Multi-scale model of transcranial electrical stimulation.

<p><b>A.1</b>: A Finite-Element Model (FEM) of tissue resistance was used to calculate electric field magnitude and direction at the cortical level in response to currents applied on the scalp. Electrodes were placed at the same locations as in the human sleep experiments (red = anodal = inward, blue = cathodal = outward). <b>A.2</b>: Applied electric fields show mixed polarities throughout cortex due to cortical folding (color indicates radial field component orthogonal to cortical surface). <b>A.3</b>: Cortical neurons in close proximity may be exposed to depolarizing (inward) and hyperpolarizing (outward) radial fields. <b>B.1</b>: The network model consists of excitatory and inhibitory neurons arranged in a 2D-lattice with long- and short-range synaptic connections respectively. Field magnitude and polarity of the stimulation applied follow the FEM computations and were applied to the network depending on the location within the lattice (applied polarity indicated with red/blue shading). <b>B.2</b>: Spiking activity of neurons in the network reproduces the typical UP and DOWN states of SWO. The LFP (black line) is determined as the average of the post-synaptic currents (gray line, LFP low-pass filtered, cut-off frequency 2.5 Hz). <b>B.3</b>: Example of network activity (LFP in white). Oscillation is in the range of human SWO (0.5–1 Hz). Spectrogram indicates power of this signal. Red curve indicates slow-oscillating ON/OFF stimulation (0.75 Hz) which is applied to the excitatory neurons in the network.