Transcranial direct current stimulation of cerebellum alters spiking precision in cerebellar cortex: A modeling study of cellular responses

Region-specific cognitive effects of HD-tDCS in older adults: M1, DLPFC, and cerebellum

While regions like the primary motor cortex (M1), left dorsolateral prefrontal cortex (DLPFC), and cerebellum are linked to cognitive functions, it is unclear which offers the greatest cognitive benefit. This study aimed to assess the effects of high-definition transcranial direct current stimulation (HD-tDCS) on these regions, focusing on inhibitory control, cognitive flexibility, and working memory. This parallel, randomized, double-blinded, and controlled trial involved 80 older adults, randomly assigned to one of four groups: anodal stimulation of M1, left DLPFC, cerebellum, or sham. Inhibitory control was assessed using reaction time (RT) and rate-correct score (RCS) from a Go/No-Go task. Trail Making Test A (TMT-A) and B (TMT-B) measured processing speed and cognitive flexibility, while the backward digit span test evaluated working memory. All assessments were conducted pre- and post-stimulation. Notably, cerebellum stimulation significantly improved working memory (p = 0.010), whereas M1, DLPFC, and sham did not. Significant interaction effects emerged for TMT-A and TMT-B, with both M1 and DLPFC stimulation enhancing performance (TMT-A: p = 0.005, p = 0.025; TMT-B: p < 0.001, p = 0.045, respectively), while cerebellum and sham had no significant impact. Additionally, RT and RCS showed no significant effects. Anodal stimulation of M1 and DLPFC improved cognitive flexibility and processing speed, whereas cerebellum stimulation selectively enhanced working memory. However, inhibitory control did not improve, highlighting the need for further tailored interventions. These findings underscore distinct region-specific effects of tDCS on cognitive performance in older adults.

Feasibility of phase-locked transcranial magnetic stimulation of cerebellum for the treatment of essential tremor

Objective.Cerebellar transcranial magnetic stimulation (TMS) has been proposed to suppress limb tremors in essential tremor (ET), but mixed results have been reported so far, both when pulses are applied repetitively TMS (rTMS) and in bursts. We aim to investigate the cellular effects of TMS on the cerebellum under ET through numerical simulations.Approach.A computational model of the olivo-cerebello-thalamocortical pathways exhibiting the main neural biomarkers of ET (i.e. circuit-wide tremor-locked neural oscillations) was expanded to incorporate the effects of TMS-induced electric field (E-field) on Purkinje cells. TMS pulse amplitude, frequency, and temporal pattern were varied, and the resultant effects on ET biomarkers were assessed. Four levels of cellular response to TMS were considered, ranging from low to high cell recruitment underneath the coil, and three stimulation patterns were tested, i.e. rTMS, irregular TMS (ir-TMS, pulses were arranged according to Sobol sequences with average frequency matching rTMS), and phase-locked TMS (PL-TMS).Main results.rTMS can suppress ET oscillations, but its efficacy depends on tremor frequency and recruitment level, with these factors shaping a narrow range of effective settings. The ratio between tremor and rTMS frequencies also affects the neural response and further narrows the span of viable settings, while ir-TMS is ineffective. PL-TMS is highly effective and robust against changes to cell recruitment level and tremor frequency. Across all scenarios, PL-TMS provides a rapid (i.e. within seconds) suppression of tremor oscillations and, when both PL-TMS and rTMS are effective, the time to tremor suppression decreases by 50% or more in PL-TMS versus rTMS. At the cellular level, PL-TMS operates by disrupting the synchronization along the olivo-cerebellar loop, and the preferred phases map onto the mid-region of the silent period between complex spikes of the Purkinje cells.Significance.Cerebellar PL-TMS can provide robust suppression of ET oscillations while operating within safety boundaries.

Effects of transcranial electrical stimulation of the cerebellum, parietal cortex, anterior cingulate, and motor cortex on postural adaptation

Several cortical regions, such as the cerebellum, posterior parietal cortex (PPC), anterior cingulate cortex (ACC), and primary motor cortex (M1), play critical roles in postural adaptation. However, studies examining the effects of transcranial direct current stimulation (tDCS) on postural adaptation in healthy individuals are limited and often yield inconsistent findings, making it challenging to draw definitive conclusions. Most research has focused on individual brain regions, leaving a gap in understanding how the cerebellum, PPC, ACC, and M1 differentially contribute to postural adaptation. Identifying the most effective brain regions for postural adaptation could optimize rehabilitation strategies for individuals with postural control impairments. Thus, this study compared the effects of tDCS over these specific brain regions on postural adaptation. This parallel, randomized, double-blinded, controlled trial involved 75 participants, divided into five groups: anodal stimulation of the PPC, cerebellum, M1, ACC, or a sham group. Each group received 20 min of direct current stimulation in a single session. Center of pressure (COP) displacement, path length, velocity, and standard deviation (SD) were measured across three trials in the anteroposterior (AP) direction during standing disturbed using vibrators attached to bilateral Achilles tendons. A repeated measure ANOVA was used to assess within-group effects, while one-way ANOVA compared between-group differences. Between-group analysis did not reveal statistically significant differences during both the vibration and post-vibration phases. Nonetheless, the within-group analysis revealed significant enhancements in postural adaptation for the PPC and cerebellum groups during the vibration phase. Specifically, the PPC group demonstrated significant reductions in COP displacement (P = 0.005), path length (P = 0.018), and SD of COP displacement (P = 0.045) across trials. Similarly, in the cerebellar group, significant improvements were noted in COP displacement (P = 0.044), velocity (P = 0.006), and phase plane (P = 0.016) across trials. In contrast, no significant changes were found in the M1, ACC, or sham groups during either the vibration or post-vibration phases. In conclusion, while intergroup comparisons were not significant, intra-group analysis revealed that PPC and cerebellar stimulation significantly enhanced postural adaptation. Incorporating tDCS over the PPC or cerebellum in postural training programs could improve postural control, potentially reducing fall risk in clinical populations such as older adults or individuals with neurological dysfunction.RCT registration: On the Iranian Registry of Clinical Trials (IRCT20220819055745N1). Registration date: 15/11/2022.

Somatodendritic orientation determines tDCS-induced neuromodulation of Purkinje cell activity in awake mice

Transcranial direct-current stimulation (tDCS) of the cerebellum is a promising non-invasive neuromodulatory technique being proposed for the treatment of neurological and neuropsychiatric disorders. However, there is a lack of knowledge about how externally applied currents affect neuronal spiking activity in cerebellar circuits in vivo. We investigated how Cb-tDCS affects the firing rate of Purkinje cells (PC) and non-PC in the mouse cerebellar cortex to understand the underlying mechanisms behind the polarity-dependent modulation of neuronal activity induced by tDCS. Mice (n = 9) were prepared for the chronic recording of LFPs to assess the actual electric field gradient imposed by Cb-tDCS in our experimental design. Single-neuron extracellular recording of PCs in awake (n = 24) and anesthetized (n = 27) mice was combined with juxtacellular recordings and subsequent staining of PC with neurobiotin under anesthesia (n = 8) to correlate their neuronal orientation with their response to Cb-tDCS. Finally, a high-density Neuropixels recording system was used to demonstrate the relevance of neuronal orientation during the application of Cb-tDCS in awake mice (n = 6). In this study, we observe that Cb-tDCS induces a heterogeneous polarity-dependent modulation of the firing rate of Purkinje cells (PC) and non-PC in the mouse cerebellar cortex. We demonstrate that the apparently heterogeneous effects of tDCS on PC activity can be explained by taking into account the somatodendritic orientation relative to the electric field. Our findings highlight the need to consider neuronal orientation and morphology to improve tDCS computational models, enhance stimulation protocol reliability, and optimize effects in both basic and clinical applications.

Cerebellar Transcranial AC Stimulation Produces a Frequency-Dependent Bimodal Cerebellar Output Pattern

Cerebellar transcranial alternating current stimulation (ctACS) has the potential to be an appealing, non-invasive treatment option for psychiatric and neurological disorders. However, realization of this potential has been limited by gaps in our knowledge of how ctACS affects cerebellar output on single cell and population levels. Previously, we showed that AC stimulation applied to the cerebellar surface produced a strong, frequency-dependent modulation of Purkinje cell (PC) and cerebellar nuclear (CN) cell activity. Here, to approximate more closely the ctACS conditions, we investigated how AC stimulation applied to the external skull surface overlying crus 1 altered PC and CN activity in anesthetized adult female Sprague-Dawley rats. PC and CN activity showed a frequency-dependent modulation in response to ctACS at frequencies ranging from 0.5 to 80 Hz. A unimodal response was seen for most PCs across all frequencies, whereas most CN cells transitioned to bimodal patterns as stimulus frequency increased. The frequency-dependence of the phases of the local minima of the CN cell modulation were consistent with CN cells being driven synaptically by PC activity. Furthermore, comparison of responses with ipsilateral and contralateral placement of the stimulus electrode with respect to the recording site showed that the strength and pattern of the entrainment depended on the stimulus electrode location, suggesting that ctACS electrode placement could be used to target specific cerebellar output channels. In sum, the results show that transcranial stimulation of the cerebellar cortex can modulate cerebellar output, which has potential implications for its use in treating neurological and psychiatric disorders.

Transsynaptic modulation of cerebellar nuclear cells: theta AC-burst stimulation

Objectives.Transcranial alternating current stimulation (tACS) and its variants are being tested in clinical trials for treatment of neurological disorders, and cerebellar tACS (ctACS) in particular has garnered much interest because of the involvement of the cerebellum in these disorders. The main objective of this study was to investigate the frequency tuning curves for the entrainment of the Purkinje cells (PCs) and the cerebellar nuclear (CN) cells by their axonal projections. In addition, we aimed to investigate the temporal and steady-state characteristics of the PC-CN transsynaptic modulation under clinically relevant stimulation waveforms.Approach.Experiments were conducted in anesthetized rats with the electrical stimulations applied to the cerebellar cortex while the spiking activity of PC and CN cells were recorded extracellularly. The PC-CN modulation was tested in a wide range of AC frequencies (1-1000 Hz). Furthermore, high-frequency AC stimulation (40-400 Hz) repeated at 4 Hz, that we termedtheta AC-Burst Stimulation, was tested for its transient and steady-state responses.Main results. The CN cell firing patterns suggest that the population of projecting PCs that is entrained by the surface stimulation consists of the cells that are entrained in 180° opposite phases to each other. The CN cell spiking activity in general follows the entrainment pattern of the projecting PCs in the transient response. The CN entrainment during the steady-state turns into suppression at high frequencies of the stimulation. The PC responses could be explained with a simple statistical model that suggested that low-frequency (as well as DC) and high-frequency AC modulation may be operating through different neural mechanisms.Significance.High-frequency AC stimulation with a low-frequency envelope can be leveraged to induce CN modulation at theta frequencies. These results may explain some of the clinical findings and provide insight for future clinical trials of ctACS.

Cerebellar transcranial AC stimulation produces a frequency-dependent bimodal cerebellar output pattern

Transcranial alternating current stimulation (ctACS) has the potential to be an appealing, non-invasive treatment option for psychiatric and neurological disorders. However, its potential has been limited by significant knowledge gaps in the details and mechanisms of how ctACS affects cerebellar output on single cell and population levels. We investigated this issue by making single-unit recordings of Purkinje cells (PC) and lateral cerebellar nuclear (Lat CN) cells in response to ctACS in anesthetized adult female Sprague-Dawley rats. The ctACS electrode was positioned directly on the skull above crus 1, either ipsilaterally just medial to the recording site or contralaterally. The return electrode was placed under the skin of the shoulder ipsilateral to the recorded cell. In response to ctACS at frequencies ranging from 0.5 to 80 Hz, PC and CN activity was modulated in a frequency-dependent manner. PC and CN entrainment strength increased with stimulation frequency. Moreover, a unimodal response was seen for most PCs across all frequencies, whereas most CN cells transitioned to bimodal patterns as stimulus frequency increased. The phase of the local minima CN cells, and its change with frequency, was consistent with CN cells being driven synaptically by PC activity. Furthermore, the nearer ctACS location to the recording site, the stronger the entrainment, suggesting that ctACS electrode placement could be used to target specific cerebellar output channels. In sum, the results show that transcranial stimulation of the cerebellar cortex can modulate cerebellar output, which has potential implications for its use in treating neurological and psychiatric disorders.

Cerebellar Non-Invasive Brain Stimulation: A Frontier in Chronic Pain Therapy

Chronic pain poses a widespread and distressing challenge; it can be resistant to conventional therapies, often having significant side effects. Non-invasive brain stimulation (NIBS) techniques offer promising avenues for the safe and swift modulation of brain excitability. NIBS approaches for chronic pain management targeting the primary motor area have yielded variable outcomes. Recently, the cerebellum has emerged as a pivotal hub in human pain processing; however, the clinical application of cerebellar NIBS in chronic pain treatment remains limited. This review delineates the cerebellum's role in pain modulation, recent advancements in NIBS for cerebellar activity modulation, and novel biomarkers for assessing cerebellar function in humans. Despite notable progress in NIBS techniques and cerebellar activity assessment, studies targeting cerebellar NIBS for chronic pain treatment are limited in number. Nevertheless, positive outcomes in pain alleviation have been reported with cerebellar anodal transcranial direct current stimulation. Our review underscores the potential for further integration between cerebellar NIBS and non-invasive assessments of cerebellar function to advance chronic pain treatment strategies.

Neurorestorative effects of cerebellar transcranial direct current stimulation on social prediction of adolescents and young adults with congenital cerebellar malformations

BACKGROUND

Converging evidence points to impairments of the predictive function exerted by the cerebellum as one of the causes of the social cognition deficits observed in patients with cerebellar disorders.

OBJECTIVE

We tested the neurorestorative effects of cerebellar transcranial direct current stimulation (ctDCS) on the use of contextual expectations to interpret actions occurring in ambiguous sensory sceneries in a sample of adolescents and young adults with congenital, non-progressive cerebellar malformation (CM).

METHODS

We administered an action prediction task in which, in an implicit-learning phase, the probability of co-occurrence between actions and contextual elements was manipulated to form either strongly or moderately informative expectations. Subsequently, in a testing phase, we probed the use of these contextual expectations for predicting ambiguous (i.e., temporally occluded) actions. In a sham-controlled, within-subject design, participants received anodic or sham ctDCS during the task.

RESULTS

Anodic ctDCS, compared to sham, improved patients' ability to use contextual expectations to predict the unfolding of actions embedded in moderately, but not strongly, informative contexts.

CONCLUSIONS

These findings corroborate the role of the cerebellum in using previously learned contextual associations to predict social events and document the efficacy of ctDCS to boost social prediction in patients with congenital cerebellar malformation. The study encourages the further exploration of ctDCS as a neurorestorative tool for the neurorehabilitation of social cognition abilities in neurological, neuropsychiatric, and neurodevelopmental disorders featured by macro- or micro-structural alterations of the cerebellum.

Outcome measures for electric field modeling in tES and TMS: A systematic review and large-scale modeling study

BACKGROUND

Electric field (E-field) modeling is a potent tool to estimate the amount of transcranial magnetic and electrical stimulation (TMS and tES, respectively) that reaches the cortex and to address the variable behavioral effects observed in the field. However, outcome measures used to quantify E-fields vary considerably and a thorough comparison is missing.

OBJECTIVES

This two-part study aimed to examine the different outcome measures used to report on tES and TMS induced E-fields, including volume- and surface-level gray matter, region of interest (ROI), whole brain, geometrical, structural, and percentile-based approaches. The study aimed to guide future research in informed selection of appropriate outcome measures.

METHODS

Three electronic databases were searched for tES and/or TMS studies quantifying E-fields. The identified outcome measures were compared across volume- and surface-level E-field data in ten tES and TMS modalities targeting two common targets in 100 healthy individuals.

RESULTS

In the systematic review, we extracted 308 outcome measures from 202 studies that adopted either a gray matter volume-level (n = 197) or surface-level (n = 111) approach. Volume-level results focused on E-field magnitude, while surface-level data encompassed E-field magnitude (n = 64) and normal/tangential E-field components (n = 47). E-fields were extracted in ROIs, such as brain structures and shapes (spheres, hexahedra and cylinders), or the whole brain. Percentiles or mean values were mostly used to quantify E-fields. Our modeling study, which involved 1,000 E-field models and > 1,000,000 extracted E-field values, revealed that different outcome measures yielded distinct E-field values, analyzed different brain regions, and did not always exhibit strong correlations in the same within-subject E-field model.

CONCLUSIONS

Outcome measure selection significantly impacts the locations and intensities of extracted E-field data in both tES and TMS E-field models. The suitability of different outcome measures depends on the target region, TMS/tES modality, individual anatomy, the analyzed E-field component and the research question. To enhance the quality, rigor, and reproducibility in the E-field modeling domain, we suggest standard reporting practices across studies and provide four recommendations.

Recent data on the cerebellum require new models and theories

The cerebellum has been a popular topic for theoretical studies because its structure was thought to be simple. Since David Marr and James Albus related its function to motor skill learning and proposed the Marr-Albus cerebellar learning model, this theory has guided and inspired cerebellar research. In this review, we summarize the theoretical progress that has been made within this framework of error-based supervised learning. We discuss the experimental progress that demonstrates more complicated molecular and cellular mechanisms in the cerebellum as well as new cell types and recurrent connections. We also cover its involvement in diverse non-motor functions and evidence of other forms of learning. Finally, we highlight the need to explain these new experimental findings into an integrated cerebellar model that can unify its diverse computational functions.

The Therapeutic Potential of Non-Invasive and Invasive Cerebellar Stimulation Techniques in Hereditary Ataxias

The degenerative ataxias comprise a heterogeneous group of inherited and acquired disorders that are characterized by a progressive cerebellar syndrome, frequently in combination with one or more extracerebellar signs. Specific disease-modifying interventions are currently not available for many of these rare conditions, which underscores the necessity of finding effective symptomatic therapies. During the past five to ten years, an increasing number of randomized controlled trials have been conducted examining the potential of different non-invasive brain stimulation techniques to induce symptomatic improvement. In addition, a few smaller studies have explored deep brain stimulation (DBS) of the dentate nucleus as an invasive means to directly modulate cerebellar output, thereby aiming to alleviate ataxia severity. In this paper, we comprehensively review the clinical and neurophysiological effects of transcranial direct current stimulation (tDCS), repetitive transcranial magnetic stimulation (rTMS), and dentate nucleus DBS in patients with hereditary ataxias, as well as the presumed underlying mechanisms at the cellular and network level and perspectives for future research.

Spherical harmonics representation of the steady-state membrane potential shift induced by tDCS in realistic neuron models

Objective.We provide a systematic framework for quantifying the effect of externally applied weak electric fields on realistic neuron compartment models as captured by physiologically relevant quantities such as the membrane potential or transmembrane current as a function of the orientation of the field.Approach.We define a response function as the steady-state change of the membrane potential induced by a canonical external field of 1 V m^-1as a function of its orientation. We estimate the function values through simulations employing reconstructions of the rat somatosensory cortex from the Blue Brain Project. The response of different cell types is simulated using the NEURON simulation environment. We represent and analyze the angular response as an expansion in spherical harmonics.Main results.We report membrane perturbation values comparable to those in the literature, extend them to different cell types, and provide their profiles as spherical harmonic coefficients. We show that at rest, responses are dominated by their dipole terms (ℓ=1), in agreement with experimental findings and compartment theory. Indeed, we show analytically that for a passive cell, only the dipole term is nonzero. However, while minor, other terms are relevant for states different from resting. In particular, we show howℓ=0andℓ=2terms can modify the function to induce asymmetries in the response.Significance.This work provides a practical framework for the representation of the effects of weak electric fields on different neuron types and their main regions-an important milestone for developing micro- and mesoscale models and optimizing brain stimulation solutions.

A Systematic Review and Large-Scale tES and TMS Electric Field Modeling Study Reveals How Outcome Measure Selection Alters Results in a Person- and Montage-Specific Manner

Background

Electric field (E-field) modeling is a potent tool to examine the cortical effects of transcranial magnetic and electrical stimulation (TMS and tES, respectively) and to address the high variability in efficacy observed in the literature. However, outcome measures used to report E-field magnitude vary considerably and have not yet been compared in detail.

Objectives

The goal of this two-part study, encompassing a systematic review and modeling experiment, was to provide an overview of the different outcome measures used to report the magnitude of tES and TMS E-fields, and to conduct a direct comparison of these measures across different stimulation montages.

Methods

Three electronic databases were searched for tES and/or TMS studies reporting E-field magnitude. We extracted and discussed outcome measures in studies meeting the inclusion criteria. Additionally, outcome measures were compared via models of four common tES and two TMS modalities in 100 healthy younger adults.

Results

In the systematic review, we included 118 studies using 151 outcome measures related to E-field magnitude. Structural and spherical regions of interest (ROI) analyses and percentile-based whole-brain analyses were used most often. In the modeling analyses, we found that there was an average of only 6% overlap between ROI and percentile-based whole-brain analyses in the investigated volumes within the same person. The overlap between ROI and whole-brain percentiles was montage- and person-specific, with more focal montages such as 4Ã-1 and APPS-tES, and figure-of-eight TMS showing up to 73%, 60%, and 52% overlap between ROI and percentile approaches respectively. However, even in these cases, 27% or more of the analyzed volume still differed between outcome measures in every analyses.

Conclusions

The choice of outcome measures meaningfully alters the interpretation of tES and TMS E-field models. Well-considered outcome measure selection is imperative for accurate interpretation of results, valid between-study comparisons, and depends on stimulation focality and study goals. We formulated four recommendations to increase the quality and rigor of E-field modeling outcome measures. With these data and recommendations, we hope to guide future studies towards informed outcome measure selection, and improve the comparability of studies.

Modulating mental state recognition by anodal tDCS over the cerebellum

Increasing evidence from neuroimaging and clinical studies has demonstrated cerebellar involvement in social cognition components, including the mentalizing process. The aim of this study was to apply transcranial direct current stimulation (tDCS) to modulate cerebellar excitability to investigate the role the cerebellum plays in mental state recognition. Forty-eight healthy subjects were randomly assigned to different groups in which anodal, cathodal, or sham tDCS (2 mA for 20 min) was delivered centering the electrode on the vermis to stimulate the posterior portion of the cerebellum. The ability to attribute mental states to others was tested before and after tDCS using a digital version of the 'Reading the Mind in the Eyes test', which includes visual perceptive and motor stimuli as control conditions. Correct response and reaction times (RTs) were recorded. The results revealed a significant reduction in RTs between the baseline and post-stimulation sessions after cerebellar anodal tDCS only for mental state stimuli (Wilcoxon test p = 0.00055), whereas no significant effect was found in the cathodal or sham conditions or for visual perceptive and motor stimuli. Overall, our study suggests that cerebellar anodal tDCS might selectively improve mental state recognition and constitute an effective strategy to positively modulate the mentalizing process.

Applications of open-source software ROAST in clinical studies: A review

BACKGROUND

Transcranial electrical stimulation (TES) is broadly investigated as a therapeutic technique for a wide range of neurological disorders. The electric fields induced by TES in the brain can be estimated by computational models. A realistic and volumetric approach to simulate TES (ROAST) has been recently released as an open-source software package and has been widely used in TES research and its clinical applications. Rigor and reproducibility of TES studies have recently become a concern, especially in the context of computational modeling.

METHODS

Here we reviewed 94 clinical TES studies that leveraged ROAST for computational modeling. When reviewing each study, we pay attention to details related to the rigor and reproducibility as defined by the locations of stimulation electrodes and the dose of stimulating current. Specifically, we compared across studies the electrode montages, stimulated brain areas, achieved electric field strength, and the relations between modeled electric field and clinical outcomes.

RESULTS

We found that over 1800 individual heads have been modeled by ROAST for more than 30 different clinical applications. Similar electric field intensities were found to be reproducible by ROAST across different studies at the same brain area under same or similar stimulation montages.

CONCLUSION

This article reviews the use cases of ROAST and provides an overview of how ROAST has been leveraged to enhance the rigor and reproducibility of TES research and its applications.

Cerebellar transcranial direct current stimulation modulates timing but not acquisition of conditioned eyeblink responses in SCA3 patients

BACKGROUND

Delay eyeblink conditioning is an extensively studied motor learning paradigm that critically depends on the integrity of the cerebellum. In healthy individuals, modulation of cerebellar excitability using transcranial direct current stimulation (tDCS) has been reported to alter the acquisition and/or timing of conditioned eyeblink responses (CRs). It remains unknown whether such effects can also be elicited in patients with cerebellar disorders.

OBJECTIVE

To investigate if repeated sessions of cerebellar tDCS modify acquisition and/or timing of CRs in patients with spinocerebellar ataxia type 3 (SCA3) and to evaluate possible associations between disease severity measures and eyeblink conditioning parameters.

METHODS

Delay eyeblink conditioning was examined in 20 mildly to moderately affected individuals with SCA3 and 31 healthy controls. After the baseline session, patients were randomly assigned to receive ten sessions of cerebellar anodal tDCS or sham tDCS (i.e., five days per week for two consecutive weeks). Patients and investigators were blinded to treatment allocation. The same eyeblink conditioning protocol was administered directly after the last tDCS session. The Scale for the Assessment and Rating of Ataxia (SARA), cerebellar cognitive affective syndrome scale (CCAS-S), and disease duration were used as clinical measures of disease severity.

RESULTS

At baseline, SCA3 patients exhibited significantly fewer CRs than healthy controls. Acquisition was inversely associated with the number of failed CCAS-S test items but not with SARA score. Onset and peak latencies of CRs were longer in SCA3 patients and correlated with disease duration. Repeated sessions of cerebellar anodal tDCS did not affect CR acquisition, but had a significant treatment effect on both timing parameters. While a shift of CRs toward the conditioned stimulus was observed in the sham group (i.e., timing became more similar to that of healthy controls, presumably reflecting the effect of a second eyeblink conditioning session), anodal tDCS induced a shift of CRs in the opposite direction (i.e., toward the unconditioned stimulus).

CONCLUSION

Our findings provide the first evidence that cerebellar tDCS is capable of modifying cerebellar function in SCA3 patients. Future studies should assess whether this intervention similarly modulates temporal processing in other degenerative ataxias.

Mathematical Model Insights into EEG Origin under Transcranial Direct Current Stimulation (tDCS) in the Context of Psychosis

Schizophrenia is a psychotic disease that develops progressively over years with a transition from prodromal to psychotic state associated with a disruption in brain activity. Transcranial Direct Current Stimulation (tDCS), known to alleviate pharmaco-resistant symptoms in patients suffering from schizophrenia, promises to prevent such a psychotic transition. To understand better how tDCS affects brain activity, we propose a neural cortico-thalamo-cortical (CTC) circuit model involving the Ascending Reticular Arousal System (ARAS) that permits to describe major impact features of tDCS, such as excitability for short-duration stimulation and electroencephalography (EEG) power modulation for long-duration stimulation. To this end, the mathematical model relates stimulus duration and Long-Term Plasticity (LTP) effect, in addition to describing the temporal LTP decay after stimulus offset. This new relation promises to optimize future stimulation protocols. Moreover, we reproduce successfully EEG-power modulation under tDCS in a ketamine-induced psychosis model and confirm the N-methyl-d-aspartate (NMDA) receptor hypofunction hypothesis in the etiopathophysiology of schizophrenia. The model description points to an important role of the ARAS and the δ-rhythm synchronicity in CTC circuit in early-stage psychosis.

Cerebellar stimulation in schizophrenia: A systematic review of the evidence and an overview of the methods

BACKGROUND

Cerebellar structural and functional abnormalities underlie widespread deficits in clinical, cognitive, and motor functioning that are observed in schizophrenia. Consequently, the cerebellum is a promising target for novel schizophrenia treatments. Here we conducted an updated systematic review examining the literature on cerebellar stimulation efficacy and tolerability for mitigating symptoms of schizophrenia. We discuss the purported mechanisms of cerebellar stimulation, current methods for implementing stimulation, and future directions of cerebellar stimulation for intervention development with this population.

METHODS

Two independent authors identified 20 published studies (7 randomized controlled trials, 7 open-label studies, 1 pilot study, 4 case reports, 1 preclinical study) that describe the effects of cerebellar circuitry modulation in patients with schizophrenia or animal models of psychosis. Published studies up to October 11, 2022 were identified from a search within PubMed, Scopus, and PsycInfo.

RESULTS

Most studies stimulating the cerebellum used transcranial magnetic stimulation or transcranial direct-current stimulation, specifically targeting the cerebellar vermis/midline. Accounting for levels of methodological rigor across studies, these studies detected post-cerebellar modulation in schizophrenia as indicated by the alleviation of certain clinical symptoms (mainly negative and depressive symptoms), as well as increased frontal-cerebellar connectivity and augmentation of canonical neuro-oscillations known to be abnormal in schizophrenia. In contrast to a prior review, we did not find consistent evidence for cognitive improvements following cerebellar modulation stimulation. Modern cerebellar stimulation methods appear tolerable for individuals with schizophrenia, with only mild and temporary side effects.

CONCLUSION

Cerebellar stimulation is a promising intervention for individuals with schizophrenia that may be more relevant to some symptom domains than others. Initial results highlight the need for continued research using more methodologically rigorous designs, such as additional longitudinal and randomized controlled trials.

SYSTEMATIC REVIEW REGISTRATION

[https://www.crd.york.ac.uk/prospero/], identifier [CRD42022346667].