Abstract
Transcranial direct and alternating current simulations (tDCS and tACS) are non-invasive neuromodulators with emerging evidence in neuropsychiatric disorders. This review compares their efficacy and tolerability in head-on trials. Following PRISMA guidelines, 955 studies were identified from Scopus, MEDLINE, and trial registries. After screening and reviewing abstracts and full texts by 2 independent authors, 11 studies comparing tACS and tDCS in neuropsychiatric disorders were included. Four studies in schizophrenia, 1 each in depression, mild cognitive impairment (MCI), and ataxia; 2 in tinnitus and epilepsy. Sessions ranged from 1-10 delivering 1-2 mA, with most sessions lasting 20 minutes. tACS frequency varied from delta to high gamma, and were symptom specific. In psychiatric disorders, alpha-tACS was associated with improvements in auditory hallucinations, whereas tDCS showed relatively greater cognitive benefits. However, delta-high-definition-tACS was linked to improvements in cognitive deficits and general psychopathology, while theta-tACS fared better in depression. In neurological disorders, gamma-tACS was associated with improvement in MCI and epilepsy. In contrast, bifrontal-tDCS was associated with improvement in tinnitus, while cerebellar-tDCS significantly outperformed gamma-tACS in neurodegenerative ataxia. Most studies had some or major risk of bias. This review suggests both tACS and tDCS are safe and promising but vary in effectiveness across disorders. Different parameter spaces of these two techniques, especially the montage and stimulation frequency, would drive the outcomes. In view of single-session protocols, especially in neurological disorders, we need larger, replicable and rigorously designed studies to confirm differential superiority and guide clinical utility.