The brain has the innate ability to undergo neuronal plasticity, which refers to changes in its structure and functions in response to continued changes in the environment. Although these concepts are well established in animal slice preparation models, their application to a large number of human subjects could only be achieved using noninvasive brain stimulation (NIBS) techniques. In this review, we discuss the mechanisms of plasticity induction using NIBS techniques including transcranial magnetic stimulation (TMS), transcranial direct current stimulation (tDCS), transcranial alternating current stimulation (tACS), random noise stimulation (RNS), transcranial ultrasound stimulation (TUS), vagus nerve stimulation (VNS), and galvanic vestibular stimulation (GVS). We briefly introduce these techniques, explain the stimulation parameters and potential clinical implications. Although their mechanisms are different, all these NIBS techniques can be used to induce plasticity at the systems level, to examine the neurophysiology of brain circuits and have potential therapeutic use in psychiatric and neurological disorders. TMS is the most established technique for the treatment of brain disorders, and repetitive TMS is an approved treatment for medication-resistant depression. Although the data on the clinical utility of the other modes of stimulation are more limited, the electrical stimulation techniques (tDCS, tACS, RNS, VNS, GVS) have the advantage of lower cost, portability, applicability at home, and can readily be combined with training or rehabilitation. Further research is needed to expand the clinical utility of NIBS and test the combination of different modes of NIBS to optimize neuromodulation induced clinical benefits.

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Objectives: Transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) have been used as neuromodulators in neuropsychiatric conditions. This study is aimed to find the effects of a single session of priming cathodal tDCS with intermittent theta-burst stimulation (iTBS) over left dorsolateral prefrontal cortex on heart rate variability (HRV) and cortical excitability parameters before and after perturbation. Materials and Methods: The neuromodulatory techniques used in the study were Cathodal tDCS for 20 min followed by iTBS for 3 min on the left dorsolateral prefrontal cortex (DLPFC). HRV variables and TMS parameters were recorded before and after this intervention of combined neuromodulation in 31 healthy volunteers (20 males and 11 females; age range of 19–35 years with Mean ± SD = 24.2 ± 4.7 years). Results: The results showed an overall increase in cortical excitability and parasympathetic dominance in healthy volunteers. Other measures of cortical excitability and HRV did not change significantly following single session of combined neuromodulation. Conclusion: This study showed that there is an overall increase in cortical excitability and parasympathetic dominance in the cohort of healthy volunteers following a combination of neuromodulation involving cathodal tDCS followed by iTBS over left DLPFC. Future studies exploring the effects of other possible combinations with sham stimulation could be carried out to explore the utility of dual stimulation as add-on therapy in disorders.

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