Neural mass model analysis of online modulation of electroencephalogram with transcranial direct current stimulation

Neural mass models (NMM) provide insights into the neuromodulatory mechanisms underlying alterations of cortical activity as recorded by electroencephalography (EEG). In the human primary motor cortex (M1), neuromodulation can be induced by non-invasive brain stimulation (NIBS). We aimed to capture the origin of NIBS-induced alterations in the EEG power spectrum using a thalamocortical NMM. We found that anodal transcranial direct current stimulation (tDCS) modulated primarily the synaptic impulse response function (sIRF) of the dendritic tree of the excitatory pyramidal neurons (ePN) in cortical NMM. We also investigated cortico-muscular coherence during anodal tDCS by adding a resonant (~8Hz) spinal-musculoskeletal system to the thalamocortical NMM that was driven by the cortical NMM, and it fed back to the thalamic NMM. The coupled NMM was fitted to the EEG power spectrum during anodal tDCS, which showed changes not only in sIRF time constant but also in cortico-thalamic and thalamo-cortical connectivity values during anodal tDCS. Based on these results, we postulate that a decrease in the time constant of sIRF represents a drop in the average membrane resistance, possibly due to a greater fraction of ion channels being open at any instant. Faster time constants of sIRF of the dendritic tree, possibly due to AMPA-mediated synapses, would lead to an enhanced responsiveness of the ePN and enhanced cortico-thalamic connectivity. This supports the hypothesis that anodal tDCS enhances responsiveness of ePN to afferent feedback in a non-specific way and also increases the coherence between thalamus, cortex and muscle where the spinal-musculoskeletal system bandpass filtered (around 8Hz) perturbations from the cortical NMM, leading to physiological tremor, which was fed back by sensory afferents to the cortex via the thalamic NMM.