What Does tDCS Stand For?
tDCS stands for "transcranial direct-current stimulation" - and it looks like something out of a 90's sci-fi movie.
You attach electrodes to your scalp and apply a weak electrical current. This current flows through the brain, altering neural activity.
The hope is that tDCS may alleviate depression, anxiety, and even improve cognitive function in healthy subjects. The Fisher Wallace Stimulator is a device that's FDA-approved for depression, anxiety, and insomnia. However, this device isn't technically tDCS because it uses an alternating current.
tDCS is popular among hobbyists and DIYers. The practice has spawned a whole community and even has its own subreddit.
tDCS and Cognitive Enhancement
Carter's group10 conducted the largest meta-analysis of its kind on tDCS, incorporating results from hundreds of experiments. The authors reported that there was no evidence to support a pro-cognitive effect of a single session of tDCS10.
A second study further found that tDCS had no effect on any neurophysiologic outcome 11. However, other studies suggest that tDCS is effective for depression. This was the finding in Moffa's meta-analysis, where lower doses and treatment resistance were positively correlated with the efficacy of tDCS for depression 12.
For an overview of the skeptic's position on tDCS, see this post.
What Is A tDCS Device?
From an engineering perspective, tDCS is extremely simple.
There are only a few components:
- electrodes (usually backed with foam/sponges)
- a power supply
- a headband
Human skin and hair aren't fantastic conductors of electricity. tDCS requires foam pads because they can be dipped in saline solution which improves skin's ability to conduct electricity.
Here's what some tDCS sponges look like:
How tDCS works
What happens when you don a tDCS device and flip the on switch? In vague terms, tDCS applies a weak electrical current to the brain.
A pair of electrodes is placed on the scalp to stimulate the brain region below the surface. This electrical stimulation can modulate the polarization and excitability of the neural membrane. Electrical stimulation also promotes neurotransmitter release (GABA, glutamate, acetylcholine, serotonin and dopamine).
Anodal vs Cathodal tDCS
The polarity of the stimulation, identified as anodal or cathodal, can excite or inhibit neurotransmission. An anode is an electrode with a positive charge. A cathode is an electrode with a negative charge.
Anodal tDCS increases cortical excitability and could improve cognitive or motor performance. Cathodal tDCS decreases excitability and should reduce performance. But reality is not so cut-and-dry and many variables influence the outcome of tDCS.
What is "Sham tDCS"?
Lot's of studies will mention sham tDCS.
To test the efficacy of a drug in clinical trials researchers give a placebo to the control arm of the study. The placebo is usually just a pill with inactive ingredients like cellulose.
Sham tDCS is analogous to administering placebo to your control group. You bring in participants in the control group, have them don a tDCS device, but no current is ever delivered to the subjects.
tDCS and Intensity
Several factors affect the degree of electrical stimulation:
- The intensity of the electric current
- The location, and size of the electrodes
- The conductance of your scalp
Methodological differences across tDCS studies present in literature are another confound.
Intra-subject variability also plays a role. Baseline levels of cortical excitability and the ability to perform a certain task affect outcomes in tDCS studies.
Site of Stimulation
The site of stimulation is also a big determinant of the effect.
Anodal tDCS can impair a linguistic performance when applied to the temporal lobes. Since this area is involved in lexical activation and retrieval, its activation may increase interfere with signal coherence.
But the same anodal stimulation can enhance linguistic performance when applied to the frontal lobe. As the frontal cortex is involved in selection, it’s activation can indeed increase linguistic control.
A limitation of tDCS devices is that they aren't targeted. Electrical stimulation can interfere with large areas of the cortex. This implies recruiting functionally or anatomically connected regions. Making tDCS interventions
To predict the effect of the stimulation, the prior state of the cortex needs to be taken into account.
The classical idea of non-invasive brain stimulation was that a brain region can, more or less, be mapped to a specific brain function. By enhancing or disrupting such region (and its related network), it is possible to enhance or disrupt the underlying function.
However, an accurate control of the brain state seems essential for a precise brain stimulation. Online and prior brain conditions are both essential.
Information-Based Brain Stimulation
Information-based brain stimulation refers to the idea that using prior knowledge about the functional, physiological, and anatomical properties of the cortex is crucial to achieving optimal results from brain stimulation.
Participants are not conceived anymore as passive receivers of an external stimulation. And brain regions are not just structures to disrupt or enhance. A detailed prior knowledge of the state of the targeted brain is a key requirement in this conception.
One way to better control tDCS is to ask the participant to perform a specific task before or even during the stimulation session. The rationale is to induce a known activation that should favor the following (or simultaneous) stimulation.
Another possibility is to use a rhythmic stimulation protocol. Rhythmic stimulation can target oscillatory brain activity (like alpha or theta power). Some frequency bands participate in specific cognitive functions. Enhancing or disrupting a frequency band should result in the same effect on the related brain function.
What is tDCS Treatment?
The idea here is that tDCS - or trans-cranial direct-current stimulation - can be used to treat neurological conditions. Tentative evidence suggests tDCS can benefit:
- motor learning - the acquisition and retention of new motor skills
- neurorehabilitation - e.g., to facilitate stroke recovery
- neuropsychiatric conditions - e.g., depression, anxiety, insomnia
The effects of tDCS on language, learning, and memory are controversial. At this point it's likely that tDCS does not benefit healthy subjects.
Interestingly, when tDCS is combined with motor training, it may facilitate stroke recovery. Both anodal tDCS over the affected motor cortex and cathodal tDCS over the intact motor cortex obtained such result.
tDCS has shown promise as a treatment for neuropsychiatric conditions.
One study reported that inhibitory stimulation over the left temporo-parietal cortex reduced hallucinations in patients with schizophrenia. Moreover, excitatory stimulation over the left dorsolateral prefrontal cortex alleviated some negative symptoms of schizophrenia.
Emerging evidence also points to the use of tDCS in language training. Facilitating effects were observed in both healthy individuals and aphasic patients.
In a study on depression, participants were tested on a task involving sustained attention and working memory. Patients showed better antidepressant responses after non-invasive brain stimulation. The rationale was to enhanced frontal theta power by performing this task. Frontal theta power was, indeed, associated with better antidepressant effect of TMS in previous studies.
TDCs and Cognitive Enhancement
The use of tDCS in the healthy has been criticized heavily.
In a simple motor task, for example, faster reaction times followed anodal tDCS stimulation over the primary motor cortex.
Some studies have found that decision-making and working memory can be influenced, too, by stimulation of the prefrontal or parietal cortex. But recent meta-analyses have raised doubts about these findings.
One hypothesis is that tDCS, when applied correctly, can help heal brain pathology. Where neuronal communication is dysfunctional, there is room for improvement. But it's not at all clear that already optimized, healthy brain stands to benefit from tDCS.
Many questions about tDCS remained unanswered. How exactly do training-related neural processes work? What is the duration of tDCS induced modulations? These answers are necessary to fine-tune interventions and maximize treatment benefits.
Boosting Video Game Play? Really?
Some companies sell easy-to-use sets which claim to boost your gaming and sport performance with tDCS.
By stimulating the prefrontal cortex - which has a well-defined role in executive function - these tools promise to increase brain communication and help users become more effective players. Such applications are as interesting as they are scary.
Scientists have issues warnings against do-it-yourself tDCS devices.
The California Public Health Department issued a warning to consumers in 2013 not to use unapproved medical devices sold on the internet.
TDCS Device Kit, Inc. of Petaluma, Calif., is voluntarily recalling the TDCS Home Device Kits because the product has not been federally approved to market in the United States, and has not been determined to be safe and effective for their intended use. During a recent inspection, CDPH determined that the devices had not been manufactured in compliance with good manufacturing practices for medical devices. Also, the devices were found to be labeled without adequate directions for use and without adequate warnings against uses that may be dangerous to health.
The risk is to confuse controlled scientific protocols with improvised devices claiming to improve your brain activity. Unusual applications can put subjects at risk and distort the long-term validation of good tDCS practice.
Many parameters of the electrical brain stimulation need to be taken into account for a safe use of tDCS. Institutional ethics boards ensure that the necessary guidelines are followed in controlled environments.
Reported short and long-term risks of tDCS don’t seem worrisome. But this only applies to the scientific and controlled implementations of tDCS. The effects of a home-made use of tDCS are quite hard to predict. Until we know, our current brain abilities may work just fine.
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