Network remodeling induced by transcranial brain stimulation: A computational model of tDCS-triggered cell assembly formation

Transcranial direct current stimulation (tDCS) is a variant of non-invasive neuro-modulation, which promises treatment for diseases like major depressive disorder or chronic pain for patients resistant to conventional therapies. In experiments, long lasting after-effects were observed, suggesting that plastic changes were induced. The exact mechanism underlying the emergence and maintenance of these after-effects, however, remains elusive. Here we propose a model to explain how transcranial stimulation triggers a homeostatic response of the network involving growth and decay of synapses. In our model, the cortical tissue underneath the electrodes is conceived as a recurrent network of excitatory and inhibitory neurons, in which excitatory-to-excitatory synapses are subject to structural plasticity. Various aspects of stimulation were tested via numerical simulations of such networks, including size and montage of the electrode, as well as intensity and duration of the stimulation. Our results suggest that stimulation indeed perturbs the homeostatic equilibrium and leads to cell assembly formation. Strong focal stimulation, for example, enhances the connectivity of new cell assemblies by increasing the rate of synaptic remodeling. Repetitive stimulation with well-chosen duty cycles increases the impact of stimulation as well. The long-term goal of our work is to optimize the impact of tDCS in clinical applications.