Noninvasive Brain Stimulation with Low-Intensity Electrical Currents: Putative Mechanisms of Action for Direct and Alternating Current Stimulation

Abstract

Transcranial stimulation with weak direct current (DC) has been valuable in exploring the effect of cortical modulation on various neural networks. Less attention has been given, however, to cranial stimulation with low-intensity alternating current (AC). Reviewing and discussing these methods simultaneously with special attention to what is known about their mechanisms of action may provide new insights for the field of noninvasive brain stimulation. Direct current appears to modulate spontaneous neuronal activity in a polarity-dependent fashion with site-specific effects that are perpetuated throughout the brain via networks of interneuronal circuits, inducing significant effects on high-order cortical processes implicated in decision making, language, memory, sensory perception, and pain. AC stimulation has also been associated with a significant behavioral and clinical impact, but the mechanism of AC stimulation has been underinvestigated in comparison with DC stimulation. Even so, preliminary studies show that although AC stimulation has only modest effects on cortical excitability, it has been shown to induce synchronous changes in brain activity as measured by EEG activity. Thus, cranial AC stimulation may render its effects not by polarizing brain tissue, but rather via rhythmic stimulation that synchronizes and enhances the efficacy of endogenous neurophysiologic activity. Alternatively, secondary nonspecific central and peripheral effects may explain the clinical outcomes of DC or AC stimulation. Here the authors review what is known about DC and AC stimulation, and they discuss features that remain to be investigated.

Parietal disruption alters audiovisual binding in the sound-induced flash illusion

“…Although this might be taken to suggest that parietal cortex is not involved in the flash illusion, it is possible that placement of the electrodes over P4 (which sits roughly above the intraparietal sulcus) and the contralateral supraorbital area induced current flow that failed to perturb activity within the inferior parietal lobule, which includes the AG and SMG. It is also possible that due to the low spatial resolution of tDCS (Stagg and Nitsche, 2011;Zaghi et al, 2010), widespread cortical disruption was induced that affected several parietal subregions, each of which plays a distinct -and potentially opposing -role in attention and multisensory integration (Chambers et al, 2004a;Chambers et al, 2004b;Gobel et al, 2001). Here, we use MRI-guided TMS to investigate the unique contributions of two distinct subregions within the inferior parietal lobule, the AG and SMG, in the sound-induced flash illusion.…”

A network approach for modulating memory processes via direct and indirect brain stimulation: Toward a causal approach for the neural basis of memory

“…tDCS delivers charge densities that do not directly lead to increases in neural spike rate but instead are thought to modulate population neuronal excitability (Coslett, Hamilton, Nitsche, & Paulus, 2011;Nitsche et al, 2008;Wagner, Valero-Cabre, & Pascual-Leone, 2007;Zaghi, Acar, Hultgren, Boggio, & Fregni, 2010). Regional excitability is increased by anodal (positive charge) stimulation and decreased by cathodal (negative charge) stimulation.…”

Physiology, Psychoacoustics and Cognition in Normal and Impaired Hearing

“…Its application involves inducing a weak electric current between electrodes at the scalp (Nitsche et al 2008;Zaghi et al 2010;Paulus 2011). The current partially penetrates the brain, where it propagates widely and forces neuronal excitability to oscillate along with its alternating waveform (Frohlich and McCormick 2010;Ozen et al 2010).…”