Is transcranial Direct Current Stimulation beneficial for a healthy human brain?

Question

I recently ran into a website making some pretty interesting claims about "transcranial Direct Current Stimulation (tDCS)". The claims are as follows:

Overclock your brain using transcranial Direct Current Stimulation (tDCS) to increase the plasticity of your brain. Make your synapses fire faster.

While this seemed to be to be pretty obvious snake-oil pitch, I was curious as to whether there was any scientific basis for the claims made. There is a Wikipedia entry for Transcranial direct-current stimulation, indicating that it is "form of neurostimulation originally developed to help patients with brain injuries such as strokes." It also makes similar claims of beneficial effects in the article's initial summary, but they don't seem to be appropriately referenced:

Tests on healthy adults demonstrated that tDCS can increase cognitive performance on a variety of tasks [citation needed], depending on the area of the brain being stimulated. tDCS has been utilized to enhance language and mathematical ability, attention span, problem solving, memory, and coordination.

While I haven't conducted a thorough review of every scientific article referenced, most seem to relate to treatment of brain disorders rather than any effects on a healthy adult brain.

Is there any scientifically verifiable evidence that passing an electrical current across certain parts of a healthy person's brain can either a) increase the plasticity of the brain, b) make synapses fire faster, or c) otherwise improve a person's cognitive ability.

Answer 1

Research on tDCS actually dates back to 1960's when researchers performed DC stimulation on rats and found that stimulation induced lasting changes on the neuron's membrane potential, which effects its spontaneous firing activity.

I would recommend you check out this scientific peer-reviewed article on it which explains all of the research conducted with tDCS on human subjects over the course of 10-15 years:

Transcranial direct current stimulation: State of the art 2008

Summary

Effects of weak electrical currents on brain and neuronal function were first described decades ago. Recently, DC polarization of the brain was reintroduced as a noninvasive technique to alter cortical activity in humans. Beyond this, transcranial direct current stimulation (tDCS) of different cortical areas has been shown, in various studies, to result in modifications of perceptual, cognitive, and behavioral functions. Moreover, preliminary data suggest that it can induce beneficial effects in brain disorders. Brain stimulation with weak direct currents is a promising tool in human neuroscience and neurobehavioral research. To facilitate and standardize future tDCS studies, we offer this overview of the state of the art for tDCS.

Answer 2

Having looked through a few meta-analyses this looks like modern neuro snake oil. Transcranial direct current stimulation has no effect.

The overview (Transcranial direct current stimulation: State of the art 2008) in @asif's answer treats every single study like it's a special flower with its own peculiarities and statistically significant effects all mean something like there's no such thing as a Type I error rate. I recognize that it's not a meta-analysis but one must be careful reading a paper such as that. Most of the time the reason one study shows a significant effect and one does not is simply random chance and a meta-analysis helps with confusing literature, especially one with many advocates.

Check out Horvath, Forte & Carter (2015) Quantitative Review Finds No Evidence of Cognitive Effects in Healthy Populations From Single-session Transcranial Direct Current Stimulation (tDCS):

BACKGROUND: Over the last 15-years, transcranial direct current stimulation (tDCS), a relatively novel form of neuromodulation, has seen a surge of popularity in both clinical and academic settings. Despite numerous claims suggesting that a single session of tDCS can modulate cognition in healthy adult populations (especially working memory and language production), the paradigms utilized and results reported in the literature are extremely variable. To address this, we conduct the largest quantitative review of the cognitive data to date.

[...]

RESULTS: Of the 59 analyses conducted, tDCS was found to not have a significant effect on any - regardless of inclusion laxity. This includes no effect on any working memory outcome or language production task.

Consider briefly one claim of advocates of tDCS, that the polarity of the connection matters. The direction electrons flow through the brain cannot possibly matter given that the neurons, axons, dendrites, synapses, etc. are randomly oriented in the brain with respect to that current flow. Should one neuron be affected such that it is excited by current in one direction an adjacent one will be affected in just the opposite way.