Computational models of electroconvulsive therapy and transcranial direct current stimulation for treatment of depression

Abstract

Electroconvulsive therapy (ECT) and transcranial direct current stimulation (tDCS) are two important forms of transcranial electrical stimulation in clinical psychiatry. They have shown impressive therapeutic results in the treatment of major depression and other psychiatric disorders. The aim of this thesis was to develop novel computational models of ECT and tDCS to assist in the further understanding of these two brain-stimulation techniques, to explore possible refinements and improvements in treatment efficacy.Head models of three different subjects were reconstructed from corresponding computed tomography (CT) or magnetic resonance imaging (MRI) scans. One was a low-resolution model rendered from a set of CT scans, incorporating skull conductivity anisotropy. The other two were high-resolution models reconstructed from MRI scans, with one incorporating white matter conductivity anisotropy. In both high-resolution models, several brain cortical regions of interest were segmented and defined; these are known to be involved in therapeutic or adverse stimulation outcomes. In one set of simulations, these structural head models were taken to be passive volume conductors, to investigate the effect of various electrode montages on the distribution of current density or electric field within the head. Results showed that current distribution in the brain was highly dependent on the electrode placement on the scalp. For example, when simulating three different right unilateral (RUL) ECT montages, the non-conventional montages with an electrode on the forehead appeared to have superiority over conventional RUL, because stimulation strength was stronger in regions believed responsible for the treatment efficacy, such as the anterior cingulate gyrus, and was weaker in regions that have been speculated to exert adverse effects, such as the hippocampus.In addition, a continuum active model of neural excitation was also developed to simulate direct activation of the brain following an ECT stimulus. This model was integrated into the passive head model to investigate the influence of different electrode placements, as well as the time-dependent effects of ECT stimulus parameters on brain activation. For instance, when the stimulus pulse width was reduced, maximum current density was unchanged but the spatial extent of activation was reduced. Moreover, results showed that stimulus frequency influenced the stimulus efficiency, that is, of all the brain neurons that were able to be directly activated by a single pulse, 80%, 10% and 0% were capable of being activated by both of two consecutive pulses with frequencies of 60 Hz, 90 Hz and 120 Hz, respectively