Head models of healthy and depressed adults for simulating the electric fields of non-invasive electric brain stimulation

During the past decade, it became clear that the electric field elicited by non-invasive brain stimulation (NIBS) techniques such as transcranial direct current stimulation (tDCS) and transcranial magnetic stimulation (TMS) are substantially influenced by variations in individual head and brain anatomy. In addition to structural variations in the healthy, several psychiatric disorders are characterized by anatomical alterations that are likely to further constrain the intracerebral effects of NIBS. Here, we present high-resolution realistic head models derived from structural magnetic resonance imaging data of 19 healthy adults and 19 patients diagnosed with major depressive disorder (MDD). By using a freely available software package for modelling the electric fields induced by different NIBS protocols, we show that our head models are well-suited for assessing inter-individual and between-group variability in the magnitude and focality of tDCS-induced electric fields for two protocols targeting the left dorsolateral prefrontal cortex

Effects of transcranial direct current stimulation for treating depression: A modeling study

Background: Transcranial direct current stimulation (tDCS) above the left dorsolateral prefrontal cortex (lDLPFC) has been widely used to improve symptoms of major depressive disorder (MDD). However, the effects of different stimulation protocols in the entire frontal lobe have not been investigated in a large sample including patient data. Methods: We used 38 head models created from structural magnetic resonance imaging data of 19 healthy adults and 19 MDD patients and applied computational modeling to simulate the spatial distribution of tDCS-induced electric fields (EFs) in 20 frontal regions. We evaluated effects of seven bipolar and two multi-electrode 4 × 1 tDCS protocols. Results: For bipolar montages, EFs were of comparable strength in the lDLPFC and in the medial prefrontal cortex (MPFC). Depending on stimulation parameters, EF cortical maps varied to a considerable degree, but were found to be similar in controls and patients. 4 × 1 montages produced more localized, albeit weaker effects. Limitations: White matter anisotropy was not modeled. The relationship between EF strength and clinical response to tDCS could not be evaluated. Conclusions: In addition to lDLPFC stimulation, excitability changes in the MPFC should also be considered as a potential mechanism underlying clinical efficacy of bipolar montages. MDD-associated anatomical variations are not likely to substantially influence current flow. Individual modeling of tDCS protocols can substantially improve cortical targeting. We make recommendations for future research to explicitly test the contribution of lDLPFC vs. MPFC stimulation to therapeutic outcomes of tDCS in this disorder

The interplay between executive control, behavioral variability and mind wandering: Insights from a high-definition transcranial direct-current stimulation study

While the involvement of executive processes in mind wandering is largely undebated, their exact relationship is subject to an ongoing debate and rarely studied dynamically within‐subject. Several brain‐stimulation studies using transcranial direct current stimulation (tDCS) have attempted to modulate mind‐wandering propensity by stimulating the left dorsolateral prefrontal cortex (DLPFC) which is an important hub in the prefrontal control network. In a series of three studies testing a total of N = 100 participants, we develop a novel task that allows to study the dynamic interplay of mind wandering, behavioural varibility and the flexible recruitment of executive resources as indexed by the randomness (entropy) of movement sequences generated by our participants. We consistently find that behavioural variability is increased and randomness is decreased during periods of mind wandering. Interestingly, we also find that behavioural variability interacts with the entropy‐MW effect, opening up the possibility to detect distinct states of off‐focus cognition. When applying a high‐definition transcranial direct‐current stimulation (HD‐tDCS) montage to the left DLPFC, we find that propensity to mind wander is reduced relative to a group receiving sham stimulation

Commentary:Transcranial stimulation of the frontal lobes increases propensity of mind-wandering without changing meta-awareness

A Commentary on Transcranial stimulation of the frontal lobes increases propensity of mind-wandering without changing meta-awareness by Axelrod, V., Zhu, X., & Qui, J. (2018). Scientific Reports, 8:15975. doi: 10.1038/s41598-018-34098-

Understanding the Neural and Behavioral Correlates of Mind Wandering Through Transcranial Direct Current Stimulation

The mind’s tendency to wander is an integral part of the human experience. Recent studies suggest that high-level cognitive functions such as mind wan- dering (MW) can be modulated by non-invasive brain stimulation (NIBS) techniques such as transcranial direct current stimulation (tDCS). However, the effectiveness of tDCS in the cognitive domain remains an issue of debate. This thesis aimed to understand if tDCS is effective in modulating MW, either on the behavioral or neural levels, by employing rigorous, transparent, open science practices that include open availability of data and materials, such as analysis scripts. In a high-powered (N = 192) preregistered replication attempt in Paper I, we fail to replicate the finding that anodal tDCS applied to the left dorsolateral prefrontal cortex (DLPFC) increases MW propensity. In contrast, a small effect was found in the opposite direction, though this was not robust. Further, tDCS did not impact any of our task performance measures. In Paper II, we showed that bipolar montages targeting the left DLPFC induce widespread effects extending far beyond the target site by simulation of tDCS-induced electric field (E-field) in the brain. However, E-field elicited by multi-electrode 4 × 1 HD-tDCS montages tended to be more focal, generally confined within the ring created by the four return electrodes. In Paper III, 4 × 1 HD-tDCS targeting the left DLPFC combined with our novel task showed reduced MW propensity for the group receiving active stimulation when compared with the sham group, without impacting task performance. These results highlight the value of preregistered replications in tDCS research in general, and the effectiveness of 4 × 1 HD-tDCS in modulating MW in particular. A NIBS method that can reliably regulate MW will have implications for conditions that are associated with the unfavorable behavioral effects of MW

Tracking the current in the Alzheimer's brain - Systematic differences between patients and healthy controls in the electric field induced by tDCS

Background: Several studies on patients with Alzheimer’s disease (AD) have used transcranial direct current stimulation (tDCS) to enhance neural excitability in the left dorsolateral prefrontal cortex (lDLPFC). Interindividual differences in brain anatomy in AD patients pose a challenge to efficiently target the lDLPFC using scalpbased coordinates, calling for new and more precise tDCS protocols. Objective: The purpose of this study was to explore how AD-related neuropathology affects the tDCS-induced electric field (EF) across different DLPFC montages using computational modeling. Method: Forty-eight realistic head models were created from structural magnetic resonance scans of AD patients and healthy controls collected from a publicly available database. We compared the tDCS-induced EF in different montages applied in the literature, in addition to a high definition (HD)-tDCS montage centered at electrode F3. Results: There was an overall global reduction in EF strength in the patient group, probably due to structural alterations that were also identified in the patient group. A widespread distribution of the EF was found across the frontal lobe for bipolar montages, while HD-tDCS yielded more focal stimulation, mainly restricted to the lDLPFC. Minor differences in the EF distribution were found when comparing the HD-tDCS montages. Conclusion: Neurodegenerative alterations present in patients with AD affect the magnitude, distribution and variability of the EF. HD-tDCS montages provide more focal stimulation of the target area, compared to bipolar montages with to pronounced group differences between AD patients and healthy matched controls. This finding poses substantial limitations to the comparison of cognitive effects of tDCS both between patients and controls and within patients at different stages of disease progression

Head models of healthy and depressed adults for simulating the electric fields of non-invasive electric brain stimulation

During the past decade, it became clear that the electric field elicited by non-invasive brain stimulation (NIBS) techniques such as transcranial direct current stimulation (tDCS) and transcranial magnetic stimulation (TMS) are substantially influenced by variations in individual head and brain anatomy. In addition to structural variations in the healthy, several psychiatric disorders are characterized by anatomical alterations that are likely to further constrain the intracerebral effects of NIBS. Here, we present high-resolution realistic head models derived from structural magnetic resonance imaging data of 19 healthy adults and 19 patients diagnosed with major depressive disorder (MDD). By using a freely available software package for modelling the electric fields induced by different NIBS protocols, we show that our head models are well-suited for assessing inter-individual and between-group variability in the magnitude and focality of tDCS-induced electric fields for two protocols targeting the left dorsolateral prefrontal cortex

Probing the neural signature of mind wandering with simultaneous fMRI-EEG and pupillometry

Mind wandering reflects the shift in attentional focus from task-related cognition driven by external stimuli toward self-generated and internally-oriented thought processes. Although such task-unrelated thoughts (TUTs) are pervasive and detrimental to task performance, their underlying neural mechanisms are only modestly understood. To investigate TUTs with high spatial and temporal precision, we simultaneously measured fMRI, EEG, and pupillometry in healthy adults while they performed a sustained attention task with experience sampling probes. Features of interest were extracted from each modality at the single-trial level and fed to a support vector machine that was trained on the probe responses. Compared to task-focused attention, the neural signature of TUTs was characterized by weaker activity in the default mode network but elevated activity in its anticorrelated network, stronger functional coupling between these networks, widespread increase in alpha, theta, delta, but not beta, frequency power, predominantly reduced amplitudes of late, but not early, event-related potentials, and larger baseline pupil size. Particularly, information contained in dynamic interactions between large-scale cortical networks was predictive of transient changes in attentional focus above other modalities. Together, our results provide insight into the spatiotemporal dynamics of TUTs and the neural markers that may facilitate their detection

Probing the neural signature of mind wandering with simultaneous fMRI-EEG and pupillometry

Mind wandering reflects the shift in attentional focus from task-related cognition driven by external stimuli toward self-generated and internally-oriented thought processes. Although such task-unrelated thoughts (TUTs) are pervasive and detrimental to task performance, their underlying neural mechanisms are only modestly understood. To investigate TUTs with high spatial and temporal precision, we simultaneously measured fMRI, EEG, and pupillometry in healthy adults while they performed a sustained attention task with experience sampling probes. Features of interest were extracted from each modality at the single-trial level and fed to a support vector machine that was trained on the probe responses. Compared to task-focused attention, the neural signature of TUTs was characterized by weaker activity in the default mode network but elevated activity in its anticorrelated network, stronger functional coupling between these networks, widespread increase in alpha, theta, delta, but not beta, frequency power, predominantly reduced amplitudes of late, but not early, event-related potentials, and larger baseline pupil size. Particularly, information contained in dynamic interactions between large-scale cortical networks was predictive of transient changes in attentional focus above other modalities. Together, our results provide insight into the spatiotemporal dynamics of TUTs and the neural markers that may facilitate their detection