Abstract
Transcranial direct current stimulation (tDCS) is a variant of noninvasive neuromodulation, which promises treatment for brain diseases like major depressive disorder. In experiments, long-lasting aftereffects were observed, suggesting that persistent plastic changes are induced. The mechanism underlying the emergence of lasting aftereffects, however, remains elusive. Here we propose a model, which assumes that tDCS triggers a homeostatic response of the network involving growth and decay of synapses. The cortical tissue exposed to tDCS is conceived as a recurrent network of excitatory and inhibitory neurons, with synapses subject to homeostatically regulated structural plasticity. We systematically tested various aspects of stimulation, including electrode size and montage, as well as stimulation intensity and duration. Our results suggest that transcranial stimulation perturbs the homeostatic equilibrium and leads to a pronounced growth response of the network. The stimulated population eventually eliminates excitatory synapses with the unstimulated population, and new synapses among stimulated neurons are grown to form a cell assembly. Strong focal stimulation tends to enhance the connectivity within new cell assemblies, and repetitive stimulation with well-chosen duty cycles can increase the impact of stimulation even further. One long-term goal of our work is to help in optimizing the use of tDCS in clinical applications.
Highlights
Transcranial direct current stimulation is a noninvasive brain stimulation technique, where a weak constant current (1−2 mA) is applied to the brain via large electrodes attached to the scalp (Edwards et al, 2013)
Our results suggest that Transcranial direct current stimulation (tDCS) can induce substantial network remodeling and cell assembly formation, and focused strong and/or repetitive stimulation with well-chosen duty cycles can effectively boost the connectivity of the cell assemblies formed
Even for a polarization as weak as ±0.1 mV, which is about the weakest depolarization known to cause observable physiological effects in tDCS experiments (Vöröslakos et al, 2018), the firing rate change was found to be larger than ±10% (Figure 1B, light gray curve)
Summary
Transcranial direct current stimulation (tDCS) is a noninvasive brain stimulation technique, where a weak constant current (1−2 mA) is applied to the brain via large electrodes attached to the scalp (Edwards et al, 2013). It induces weak electric fields that are typically not sufficient to trigger action potentials directly, but can polarize the membrane of neurons by fractions of millivolts (Joucla & Yvert, 2009), depending on the orientation of the electric field vector relative to the somato-dendritic axis of the neuron (Gluckman et al, 1996; Radman, Ramos, Brumberg, & Bikson, 2009; Wiethoff, Hamada, & Rothwell, 2014) This membrane potential deflection can influence spike timing and firing rates of neurons that are part of an active network (Bikson, Radman, & Datta, 2006; Vöröslakos et al, 2018). Systematic transcutaneous current stimulation experiments in rats (Vöröslakos et al, 2018) could establish quantitative relations between the externally applied current, the induced electric field, the associated membrane potential deflection, and the resulting firing rate change
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