Relation between the electric field and activation of cortical neurons in transcranial electrical stimulation

Abstract

Background: To address the brain areas and circuits affected by transcranial electrical stimulation (tES), which had been used widely to treat psychiatric and neurological diseases, the stimulus-induced electric field in the cortex was calculated using a head model that reflects anatomical information. To obtain detailed information at the macroscopic and microscopic levels, multi-scale modeling was proposed that integrates the head model with multi-compartmental models of cortical neurons.

Objective: Our goal was to use multi-scale modeling to describe the relation between the stimulus-induced electric field and neuronal responses during tES.

Methods: We simulated sub- and supra-threshold neuronal responses to stimulus-induced uniform and realistic electric fields. For the realistic electric field, multi-scale models that combined the head model derived from structural MRIs and multi-compartmental models of neurons were constructed. Then, we simulated the steady-state membrane polarization for sub-threshold stimulation and the excitation threshold for supra-threshold stimulation by varying the tES montages. The electric field calculated was decomposed into two orthogonal components, the radial and tangential fields, which were compared to the neuronal responses.

Results: The stimulus-induced electric field depended strongly on stimulus parameters, and neuronal excitability showed a higher correlation with the radial field. We demonstrated that neurons exhibited linear polarization during sub-threshold stimulation depending on the local radial field intensity that resulted in a significant relation regardless of the tES montage. Supra-threshold stimulation showed a stronger relation with the radial field, but rather complex patterns of excitation thresholds depending on neurons' morphological features.

Conclusion: Our results indicated that cortical neurons are affected greatly by the relative direction of the stimulus-induced electric field, which may be a necessary step toward a detailed understanding of tES' potential mechanisms.

Keywords: Finite element method; Multi-scale modeling; Noninvasive brain stimulation; Pyramidal neuron; Transcranial electrical stimulation.