Transcranial DC Stimulation in Fibromyalgia: Optimized Cortical Target Supported by High-Resolution Computational Models

tDCS Modulation of Visually Induced Analgesia

Multisensory interactions can produce analgesic effects. In particular, viewing one's own body reduces pain levels, perhaps because of changes in connectivity between visual areas specialized for body representation, and sensory areas underlying pain perception. We tested the causal role of the extrastriate visual cortex in triggering visually induced analgesia by modulating the excitability of this region with transcranial direct current stimulation (tDCS). Anodal, cathodal, or sham tDCS (2 mA, 10 min) was administered to 24 healthy participants over the right occipital or over the centro-parietal areas thought to be involved in the sensory processing of pain. Participants were required to rate the intensity of painful electrical stimuli while viewing either their left hand or an object occluding the left hand, both before and immediately after tDCS. We found that the analgesic effect of viewing the body was enhanced selectively by anodal stimulation of the occipital cortex. The effect was specific for the polarity and the site of stimulation. The present results indicate that visually induced analgesia may depend on neural signals from the extrastriate visual cortex.

Reversal of chronic stress-induced pain by transcranial direct current stimulation (tDCS) in an animal model

Transcranial direct current stimulation (tDCS) has been suggested as a therapeutic tool for pain syndromes. Although initial results in human subjects are encouraging, it still remains unclear whether the effects of tDCS can reverse maladaptive plasticity associated with chronic pain. To investigate this question, we tested whether tDCS can reverse the specific behavioral effects of chronic stress in the pain system, and also those indexed by corticosterone and interleukin-1β levels in serum and TNFα levels in the hippocampus, in a well-controlled rat model of chronic restraint stress (CRS). Forty-one adult male Wistar rats were divided into two groups control and stress. The stress group was exposed to CRS for 11 weeks for the establishment of hyperalgesia and mechanical allodynia as shown by the hot plate and von Frey tests, respectively. Rats were then divided into four groups control, stress, stress+sham tDCS and stress+tDCS. Anodal or sham tDCS was applied for 20min/day over 8 days and the tests were repeated. Then, the animals were killed, blood collected and hippocampus removed for ELISA testing. This model of CRS proved effective to induce chronic pain, as the animals exhibited hyperalgesia and mechanical allodynia. The hot plate test showed an analgesic effect, and the von Frey test, an anti-allodynic effect after the last tDCS session, and there was a significant decrease in hippocampal TNFα levels. These results support the notion that tDCS reverses the detrimental effects of chronic stress on the pain system and decreases TNFα levels in the hippocampus.

Bilateral temporal cortex transcranial direct current stimulation worsens male performance in a multisensory integration task

Somatosensory integration is a critical cognitive function for human social interaction. Though somatosensory integration has been highly explored in cognitive studies; only a few studies have explored focal modulation of cortical excitability using a speech perception paradigm. In the current study, we aimed to investigate the effects of tDCS applied over the temporal cortex of healthy subjects during a go-no-go task in which stimuli were shapes and non-words. Twenty-eight subjects were randomized to receive cathodal, anodal or sham tDCS bilaterally over the superior temporal cortex (the reference electrode was on deltoid) in a counterbalanced order. The effects on judgment of congruency between shapes and non-words in healthy volunteers were measured by a go-no-go task. Our findings show a significant modification of performance according to the polarity of stimulation, task and subject gender. We found that men performed worse on the no-go condition for congruent stimuli during cathodal tDCS. For reaction time, on the other hand, there was a similar effect for anodal and cathodal stimulation. There were significantly faster responses on incongruent trials during both anodal and cathodal tDCS. Along with previous literature showing gender differences in tasks associated with speech perception, the findings of this study provide additional evidence suggesting that men may have a more focal and restricted neural processing in this multisensory integration task.

High-resolution modeling assisted design of customized and individualized transcranial direct current stimulation protocols

OBJECTIVES

Transcranial direct current stimulation (tDCS) is a neuromodulatory technique that delivers low-intensity currents facilitating or inhibiting spontaneous neuronal activity. tDCS is attractive since dose is readily adjustable by simply changing electrode number, position, size, shape, and current. In the recent past, computational models have been developed with increased precision with the goal to help customize tDCS dose. The aim of this review is to discuss the incorporation of high-resolution patient-specific computer modeling to guide and optimize tDCS.

METHODS

In this review, we discuss the following topics: 1) The clinical motivation and rationale for models of transcranial stimulation is considered pivotal in order to leverage the flexibility of neuromodulation; 2) the protocols and the workflow for developing high-resolution models; 3) the technical challenges and limitations of interpreting modeling predictions; and 4) real cases merging modeling and clinical data illustrating the impact of computational models on the rational design of rehabilitative electrotherapy.

CONCLUSIONS

Though modeling for noninvasive brain stimulation is still in its development phase, it is predicted that with increased validation, dissemination, simplification, and democratization of modeling tools, computational forward models of neuromodulation will become useful tools to guide the optimization of clinical electrotherapy.

After-effects of consecutive sessions of transcranial direct current stimulation (tDCS) in a rat model of chronic inflammation

Transcranial direct current stimulation (tDCS) induces cortical excitability changes in animals and humans that can last beyond the duration of stimulation. Preliminary evidence suggests that tDCS may have an analgesic effect; however, the timing of these effects, especially when associated with consecutive sessions of stimulation in a controlled animal experiment setting, has yet to be fully explored. To evaluate the effects of tDCS in inflammatory chronic pain origin immediately and 24 h after the last treatment session, complete Freund's adjuvant (CFA) was injected (100 μl) in the right footpad to induce inflammation. On the 15th day after CFA injection, rats were divided into two groups: tDCS (n = 9) and sham (n = 9). The tDCS was applied for 8 days. The hot plate and Von Frey tests were applied immediately and 24 h after the last tDCS session. Eight 20-min sessions of 500 μA anodal tDCS resulted in antinociceptive effects as assessed by the hot plate test immediately (P = 0.04) and 24 h after the last tDCS session (P = 0.006), for the active tDCS group only. There was increased withdrawal latency in the Von Frey test at 24 h after the last session (P = 0.01). Our findings confirm the hypothesis that tDCS induces significant, long-lasting, neuroplastic effects and expands these findings to a chronic pain model of peripheral inflammation, thus supporting the exploration of this technique in conditions associated with chronic pain and peripheral inflammation, such as osteoarthritis.

Computational models of transcranial direct current stimulation

During transcranial direct current stimulation (tDCS), controllable dose parameters are electrode number (typically 1 anode and 1 cathode), position, size, shape, and applied electric current. Because different electrode montages result in distinct brain current flow patterns across the brain, tDCS dose parameters can be adjusted, in an application-specific manner, to target or avoid specific brain regions. Though the tDCS electrode montage often follows basic rules of thumb (increased/decreased excitability "under" the anode/cathode electrode), computational forward models of brain current flow provide more accurate insight into detailed current flow patterns and, in some cases, can even challenge simplified electrode-placement assumptions. With the increased recognized value of computational forward models in informing tDCS montage design and interpretation of results, there have been recent advances in modeling tools and a greater proliferation of publications.  In addition, the importance of customizing tDCS for potentially vulnerable populations (eg, skull defects, brain damage/stroke, and extremes of age) can be considered. Finally, computational models can be used to design new electrode montages, for example, to improve spatial targeting such as high-definition tDCS. Pending further validation and dissemination of modeling tools, computational forward models of neuromodulation will become standard tools to guide the optimization of clinical trials and electrotherapy.

tDCS-induced analgesia and electrical fields in pain-related neural networks in chronic migraine

OBJECTIVE

We investigated in a sham-controlled trial the analgesic effects of a 4-week treatment of transcranial direct current stimulation (tDCS) over the primary motor cortex in chronic migraine. In addition, using a high-resolution tDCS computational model, we analyzed the current flow (electric field) through brain regions associated with pain perception and modulation.

METHODS

Thirteen patients with chronic migraine were randomized to receive 10 sessions of active or sham tDCS for 20 minutes with 2 mA over 4 weeks. Data were collected during baseline, treatment and follow-up. For the tDCS computational analysis, we adapted a high-resolution individualized model incorporating accurate segmentation of cortical and subcortical structures of interest.

RESULTS

There was a significant interaction term (time vs group) for the main outcome (pain intensity) and for the length of migraine episodes (ANOVA, P < .05 for both analyses). Post-hoc analysis showed a significant improvement in the follow-up period for the active tDCS group only. Our computational modeling studies predicted electric current flow in multiple cortical and subcortical regions associated with migraine pathophysiology. Significant electric fields were generated, not only in targeted cortical regions but also in the insula, cingulate cortex, thalamus, and brainstem regions.

CONCLUSIONS

Our findings give preliminary evidence that patients with chronic migraine have a positive, but delayed, response to anodal tDCS of the primary motor cortex. These effects may be related to electrical currents induced in pain-related cortical and subcortical regions.

Evaluation of local electric fields generated by transcranial direct current stimulation with an extracephalic reference electrode based on realistic 3D body modeling

In this study, local electric field distributions generated by transcranial direct current stimulation (tDCS) with an extracephalic reference electrode were evaluated to address extracephalic tDCS safety issues. To this aim, we generated a numerical model of an adult male human upper body and applied the 3D finite element method to electric current conduction analysis. In our simulations, the active electrode was placed over the left primary motor cortex (M1) and the reference electrode was placed at six different locations: over the right temporal lobe, on the right supraorbital region, on the right deltoid, on the left deltoid, under the chin, and on the right buccinator muscle. The maximum current density and electric field intensity values in the brainstem generated by the extracephalic reference electrodes were comparable to, or even less than, those generated by the cephalic reference electrodes. These results suggest that extracephalic reference electrodes do not lead to unwanted modulation of the brainstem cardio-respiratory and autonomic centers, as indicated by recent experimental studies. The volume energy density was concentrated at the neck area by the use of deltoid reference electrodes, but was still smaller than that around the active electrode locations. In addition, the distributions of elicited cortical electric fields demonstrated that the use of extracephalic reference electrodes might allow for the robust prediction of cortical modulations with little dependence on the reference electrode locations.

Parietal contributions to visual working memory depend on task difficulty

The nature of parietal contributions to working memory (WM) remain poorly understood but of considerable interest. We previously reported that posterior parietal damage selectively impaired WM probed by recognition (Berryhill and Olson, 2008a). Recent studies provided support using a neuromodulatory technique, transcranial direct current stimulation (tDCS) applied to the right parietal cortex (P4). These studies confirmed parietal involvement in WM because parietal tDCS altered WM performance: anodal current tDCS improved performance in a change detection task, and cathodal current tDCS impaired performance on a sequential presentation task. Here, we tested whether these complementary results were due to different degrees of parietal involvement as a function of WM task demands, WM task difficulty, and/or participants' WM capacity. In Experiment 1, we applied cathodal and anodal tDCS to the right parietal cortex and tested participants on both previously used WM tasks. We observed an interaction between tDCS (anodal, cathodal), WM task difficulty, and participants' WM capacity. When the WM task was difficult, parietal stimulation (anodal or cathodal) improved WM performance selectively in participants with high WM capacity. In the low WM capacity group, parietal stimulation (anodal or cathodal) impaired WM performance. These nearly equal and opposite effects were only observed when the WM task was challenging, as in the change detection task. Experiment 2 probed the interplay of WM task difficulty and WM capacity in a parametric manner by varying set size in the WM change detection task. Here, the effect of parietal stimulation (anodal or cathodal) on the high WM capacity group followed a linear function as WM task difficulty increased with set size. The low WM capacity participants were largely unaffected by tDCS. These findings provide evidence that parietal involvement in WM performance depends on both WM capacity and WM task demands. We discuss these findings in terms of alternative WM strategies employed by low and high WM capacity individuals. We speculate that low WM capacity individuals do not recruit the posterior parietal lobe for WM tasks as efficiently as high WM capacity individuals. Consequently, tDCS provides greater benefit to individuals with high WM capacity.

Inter-Individual Variation during Transcranial Direct Current Stimulation and Normalization of Dose Using MRI-Derived Computational Models

BACKGROUND

Transcranial Direct Current Stimulation (tDCS) is a non-invasive, versatile, and safe neuromodulation technology under investigation for the treatment of neuropsychiatric disorders, adjunct to rehabilitation, and cognitive enhancement in healthy adults. Despite promising results, there is variability in responsiveness. One potential source of variability is the intensity of current delivered to the brain which is a function of both the operator controlled tDCS dose (electrode montage and total applied current) and subject specific anatomy. We are interested in both the scale of this variability across anatomical typical adults and methods to normalize inter-individual variation by customizing tDCS dose. Computational FEM simulations are a standard technique to predict brain current flow during tDCS and can be based on subject specific anatomical MRI.

OBJECTIVE

To investigate this variability, we modeled multiple tDCS montages across three adults (ages 34-41, one female).

RESULTS

Conventional pad stimulation led to diffuse modulation with maximum current flow between the pads across all subjects. There was high current flow directly under the pad for one subject while the location of peak induced cortical current flow was variable. The High-Definition tDCS montage led to current flow restricted to within the ring perimeter across all subjects. The current flow profile across all subjects and montages was influenced by details in cortical gyri/sulci.

CONCLUSION

This data suggests that subject specific modeling can facilitate consistent and more efficacious tDCS.

Translational research in transcranial direct current stimulation (tDCS): a systematic review of studies in animals

Recent therapeutic human studies testing transcranial direct current stimulation (tDCS) has shown promising results, although many questions remain unanswered. Translational research with experimental animals is an appropriate framework for investigating its mechanisms of action that are still undetermined. Nevertheless, animal and human studies are often discordant. Our aim was to review tDCS animal studies, examining and comparing their main findings with human studies. We performed a systematic review in Medline and other databases, screening for animal studies in vivo that delivered tDCS. Studies in vitro and using other neuromodulatory techniques were excluded. We extracted data according to Animal Research: Reporting In Vivo Experiments (ARRIVE) guidelines for reporting in vivo animal research. Thus, we collected data on sample characteristics (size, gender, weight and specimen) and methodology (experimental procedures, experimental animals, housing and husbandry, as well as analysis). We also collected data on methods for delivering tDCS (location, size, current and current density of electrodes and electrode montage), experimental effects (polarity-, intensity- and after-effects) and safety. Only 12 of 48 potentially eligible studies met our inclusion criteria and were reviewed. Quality assessment reporting was only moderate and studies were heterogeneous regarding tDCS montage methodology, position of active and reference electrodes, and current density used. Nonetheless, almost all studies demonstrated that tDCS had positive immediate and long-lasting effects. Vis-à-vis human trials, animal studies applied higher current densities (34.2 vs. 0.4 A/m(2), respectively), preferred extra-cephalic positions for reference electrodes (60% vs. 10%, respectively) and used electrodes with different sizes more often. Potential implications for translational tDCS research are discussed.

Polarity-dependent transcranial direct current stimulation effects on central auditory processing

Given the polarity dependent effects of transcranial direct current stimulation (tDCS) in facilitating or inhibiting neuronal processing, and tDCS effects on pitch perception, we tested the effects of tDCS on temporal aspects of auditory processing. We aimed to change baseline activity of the auditory cortex using tDCS as to modulate temporal aspects of auditory processing in healthy subjects without hearing impairment. Eleven subjects received 2mA bilateral anodal, cathodal and sham tDCS over auditory cortex in a randomized and counterbalanced order. Subjects were evaluated by the Random Gap Detection Test (RGDT), a test measuring temporal processing abilities in the auditory domain, before and during the stimulation. Statistical analysis revealed a significant interaction effect of time vs. tDCS condition for 4000 Hz and for clicks. Post-hoc tests showed significant differences according to stimulation polarity on RGDT performance: anodal improved 22.5% and cathodal decreased 54.5% subjects' performance, as compared to baseline. For clicks, anodal also increased performance in 29.4% when compared to baseline. tDCS presented polarity-dependent effects on the activity of the auditory cortex, which results in a positive or negative impact in a temporal resolution task performance. These results encourage further studies exploring tDCS in central auditory processing disorders.