Neural mass models (NMM) provide insights into 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 neuromodulation by a thalamocortical NMM.
A. Neural mass model
The NMM for a single cortical source comprised of 4 neuronal subpopulations, excitatory pyramidal neurons (ePN), excitatory interneurons (eIN), slow inhibitory interneurons (siIN), and fast inhibitory interneurons (fiIN) . The NMM for the cortical source was coupled with another representing the thalamus , which comprised of 2 neuronal subpopulations, an excitatory thalamocortical (eTCN) and an inhibitory reticular-thalamic (iRT). The details of the thalamic NMM, parameterized to generate the alpha rhythm, are presented by Sotero et al..
B. EEG data fitting
Eyes-open resting state EEG was recorded from the central site Cz using the international 10-20 system of scalp sites with Starstim (Neuroelectrics, Spain) before and immediately after 15min of anodal tDCS (current density=0.526A/m2) at the same site (Fig. 1) . For each EEG recording, the average experimental power spectrum was analyzed from 0.25Hz to 50Hz for 25 successive 4s epochs. Then the NMM was fitted in the spectral domain using the Levenberg-Marquardt method for parameter optimization under quasi-stationarity assumptions, to identify the key parameters that determine the change in EEG spectral response.
The synaptic impulse response function (sIRF) of the dendritic tree of ePN, that receives presynaptic inputs to produce postsynaptic membrane potential alterations, was primarily changed (Fig. 1).
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