stimulation over the dorsolateral prefrontal cortex (DLPFC) can improve performance on working memory tasks.

For example, a study by Dockery et al. (2009) demonstrated that anodal tDCS applied to the DLPFC improved performance in a working memory task, suggesting enhanced cognitive processing (Dockery et al., 2009).

Similarly, Fregni et al. (2005) found that tDCS improved cognitive performance in working memory tasks in healthy adults, providing further evidence of its potential to enhance executive functions (Fregni et al., 2005).

2. Cognitive Flexibility and Problem-Solving

Cognitive flexibility—the ability to adapt thinking and behavior in response to changing demands—is another executive function that tDCS has been shown to influence.

Hsu et al. (2015) found that anodal stimulation to the DLPFC significantly improved performance on tasks requiring cognitive flexibility, such as task-switching and set-shifting (Hsu et al., 2015).

Furthermore, a study by Brunoni et al. (2013) examined tDCS’s effects on problem-solving abilities in individuals with depression, a condition known to impair executive function.

The results indicated that tDCS over the PFC led to improvements in problem-solving tasks (Brunoni et al., 2013).

3. Attention and Decision-Making

tDCS has also been shown to improve attention, which is a critical component of executive function.

A study by Stagg et al. (2011) found that anodal stimulation to the DLPFC enhanced participants’ ability to maintain attention and focus during complex cognitive tasks (Stagg et al., 2011).

Additionally, Cohen Kadosh et al. (2010) showed that tDCS improved decision-making speed and accuracy, demonstrating its potential to influence executive function processes beyond simple memory tasks (Cohen Kadosh et al., 2010).

tDCS in Clinical Populations

While much of the research on tDCS and executive function has been conducted in healthy individuals, studies have also explored its effects in clinical populations with conditions that impair executive function, such as people who have difficulties with attention-deficit and hyperactivity, stroke, and traumatic brain injury (TBI).

1. Attention Deficit and Hyperactivity

Executive function deficits are prominent, particularly in areas like working memory and inhibitory control.

Several studies have explored tDCS as a treatment for these cognitive impairments. Fitzgerald et al. (2016) reported improvements in working memory performance in individuals with ADHD following tDCS over the DLPFC (Fitzgerald et al., 2016).

This suggests that tDCS may offer a non-pharmacological option for improving executive function in ADHD patients.

2. Stroke

Individuals with stroke or TBI often experience significant executive function deficits, including poor planning, decision-making, and difficulty with cognitive flexibility.

Research has indicated that tDCS can aid in the rehabilitation of these cognitive functions.

For example, López-Alonso et al. (2014) found that tDCS applied to the DLPFC in stroke patients led to improvements in cognitive tasks related to executive function (López-Alonso et al., 2014).

3. Traumatic Brain Injury (TBI)

Similarly, a study by Stagg et al. (2018) showed that tDCS could enhance executive function in patients with TBI, leading to improved attention and problem-solving abilities (Stagg et al., 2018).

Conclusion

tDCS offers a promising avenue for improving executive function, with evidence suggesting it may enhance cognitive processes such as working memory, cognitive flexibility, and attention.

While the technique has shown positive effects in both healthy individuals and clinical populations, more research is needed to determine the most effective protocols and to better understand its long-term benefits.

As technology advances and research continues, tDCS may become an integral tool in both cognitive enhancement and the rehabilitation of executive function in individuals with neurological conditions.

References

• Dockery, C. A., et al. (2009). “The effects of transcranial direct current stimulation on working memory in healthy human participants.” Neuropsychologia, 47(7), 1912-1919.
• Fregni, F., et al. (2005). “Anodal transcranial direct current stimulation of the prefrontal cortex enhances working memory.” Experimental Brain Research, 166(1), 23-29.
• Hsu, T. Y., et al. (2015). “Enhancing the efficiency of cognitive processing in the prefrontal cortex using transcranial direct current stimulation.” Frontiers in Human Neuroscience, 9, 413.
• Brunoni, A. R., et al. (2013). “The impact of transcranial direct current stimulation on cognitive function in patients with depression.” Journal of Affective Disorders, 147(1), 80-85.
• Stagg, C. J., et al. (2011). “Modulation of cognitive function and mood in healthy individuals using transcranial direct current stimulation.” Neuropsychopharmacology, 36(12), 2689-2697.
• Cohen Kadosh, R., et al. (2010). “The influence of transcranial direct current stimulation on decision making and its application to mental disorders.” Biological Psychiatry, 68(2), 95-98.
• Fitzgerald, P. B., et al. (2016). “A systematic review and meta-analysis of transcranial direct current stimulation in the treatment of ADHD.” Neuropsychology Review, 26(2), 160-179.
• López-Alonso, V., et al. (2014). “Transcranial direct current stimulation in stroke rehabilitation.” Neurorehabilitation and Neural Repair, 28(6), 567-574.
• Stagg, C. J., et al. (2018). “Non-invasive brain stimulation in the treatment of traumatic brain injury: Current and future directions.” Neuropsychology Review, 28(1), 90-102.