Figure 8. tDCS produces diffuse current flow in the brain which increases the probability of coincident pre‐ and postsynaptic inputs.

A, the electric field magnitude is plotted in a finite element model of tDCS with anode over the primary motor cortex (M1) and cathode over supraorbital region (SO). The electric field magnitude distribution in brain regions adjacent and upstream to M1 is comparable to or higher than in M1. B, the primary motor cortex is synaptically connected to the supplementary motor area (SMA) and premotor cortex (PMC), which drives motor activity. The thickness of the arrows indicates the relative connection strength from fMRI studies of this functional network. Modulating presynaptic activity in these upstream brain regions during tDCS may drive postsynaptic activity in M1. C, DCS‐induced membrane polarization changes the likelihood of firing. An increase in the presynaptic activity (or postsynaptic activity) increases the probability of coincident pre‐ and postsynaptic action potentials. The vertical grey dashed lines in the graph indicate the simulated number of coincidences in a model Integrate & Fire (I&F) neuron receiving Poisson distributed synaptic inputs. The hypergeometric probability distribution can be used to approximate the estimated number of coincidences. Combined, these results suggest the probability of coincident inputs can be directly modulated when presynaptic or postsynaptic activity is modulated by diffuse current flow. [Color figure can be viewed at wileyonlinelibrary.com]