remission, and depression improvement. the results showed statistically superior efficacy of active tdcs compared to sham for nine eligible studies (572 participants), presenting moderate/high certainty of evidence, were included. active tdcs was significantly superior to sham for response (30.9% vs. 18.9% respectively; or = 1.96, 95%ci , nnt = 9), remission (19.9% vs. 11.7%, or = 1.94 , nnt = 13) and depression improvement (effect size of β = 0.31, ). a 2016 meta-analysis showed that 34% of people treated with tdcs showed at least 50% symptom reduction (compared to 19% placebo). a 2017 study conducted by brunoni showed 6-weeks of tdcs treatment resulted in reduction of at least half of depression symptoms in 41% of depressed people (vs. 22% placebo and 47% antidepressants). in 2015, the british national institute for health and care excellence (nice) found tdcs to be safe and to appear effective for depression treatment. up until 2014, there have been several small randomized clinical trials (rct) in major depressive disorder (mdd); most found alleviation of depressive symptoms. there have been only two rcts in treatment-resistant mdd; both were small, and one found an effect and the other did not. one meta-analysis of the data focused on reduction in symptoms and found an effect compared to sham treatment, but another that was focused on relapse found no effect compared to sham. cognition there is mixed evidence about whether tdcs is useful for cognitive enhancement in healthy people. several reviews have found evidence of small yet significant cognitive improvements. other reviews found no evidence at all, although one of them has been criticized for overlooking within-subject effects and evidence from multiple-session tdcs trials. however, the original authors addressed these raised concerns in a further analysis and continued to find no evidence of impact a 2015 review of results from hundreds of tdcs experiments found that there was no statistically conclusive evidence to support any net cognitive effect, positive or negative, of single session tdcs in healthy populations – there is no evidence that tdcs is useful for cognitive enhancement. a second study by the same authors found there was little-to-no statistically reliable impact of tdcs on any neurophysiologic outcome. parkinson's, alzheimer's disease, and schizophrenia there is no strong evidence that tdcs is useful for memory deficits in alzheimer's disease, schizophrenia, non-neuropathic pain. a few clinical trials have been conducted on the use of tdcs to ameliorate memory deficits in parkinson's disease and alzheimer's disease and healthy subjects, with mixed results. research conducted as of 2013 in schizophrenia, has found that while large effect sizes were initially found for symptom improvement, later and larger studies have found smaller effect sizes (see also section on use of tdcs in psychiatric disorders below). studies have mostly concentrated on positive symptoms like auditory hallucinations; research on negative symptoms is lacking. stroke there is no strong evidence that tdcs can help improve upper limb function after stroke. in stroke, research conducted as of 2014, has found that tdcs is not effective for improving upper limb function after stroke. while some reviews have suggested an effect of tdcs for improving post-stroke aphasia, a 2015 cochrane review could find no improvement from combining tdcs with conventional treatment. research conducted as of 2013 suggests that tdcs may be effective for improve vision deficits following stroke. motor learning and memory function there is evidence that certain tdcs montages can increase learning rates for particular tasks in healthy individuals, namely motor tasks and memory function. however, reproducibility remains to be fully tested across studies and standardization for these kinds of studies has not been implemented fully, though an attempt at formalizing standards was released in 2017. other research conducted as of 2012 on the use of tdcs to treat pain, found that the research has been of low quality and cannot be used as a basis to recommend use of tdcs to treat pain. in chronic pain following spinal cord injury, research is of high quality and has found tdcs to be ineffective. tdcs has also been studied in addiction. there is some moderate (level b) evidence to indicate that, in addition to treating major depressive disorder, tdcs may also be appropriate to treat fibromyalgia, and craving disorders. tdcs has been used in neuroscience research, particularly to try to link specific brain regions to specific cognitive tasks or psychological phenomena. the cerebellum has been a focus of research, due to its high concentration of neurons, its location immediately below the skull, and its multiple reciprocal anatomical connections to motor and associative parts of the brain. most such studies focus on the impact of cerebellar tdcs on motor, cognitive, and affective functions in healthy and patient populations, but some also employ tdcs over the cerebellum to study the functional connectivity of the cerebellum to other areas of the brain. regulatory approvals tdcs is a ce approved treatment for major depressive disorder (mdd) in the uk, eu, australia, and mexico. as of 2015, tdcs has not been approved for any use by the us fda. an fda briefing document prepared in 2012 stated that "there is no regulation for therapeutic tdcs".