Brain stimulation studies of non-motor cerebellar function: A systematic review

An increase in reactive oxygen species underlies neonatal cerebellum repair

The neonatal mouse cerebellum shows remarkable regenerative potential upon injury at birth, wherein a subset of Nestin-expressing progenitors (NEPs) undergoes adaptive reprogramming to replenish granule cell progenitors that die. Here, we investigate how the microenvironment of the injured cerebellum changes upon injury and contributes to the regenerative potential of normally gliogenic-NEPs and their adaptive reprogramming. Single cell transcriptomic and bulk chromatin accessibility analyses of the NEPs from injured neonatal cerebella compared to controls show a temporary increase in cellular processes involved in responding to reactive oxygen species (ROS), a known damage-associated molecular pattern. Analysis of ROS levels in cerebellar tissue confirm a transient increased one day after injury at postanal day 1, overlapping with the peak cell death in the cerebellum. In a transgenic mouse line that ubiquitously overexpresses human mitochondrial catalase (mCAT), ROS is reduced 1 day after injury to the granule cell progenitors, and we demonstrate that several steps in the regenerative process of NEPs are curtailed leading to reduced cerebellar growth. We also provide evidence that microglia are involved in one step of adaptive reprogramming by regulating NEP replenishment of the granule cell precursors. Collectively, our results highlight that changes in the tissue microenvironment regulate multiple steps in adaptative reprogramming of NEPs upon death of cerebellar granule cell progenitors at birth, highlighting the instructive roles of microenvironmental signals during regeneration of the neonatal brain.

Time consciousness: Silence, mindfulness, and subjective time perception

Although recent theories of consciousness have emerged to define what consciousness is, an under-represented aspect within this field remains: time consciousness. However, the subjective passage of time is modulated by changing experiences within different situational contexts and by self-awareness. The experience of silence influences our awareness of self, space, and time, and it impacts on psychological well-being. The present review describes how self and time are influenced by different situations of silence (pure silence indoors and outdoors, the "just thinking" situation, and the combination of silence with deep relaxation). Also, the changes in time experience during a "forced" waiting situation due to the COVID-19 lockdown are presented in order to highlight the role of boredom in waiting situations and in situations in which we are alone with "our thoughts." Finally, in the context of the importance of creating silence through meditation practices, the alterations to one's sense of self and time during mindfulness meditation are reviewed. These studies are discussed within the framework of the cognitive models of prospective time perception, such as the attentional-gate model and the model of self-regulation and self-awareness.

The Predictive Role of the Posterior Cerebellum in the Processing of Dynamic Emotions

Recent studies have bolstered the important role of the cerebellum in high-level socio-affective functions. In particular, neuroscientific evidence shows that the posterior cerebellum is involved in social cognition and emotion processing, presumably through its involvement in temporal processing and in predicting the outcomes of social sequences. We used cerebellar transcranial random noise stimulation (ctRNS) targeting the posterior cerebellum to affect the performance of 32 healthy participants during an emotion discrimination task, including both static and dynamic facial expressions (i.e., transitioning from a static neutral image to a happy/sad emotion). ctRNS, compared to the sham condition, significantly reduced the participants' accuracy to discriminate static sad facial expressions, but it increased participants' accuracy to discriminate dynamic sad facial expressions. No effects emerged with happy faces. These findings may suggest the existence of two different circuits in the posterior cerebellum for the processing of negative emotional stimuli: a first-time-independent mechanism which can be selectively disrupted by ctRNS, and a second time-dependent mechanism of predictive "sequence detection" which can be selectively enhanced by ctRNS. This latter mechanism might be included among the cerebellar operational models constantly engaged in the rapid adjustment of social predictions based on dynamic behavioral information inherent to others' actions. We speculate that it might be one of the basic principles underlying the understanding of other individuals' social and emotional behaviors during interactions.

Smaller total and subregional cerebellar volumes in posttraumatic stress disorder: a mega-analysis by the ENIGMA-PGC PTSD workgroup

Although the cerebellum contributes to higher-order cognitive and emotional functions relevant to posttraumatic stress disorder (PTSD), prior research on cerebellar volume in PTSD is scant, particularly when considering subregions that differentially map on to motor, cognitive, and affective functions. In a sample of 4215 adults (PTSD n = 1642; Control n = 2573) across 40 sites from the ENIGMA-PGC PTSD working group, we employed a new state-of-the-art deep-learning based approach for automatic cerebellar parcellation to obtain volumetric estimates for the total cerebellum and 28 subregions. Linear mixed effects models controlling for age, gender, intracranial volume, and site were used to compare cerebellum volumes in PTSD compared to healthy controls (88% trauma-exposed). PTSD was associated with significant grey and white matter reductions of the cerebellum. Compared to controls, people with PTSD demonstrated smaller total cerebellum volume, as well as reduced volume in subregions primarily within the posterior lobe (lobule VIIB, crus II), vermis (VI, VIII), flocculonodular lobe (lobule X), and corpus medullare (all p-FDR < 0.05). Effects of PTSD on volume were consistent, and generally more robust, when examining symptom severity rather than diagnostic status. These findings implicate regionally specific cerebellar volumetric differences in the pathophysiology of PTSD. The cerebellum appears to play an important role in higher-order cognitive and emotional processes, far beyond its historical association with vestibulomotor function. Further examination of the cerebellum in trauma-related psychopathology will help to clarify how cerebellar structure and function may disrupt cognitive and affective processes at the center of translational models for PTSD.

Bibliometric and visualised analysis on non-invasive cerebellar stimulation from 1995 to 2021

Background

The non-invasive cerebellar stimulation (NICS) is a neural modulation technique, which shows the therapeutic and diagnostic potentials for rehabilitating brain functions in neurological or psychiatric diseases. There is a rapid growth in the clinical research related to NICS in recent years. Hence, we applied a bibliometric approach to analyze the current status, the hot spots, and the trends of NICS visually and systematically.

Methods

We searched the NICS publications from the Web of Science (Wos) between 1995 and 2021. Both VOSviewer (1.6.18) and Citespace (Version 6.1.2) software were used to generate the co-occurrence or co-cited network maps about the authors, institutions, countries, journals, and keywords.

Results

A total of 710 articles were identified in accordance with our inclusion criteria. The linear regression analysis shows a statistical increase in the number of publications per year on NICS research over time (p &lt; 0.001). The Italy and University College London ranked the first in this field with 182 and 33 publications, respectively. Koch, Giacomo was the most prolific author (36 papers). The journal of Cerebellum, Brain stimulation and Clinical neurophysiology were the most three productive journals to publish NICS-related articles.

Conclusion

Our findings provide the useful information regarding to the global trends and frontiers in NICS field. Hot topic was focused on the interaction between the transcranial direct current stimulation and functional connectivity in the brain. It could guide the future research and clinical application of NICS.

Mini-review: Transcranial Alternating Current Stimulation and the Cerebellum

Oscillatory activity in the cerebellum and linked networks is an important aspect of neuronal processing and functional implementation of behavior. So far, it was challenging to quantify and study cerebellar oscillatory signatures in human neuroscience due to the constraints of non-invasive cerebellar electrophysiological recording and interventional techniques. The emerging cerebellar transcranial alternating current stimulation technique (CB-tACS) is a promising tool, which may partially overcome this challenge and provides an exciting non-invasive opportunity to better understand cerebellar physiology.Several studies have successfully demonstrated that CB-tACS can modulate the cerebellar outflow and cerebellum-linked behavior. In the present narrative review, we summarize current studies employing the CB-tACS approach and discuss open research questions. Hereby, we aim to provide an overview on this emerging electrophysiological technique and strive to promote future research in the field. CB-tACS will contribute in the further deciphering of cerebellar oscillatory signatures and its role for motor, cognitive, or affective functions. In long term, CB-tACS could develop into a therapeutic tool for retuning disturbed oscillatory activity in cerebellar networks underlying brain disorders.

A Brief History of Cerebellar Neurostimulation

The first attempts at using electric stimulation to study human brain functions followed the experiments of Luigi Galvani and Giovanni Aldini on animal electricity during the eighteenth century. Since then, the cerebellum has been among the areas that have been studied by invasive and non-invasive forms of electrical and magnetic stimulation. During the nineteenth century, animal experiments were conducted to map the motor-related regions of cerebellar cortex by means of direct electric stimulation. As electric stimulation research on the cerebellum moved into the twentieth century, systematic research of electric cerebellar stimulation led to a better understanding of its effects and mechanism of action. In addition, the clinical potential of cerebellar stimulation in the treatment of motor diseases started to be explored. With the introduction of transcranial electric and magnetic stimulation, cerebellar research moved to non-invasive techniques. During the twenty-first century, following on groundbreaking research that linked the cerebellum to non-motor functions, non-invasive techniques have facilitated research into different aspects of cerebellar functioning. The present review provides a brief historical account of cerebellar neurostimulation and discusses current challenges and future direction in this field of research.

AIM2 inflammasome mediates apoptotic and pyroptotic death in the cerebellum following chronic hypoperfusion

Vascular dementia (VaD) is the second most common form of dementia and is caused by vascular pathologies resulting in chronic cerebral hypoperfusion (CCH)- induced brain injury, and ultimately cognitive impairment and memory loss. Several lines of evidence have demonstrated chronic inflammation may be involved in VaD disease progression. It is now recognized that a major contributor to cerebral and systemic chronic inflammation involves the activation of innate immune molecular complexes termed inflammasomes. Whilst previous studies on animal models of VaD have focused on the cortex, hippocampus and striatum, few studies have investigated the effect of CCH on the cerebellum. Emerging studies have found new roles of the cerebellum in cognition, based on its structural interconnectivity with other brain regions and clinical relevance in neuropsychological deficits. In the present study, we conducted our investigation on the cerebellum using a CCH mouse model of VaD following bilateral common carotid artery stenosis (BCAS). This study is the first to characterize an increased expression of inflammasome receptors, adaptor and effector proteins, markers of inflammasome activation, proinflammatory cytokines, and apoptotic and pyroptotic cell death proteins in the cerebellum following CCH. Furthermore, in AIM2 knockout mice, we observed attenuated inflammasome-mediated production of proinflammatory cytokines, apoptosis, and pyroptosis in the cerebellum following CCH. Collectively, our findings provide novel evidence that AIM2 inflammasome activation promotes apoptosis and pyroptosis in the cerebellum following chronic hypoperfusion in a mouse model of VaD.

Probing cerebellar involvement in cognition through a meta-analysis of TMS evidence

Traditionally, the cerebellum has been linked to motor coordination, but growing evidence points to its involvement in a wide range of non-motor functions. Though the number of studies using transcranial magnetic stimulation (TMS) to investigate cerebellar involvement in cognitive processes is growing exponentially, these findings have not yet been synthesized in a meta-analysis. Here, we used meta-analysis to estimate the effects of cerebellar TMS on performance in cognitive tasks for healthy participants. Outcomes included participants' accuracy and response times (RTs) of several non-motor tasks performed either during or after the administration of TMS. We included overall 41 studies, of which 44 single experiments reported effects on accuracy and 41 on response times (RTs). The meta-analyses showed medium effect sizes (for accuracy: d = 0.61 [95% CI = 0.48, .073]; for RTs: d = 0.40 [95% CI = 0.30, 0.49]), with leave-one-out analyses indicating that cumulative effects were robust, and with moderate heterogeneity. For both accuracy and RTs, the effect of TMS was moderated by the stimulation paradigm adopted but not by the cognitive function investigated, while the timing of the stimulation moderated only the effects on RTs. Further analyses on lateralization revealed no moderation effects of the TMS site. Taken together, these findings indicate that TMS administered over the cerebellum is able to modulate cognitive performance, affecting accuracy or RTs, and suggest that the various stimulation paradigms play a key role in determining the efficacy of cerebellar TMS.

Intermittent fasting attenuates inflammasome-associated apoptotic and pyroptotic death in the brain following chronic hypoperfusion

Chronic cerebral hypoperfusion (CCH) has been shown to initiate several inflammatory pathways that can contribute to cognitive deficits and memory loss in vascular cognitive impairment (VCI). Multi-protein complexes termed inflammasomes that may be involved in the inflammatory response to CCH has already been shown to contribute to the inflammatory process and cell death following acute cerebral ischemia. Intermittent fasting (IF) has already been shown to decrease inflammasome activation and protect the brain from ischemic stroke; however, its effects during CCH remains unknown. The present study investigated the impact of IF (16 h of food deprivation daily) for four months on inflammasome-mediated cell death in the cerebellum following CCH in a mouse model of VCI using fourteen to sixteen-week-old male C57BL/6NTac mice. Here we demonstrated that IF decreased inflammasome activation, and initiation of apoptotic and pyroptotic cell death pathways as reflected by the reduction (20-30%) in the expression levels of key effector proteins and cell death markers in the cerebellum following CCH. In summary, our results indicate that IF can attenuate the inflammatory response and cell death pathways in the brain following chronic hypoperfusion in a mouse model of VCI.

Cerebellar transcranial direct current stimulation reconfigurates static and dynamic functional connectivity of the resting-state networks

Background

Transcranial direct current stimulation (tDCS) of the cerebellum dynamically modulates cerebello-thalamo-cortical excitability in a polarity-specific manner during motor, visuo- motor and cognitive tasks. It remains to be established whether tDCS of the cerebellum impact also on resting-state intrinsically connected networks (ICNs). Such impact would open novel research and therapeutical doors for the neuromodulation of ICNs in human.

Method

We combined tDCS applied over the right cerebellum and fMRI to investigate tDCS- induced resting-state intrinsic functional reconfiguration, using a randomized, sham-controlled design. fMRI data were recorded both before and after real anodal stimulation (2 mA, 20 min) or sham tDCS in 12 right-handed healthy volunteers. We resorted to a region-of-interest static correlational analysis and to a sliding window analysis to assess temporal variations in resting state FC between the cerebellar lobule VII and nodes of the main ICNs.

Results

After real tDCS and compared with sham tDCS, functional changes were observed between the cerebellum and ICNs. Static FC showed enhanced or decreased correlation between cerebellum and brain areas belonging to visual, default-mode (DMN), sensorimotor and salience networks (SN) (p-corrected < 0.05). The temporal variability (TV) of BOLD signal was significantly modified after tDCS displaying in particular a lesser TV between the whole lobule VII and DMN and central executive network and a greater TV between crus 2 and SN. Static and dynamic FC was also modified between cerebellar lobuli.

Conclusion

These results demonstrate short- and long-range static and majorly dynamic effects of tDCS stimulation of the cerebellum affecting distinct resting-state ICNs, as well as intracerebellar functional connectivity, so that tDCS of the cerebellum appears as a non-invasive tool reconfigurating the dynamics of ICNs.

Consensus Paper: Cerebellum and Social Cognition

The traditional view on the cerebellum is that it controls motor behavior. Although recent work has revealed that the cerebellum supports also nonmotor functions such as cognition and affect, only during the last 5 years it has become evident that the cerebellum also plays an important social role. This role is evident in social cognition based on interpreting goal-directed actions through the movements of individuals (social "mirroring") which is very close to its original role in motor learning, as well as in social understanding of other individuals' mental state, such as their intentions, beliefs, past behaviors, future aspirations, and personality traits (social "mentalizing"). Most of this mentalizing role is supported by the posterior cerebellum (e.g., Crus I and II). The most dominant hypothesis is that the cerebellum assists in learning and understanding social action sequences, and so facilitates social cognition by supporting optimal predictions about imminent or future social interaction and cooperation. This consensus paper brings together experts from different fields to discuss recent efforts in understanding the role of the cerebellum in social cognition, and the understanding of social behaviors and mental states by others, its effect on clinical impairments such as cerebellar ataxia and autism spectrum disorder, and how the cerebellum can become a potential target for noninvasive brain stimulation as a therapeutic intervention. We report on the most recent empirical findings and techniques for understanding and manipulating cerebellar circuits in humans. Cerebellar circuitry appears now as a key structure to elucidate social interactions.

Cerebellar-Stimulation Evoked Prefrontal Electrical Synchrony Is Modulated by GABA

Cerebellar-prefrontal connectivity has been recognized as important for behaviors ranging from motor coordination to cognition. Many of these behaviors are known to involve excitatory or inhibitory modulations from the prefrontal cortex. We used cerebellar transcranial magnetic stimulation (TMS) with simultaneous electroencephalography (EEG) to probe cerebellar-evoked electrical activity in prefrontal cortical areas and used magnetic resonance spectroscopy (MRS) measures of prefrontal GABA and glutamate levels to determine if they are correlated with those potentials. Cerebellar-evoked bilateral prefrontal synchrony in the theta to gamma frequency range showed patterns that reflect strong GABAergic inhibitory function (r = - 0.66, p = 0.002). Stimulation of prefrontal areas evoked bilateral prefrontal synchrony in the theta to low beta frequency range that reflected, conversely, glutamatergic excitatory function (r = 0.66, p = 0.002) and GABAergic inhibitory function (r = - 0.65, p = 0.002). Cerebellar-evoked prefrontal synchronization had opposite associations with cognition and motor coordination: it was positively associated with working memory performance (r = 0.57, p = 0.008) but negatively associated with coordinated motor function as measured by rapid finger tapping (r = - 0.59, p = 0.006). The results suggest a relationship between regional GABA levels and interregional effects on synchrony. Stronger cerebellar-evoked prefrontal synchrony was associated with better working memory but surprisingly worse motor coordination, which suggests competing effects for motor activity and cognition. The data supports the use of a TMS-EEG-MRS approach to study the neurochemical basis of large-scale oscillations modulated by the cerebellar-prefrontal connectivity.

Genetic deletion of genes in the cerebellar rhombic lip lineage can stimulate compensation through adaptive reprogramming of ventricular zone-derived progenitors

BACKGROUND

The cerebellum is a foliated posterior brain structure involved in coordination of motor movements and cognition. The cerebellum undergoes rapid growth postnataly due to Sonic Hedgehog (SHH) signaling-dependent proliferation of ATOH1+ granule cell precursors (GCPs) in the external granule cell layer (EGL), a key step for generating cerebellar foliation and the correct number of granule cells. Due to its late development, the cerebellum is particularly vulnerable to injury from preterm birth and stress around birth. We recently uncovered an intrinsic capacity of the developing cerebellum to replenish ablated GCPs via adaptive reprogramming of Nestin-expressing progenitors (NEPs). However, whether this compensation mechanism occurs in mouse mutants affecting the developing cerebellum and could lead to mis-interpretation of phenotypes was not known.

METHODS

We used two different approaches to remove the main SHH signaling activator GLI2 in GCPs: 1) Our mosaic mutant analysis with spatial and temporal control of recombination (MASTR) technique to delete Gli2 in a small subset of GCPs; 2) An Atoh1-Cre transgene to delete Gli2 in most of the EGL. Genetic Inducible Fate Mapping (GIFM) and live imaging were used to analyze the behavior of NEPs after Gli2 deletion.

RESULTS

Mosaic analysis demonstrated that SHH-GLI2 signaling is critical for generating the correct pool of granule cells by maintaining GCPs in an undifferentiated proliferative state and promoting their survival. Despite this, inactivation of GLI2 in a large proportion of GCPs in the embryo did not lead to the expected dramatic reduction in the size of the adult cerebellum. GIFM uncovered that NEPs do indeed replenish GCPs in Gli2 conditional mutants, and then expand and partially restore the production of granule cells. Furthermore, the SHH signaling-dependent NEP compensation requires Gli2, demonstrating that the activator side of the pathway is involved.

CONCLUSION

We demonstrate that a mouse conditional mutation that results in loss of SHH signaling in GCPs is not sufficient to induce long term severe cerebellum hypoplasia. The ability of the neonatal cerebellum to regenerate after loss of cells via a response by NEPs must therefore be considered when interpreting the phenotypes of Atoh1-Cre conditional mutants affecting GCPs.

Insights from perceptual, sensory, and motor functioning in autism and cerebellar primary disturbances: Are there reliable markers for these disorders?

The contribution of cerebellar circuitry alterations in the pathophysiology of Autism Spectrum Disorder (ASD) has been widely investigated in the last decades. Yet, experimental studies on neurocognitive markers of ASD have not been attentively compared with similar studies in patients with cerebellar primary disturbances (e.g., malformations, agenesis, degeneration, etc). Addressing this neglected issue could be useful to underline unexpected areas of overlap and/or underestimated differences between these sets of conditions. In fact, ASD and cerebellar primary disturbances (notably, Cerebellar Cognitive Affective Syndrome, CCAS) can share atypical manifestations in perceptual, sensory, and motor functions, but neural subcircuits involved in these anomalies/difficulties could be distinct. Here, we specifically deal with this issue focusing on four paradigmatic neurocognitive functions: visual and biological motion perception, multisensory integration, and high stages of the motor hierarchy. From a research perspective, this represents an essential challenge to more deeply understand neurocognitive markers of ASD and of cerebellar primary disturbances/CCAS. Although we cannot assume definitive conclusions, and beyond phenotypical similarities between ASD and CCAS, clinical and experimental evidence described in this work argues that ASD and CCAS are distinct phenomena. ASD and CCAS seem to be characterized by different pathophysiological mechanisms and mediated by distinct neural nodes. In parallel, from a clinical perspective, this characterization may furnish insights to tackle the distinction between autistic functioning/autistic phenotype (in ASD) and dysmetria of thought/autistic-like phenotype (in CCAS).

Targeting the Cerebellum by Noninvasive Neurostimulation: a Review

Transcranial magnetic and electric stimulation of the brain are novel and highly promising techniques currently employed in both research and clinical practice. Improving or rehabilitating brain functions by modulating excitability with these noninvasive tools is an exciting new area in neuroscience. Since the cerebellum is closely connected with the cerebral regions subserving motor, associative, and affective functions, the cerebello-thalamo-cortical pathways are an interesting target for these new techniques. Targeting the cerebellum represents a novel way to modulate the excitability of remote cortical regions and their functions. This review brings together the studies that have applied cerebellar stimulation, magnetic and electric, and presents an overview of the current knowledge and unsolved issues. Some recommendations for future research are implemented as well.

Cerebellar Contributions to Language in Typical and Atypical Development: A Review

In this review, we present the growing literature suggesting, from a variety of angles, that the cerebellum contributes to higher-order cognitive functions, rather than simply sensorimotor functions, and more specifically to language and its development. The cerebellum's association with language function is determined by the specific cortico-cerebellar connectivity to the right cerebellum from the left cortical hemisphere. The findings we review suggest that the cerebellum plays an important role as part of a broader language network, and also implies that the cerebellum may be a potential new therapeutic target to treat speech and language deficits, especially during development.

Effects of cerebellar transcranial alternating current stimulation on motor cortex excitability and motor function

The cerebellum regulates several motor functions through two main mechanisms, the cerebellum-brain inhibition (CBI) and the motor surround inhibition (MSI). Although the exact cerebellar structures and functions involved in such processes are partially known, Purkinje cells (PC) and their surrounding interneuronal networks may play a pivotal role concerning CBI and MSI. Cerebellar transcranial alternating current stimulation (tACS) has been proven to shape specific cerebellar components in a feasible, safe, effective, and non-invasive manner. The aim of our study was to characterize the cerebellar structures and functions subtending CBI and MSI using a tACS approach. Fifteen healthy individuals underwent a cerebellar tACS protocol at 10, 50, and 300 Hz, or a sham-tACS over the right cerebellar hemisphere. We measured the tACS aftereffects on motor-evoked potential (MEP) amplitude, CBI induced by tACS (tiCBI) at different frequencies, MSI, and hand motor task performance. None of the participants had any side effect related to tACS. After 50-Hz tACS, we observed a clear tiCBI-50Hz weakening (about +30%, p < 0.001) paralleled by a MEP amplitude increase (about +30%, p = 0.001) and a reduction of the time required to complete some motor task (about -20%, p = 0.01), lasting up to 30 min. The 300-Hz tACS induced a selective, specific tiCBI-300Hz and tiCBI-50Hz modulation in surrounding muscles (about -15%, p = 0.01) and MSI potentiation (about +40%, p < 0.001). The 10-Hz tACS and the sham-tACS were ineffective (p > 0.6). Our preliminary data suggest that PC may represent the last mediator of tiCBI and that the surrounding interneuronal network may have an important role in updating MSI, tiCBI, and M1 excitability during tonic muscle contraction, by acting onto the PC. The knowledge of these neurophysiological issues offers new cues to design innovative, non-invasive neuromodulation protocols to shape cerebellar-cerebral functions.

Cerebellar tDCS does not improve performance in probabilistic classification learning

In this study, the role of the cerebellum in a cognitive learning task using transcranial direct current stimulation (tDCS) was investigated. Using a weather prediction task, subjects had to learn the probabilistic associations between a stimulus (a combination of cards) and an outcome (sun or rain). This task is a variant of a probabilistic classification learning task, for which it has been reported that prefrontal tDCS enhances performance. Using a between-subject design, all 30 subjects learned to improve their performance with increasing accuracies and shortened response times over a series of 500 trials. Subjects also became more confident in their prediction during the experiment. However, no differences in performance and learning were observed between subjects receiving sham stimulation (n = 10) or anodal stimulation (2 mA for 20 min) over either the right cerebellum (n = 10) or the left prefrontal cortex (n = 10). This suggests that stimulating the brain with cerebellar tDCS does not readily influence probabilistic classification performances, probably due to the rather complex nature of this cognitive task.

The sound of emotions-Towards a unifying neural network perspective of affective sound processing

Affective sounds are an integral part of the natural and social environment that shape and influence behavior across a multitude of species. In human primates, these affective sounds span a repertoire of environmental and human sounds when we vocalize or produce music. In terms of neural processing, cortical and subcortical brain areas constitute a distributed network that supports our listening experience to these affective sounds. Taking an exhaustive cross-domain view, we accordingly suggest a common neural network that facilitates the decoding of the emotional meaning from a wide source of sounds rather than a traditional view that postulates distinct neural systems for specific affective sound types. This new integrative neural network view unifies the decoding of affective valence in sounds, and ascribes differential as well as complementary functional roles to specific nodes within a common neural network. It also highlights the importance of an extended brain network beyond the central limbic and auditory brain systems engaged in the processing of affective sounds.

Cerebellar contributions to motor control and language comprehension: searching for common computational principles

The past 25 years have seen the functional domain of the cerebellum extend beyond the realm of motor control, with considerable discussion of how this subcortical structure contributes to cognitive domains including attention, memory, and language. Drawing on evidence from neuroanatomy, physiology, neuropsychology, and computational work, sophisticated models have been developed to describe cerebellar function in sensorimotor control and learning. In contrast, mechanistic accounts of how the cerebellum contributes to cognition have remained elusive. Inspired by the homogeneous cerebellar microanatomy and a desire for parsimony, many researchers have sought to extend mechanistic ideas from motor control to cognition. One influential hypothesis centers on the idea that the cerebellum implements internal models, representations of the context-specific dynamics of an agent's interactions with the environment, enabling predictive control. We briefly review cerebellar anatomy and physiology, to review the internal model hypothesis as applied in the motor domain, before turning to extensions of these ideas in the linguistic domain, focusing on speech perception and semantic processing. While recent findings are consistent with this computational generalization, they also raise challenging questions regarding the nature of cerebellar learning, and may thus inspire revisions of our views on the role of the cerebellum in sensorimotor control.

Inactivation of Cerebellar Cortical Crus II Disrupts Temporal Processing of Absolute Timing but not Relative Timing in Voluntary Movements

Several recent studies have demonstrated that the cerebellum plays an important role in temporal processing at the scale of milliseconds. However, it is not clear whether intrinsic cerebellar function involves the temporal processing of discrete or continuous events. Temporal processing during discrete events functions by counting absolute time like a stopwatch, while during continuous events it measures events at intervals. During the temporal processing of continuous events, animals might respond to rhythmic timing of sequential responses rather than to the absolute durations of intervals. Here, we tested the contribution of the cerebellar cortex to temporal processing of absolute and relative timings in voluntary movements. We injected muscimol and baclofen to a part of the cerebellar cortex of rats. We then tested the accuracy of their absolute or relative timing prediction using two timing tasks requiring almost identical reaching movements. Inactivation of the cerebellar cortex disrupted accurate temporal prediction in the absolute timing task. The rats formed two groups based on the changes to their timing accuracy following one of two distinct patterns which can be described as longer or shorter declines in the accuracy of learned intervals. However, a part of the cerebellar cortical inactivation did not affect the rats' performance of relative timing tasks. We concluded that a part of the cerebellar cortex, Crus II, contributes to the accurate temporal prediction of absolute timing and that the entire cerebellar cortex may be unnecessary in cases in which accurately knowing the absolute duration of an interval is not required for temporal prediction.

Cytoarchitectonic mapping of the human brain cerebellar nuclei in stereotaxic space and delineation of their co-activation patterns

The cerebellar nuclei are involved in several brain functions, including the modulation of motor and cognitive performance. To differentiate their participation in these functions, and to analyze their changes in neurodegenerative and other diseases as revealed by neuroimaging, stereotaxic maps are necessary. These maps reflect the complex spatial structure of cerebellar nuclei with adequate spatial resolution and detail. Here we report on the cytoarchitecture of the dentate, interposed (emboliform and globose) and fastigial nuclei, and introduce 3D probability maps in stereotaxic MNI-Colin27 space as a prerequisite for subsequent meta-analysis of their functional involvement. Histological sections of 10 human post mortem brains were therefore examined. Differences in cell density were measured and used to distinguish a dorsal from a ventral part of the dentate nucleus. Probabilistic maps were calculated, which indicate the position and extent of the nuclei in 3D-space, while considering their intersubject variability. The maps of the interposed and the dentate nuclei differed with respect to their interaction patterns and functions based on meta-analytic connectivity modeling and quantitative functional decoding, respectively. For the dentate nucleus, significant (p < 0.05) co-activations were observed with thalamus, supplementary motor area (SMA), putamen, BA 44 of Broca's region, areas of superior and inferior parietal cortex, and the superior frontal gyrus (SFG). In contrast, the interposed nucleus showed more limited co-activations with SMA, area 44, putamen, and SFG. Thus, the new stereotaxic maps contribute to analyze structure and function of the cerebellum. These maps can be used for anatomically reliable and precise identification of degenerative alteration in MRI-data of patients who suffer from various cerebellar diseases.

Schmahmann's syndrome - identification of the third cornerstone of clinical ataxiology

Schmahmann’s syndrome represents a novel clinical condition consisting of a constellation of cognitive and affective deficits following cerebellar disease. The complex was first described in 1998 as cerebellar cognitive affective syndrome (CCAS) on the basis of a careful neurological examination, detailed bedside mental state tests, neuropsychological investigations and anatomical neuroimaging of a group of 20 patients with focal cerebellar disorders. The syndrome was characterized by four clusters of symptoms including: (a) impairment of executive functions such as planning, set-shifting, verbal fluency, abstract reasoning and working memory, (b) impaired visuo-spatial cognition, (c) personality changes with blunting of affect or abnormal behaviour, and (d) language deficits including agrammatism, wordfinding disturbances, disruption of language dynamics and dysprosodia. This complex of neurocognitive and behavioural-affective symptoms was ascribed to a functional disruption of the reciprocal pathways that connect the cerebellum with the limbic circuitry and the prefrontal, temporal and parietal association cortices. With the introduction of Schmahmann’s syndrome, clinical ataxiology has found its third cornerstone, the two others being the cerebellar motor syndrome (CMS) mainly delineated by the pioneer French and English neurologists of the 19^th and early 20^th century, and the vestibulo-cerebellar syndrome (VCS) consisting of ocular instability, deficits of oculomotor movements and ocular misalignment.

Cerebellar contributions to verbal working memory

There is increasing evidence for a cerebellar role in working memory. Clinical research has shown that working memory impairments after cerebellar damage and neuroimaging studies have revealed task-specific activation in the cerebellum during working memory processing. A lateralisation of cerebellar function within working memory has been proposed with the right hemisphere making the greater contribution to verbal processing and the left hemisphere for visuospatial tasks. We used continuous theta burst stimulation (cTBS) to examine whether differences in post-stimulation performance could be observed based on the cerebellar hemisphere stimulated and the type of data presented. We observed that participants were significantly less accurate on a verbal version of a Sternberg task after stimulation to the right cerebellar hemisphere when compared to left hemisphere stimulation. Performance on a visual Sternberg task was unaffected by stimulation of either hemisphere. We discuss our results in the context of prior studies that have used cerebellar stimulation to investigate working memory and highlight the cerebellar role in phonological encoding.

The therapeutic potential of the cerebellum in schizophrenia

The cognitive role of the cerebellum is critically tied to its distributed connections throughout the brain. Accumulating evidence from anatomical, structural and functional imaging, and lesion studies advocate a cognitive network involving indirect connections between the cerebellum and non-motor areas in the prefrontal cortex. Cerebellar stimulation dynamically influences activity in several regions of the frontal cortex and effectively improves cognition in schizophrenia. In this manuscript, we summarize current literature on the cingulocerebellar circuit and we introduce a method to interrogate this circuit combining opotogenetics, neuropharmacology, and electrophysiology in awake-behaving animals while minimizing incidental stimulation of neighboring cerebellar nuclei. We propose the novel hypothesis that optogenetic cerebellar stimulation can restore aberrant frontal activity and rescue impaired cognition in schizophrenia. We focus on how a known cognitive region in the frontal cortex, the anterior cingulate, is influenced by the cerebellum. This circuit is of particular interest because it has been confirmed using tracing studies, neuroimaging reveals its role in cognitive tasks, it is conserved from rodents to humans, and diseases such as schizophrenia and autism appear in its aberrancy. Novel tract tracing results presented here provide support for how these two areas communicate. The primary pathway involves a disynaptic connection between the cerebellar dentate nuclei (DN) and the anterior cingulate cortex. Secondarily, the pathway from cerebellar fastigial nuclei (FN) to the ventral tegmental area, which supplies dopamine to the prefrontal cortex, may play a role as schizophrenia characteristically involves dopamine deficiencies. We hope that the hypothesis described here will inspire new therapeutic strategies targeting currently untreatable cognitive impairments in schizophrenia.

Transcranial cerebellar direct current stimulation and transcutaneous spinal cord direct current stimulation as innovative tools for neuroscientists

Two neuromodulatory techniques based on applying direct current (DC) non-invasively through the skin, transcranial cerebellar direct current stimulation (tDCS) and transcutaneous spinal DCS, can induce prolonged functional changes consistent with a direct influence on the human cerebellum and spinal cord. In this article we review the major experimental works on cerebellar tDCS and on spinal tDCS, and their preliminary clinical applications. Cerebellar tDCS modulates cerebellar motor cortical inhibition, gait adaptation, motor behaviour, and cognition (learning, language, memory, attention). Spinal tDCS influences the ascending and descending spinal pathways, and spinal reflex excitability. In the anaesthetised mouse, DC stimulation applied under the skin along the entire spinal cord may affect GABAergic and glutamatergic systems. Preliminary clinical studies in patients with cerebellar disorders, and in animals and patients with spinal cord injuries, have reported beneficial effects. Overall the available data show that cerebellar tDCS and spinal tDCS are two novel approaches for inducing prolonged functional changes and neuroplasticity in the human cerebellum and spinal cord, and both are new tools for experimental and clinical neuroscientists.

Cerebellar contributions to spatial memory

There is mounting evidence for a role for the cerebellum in working memory (WM). The majority of relevant studies has examined verbal WM and has suggested specialisation of the right cerebellar hemisphere for language processing. Our study used theta burst stimulation (TBS) to examine whether there is a converse cerebellar hemispheric specialisation for spatial WM. We conducted two experiments to examine spatial WM performance before and after TBS to mid-hemispheric and lateral locations in the posterior cerebellum. Participants were required to recall the order of presentation of targets on a screen or the targets' order of presentation and their locations. We observed impaired recollection of target order after TBS to the mid left cerebellar hemisphere and reduced response speed after TBS to the left lateral cerebellum. We suggest that these results give evidence of the contributions of the left cerebellar cortex to the encoding and retrieval of spatial information.

Marked reduction of cerebellar deficits in upper limbs following transcranial cerebello-cerebral DC stimulation: tremor reduction and re-programming of the timing of antagonist commands

Cerebellar ataxias represent a very heterogeneous group of disabling disorders for which we lack effective symptomatic therapies in most cases. There is currently an intense interest in the use of non-invasive transcranial DC stimulation (tDCS) to modulate the activity of the cerebellum in ataxic disorders. We performed a detailed laboratory assessment of the effects of transcranial cerebello-cerebral DC stimulation (tCCDCS, including a sham procedure) on upper limb tremor and dysmetria in 2 patients presenting a dominant spinocerebellar ataxia (SCA) type 2, one of the most common SCAs encountered during practice. Both patients had a very similar triplet expansion size in the ATXN2 gene (respectively, 39 and 40 triplets). tCCDCS reduced both postural tremor and action tremor, as confirmed by spectral analysis. Quadratical PSD (power spectral density) of postural tremor dropped to 38.63 and 41.42% of baseline values in patient 1 and 2, respectively. The integral of the subband 4-20 Hz dropped to 46.9 and 62.3% of baseline values, respectively. Remarkably, tCCDCS canceled hypermetria and reduced dramatically the onset latency of the antagonist EMG activity associated with fast goal-directed movements toward 3 aimed targets (0.2, 0.3, and 0.4 rad). Following tCCDCS, the latency dropped from 108-98 to 63-57 ms in patient 1, and from 74-87 to 41-46 ms in patient 2 (mean control values ± SD: 36 ± 8 to 45 ± 11 ms), corresponding to a major drop of z scores for the 2 patients from 7.12 ± 0.69 to 1.28 ± 1.27 (sham procedure: 6.79 ± 0.71). This is the first demonstration that tCCDCS improves upper limb tremor and hypermetria in SCA type 2. In particular, this is the first report of a favorable effect on the onset latency of the antagonist EMG activity, a neurophysiological marker of the defect in programming of timing of motor commands. Our results indicate that tCCDCS should be considered in the symptomatic management of upper limb motor deficits in cerebellar ataxias. Future studies addressing a tDCS-based neuromodulation to improve motor control of upper limbs are required (a) in a large group of cerebellar disorders, and (b) in different subgroups of ataxic patients. The anatomical location of the cerebellum below the skull is particularly well suited for such studies.

Restoring cognitive functions using non-invasive brain stimulation techniques in patients with cerebellar disorders

Numerous studies have highlighted the possibility of modulating the excitability of cerebro-cerebellar circuits bi-directionally using transcranial electrical brain stimulation, in a manner akin to that observed using magnetic stimulation protocols. It has been proposed that cerebellar stimulation activates Purkinje cells in the cerebellar cortex, leading to inhibition of the dentate nucleus, which exerts a tonic facilitatory drive onto motor and cognitive regions of cortex through a synaptic relay in the ventral-lateral thalamus. Some cerebellar deficits present with cognitive impairments if damage to non-motor regions of the cerebellum disrupts the coupling with cerebral cortical areas for thinking and reasoning. Indeed, white matter changes in the dentato-rubral tract correlate with cognitive assessments in patients with Friedreich ataxia, suggesting that this pathway is one component of the anatomical substrate supporting a cerebellar contribution to cognition. An understanding of the physiology of the cerebro-cerebellar pathway previously helped us to constrain our interpretation of results from two recent studies in which we showed cognitive enhancements in healthy participants during tests of arithmetic after electrical stimulation of the cerebellum, but only when task demands were high. Others studies have also shown how excitation of the prefrontal cortex can enhance performance in a variety of working memory tasks. Thus, future efforts might be guided toward neuro-enhancement in certain patient populations, using what is commonly termed "non-invasive brain stimulation" as a cognitive rehabilitation tool to modulate cerebro-cerebellar circuits, or for stimulation over the cerebral cortex to compensate for decreased cerebellar drive to this region. This article will address these possibilities with a review of the relevant literature covering ataxias and cerebellar cognitive affective disorders, which are characterized by thalamo-cortical disturbances.

The cerebellum and cognitive function: 25 years of insight from anatomy and neuroimaging

Twenty-five years ago the first human functional neuroimaging studies of cognition discovered a surprising response in the cerebellum that could not be attributed to motor demands. This controversial observation challenged the well-entrenched view that the cerebellum solely contributes to the planning and execution of movement. Recurring neuroimaging findings combined with key insights from anatomy and case studies of neurological patients motivated a reconsideration of the traditional model of cerebellar organization and function. The majority of the human cerebellum maps to cerebral association networks in an orderly manner that includes a mirroring of the prominent cerebral asymmetries for language and attention. These findings inspire exploration of the cerebellum's contributions to a diverse array of functional domains and neuropsychiatric disorders.

Alterations in the sense of time, space, and body in the mindfulness-trained brain: a neurophenomenologically-guided MEG study

Meditation practice can lead to what have been referred to as “altered states of consciousness.”One of the phenomenological characteristics of these states is a joint alteration in the sense of time, space, and body. Here, we set out to study the unique experiences of alteration in the sense of time and space by collaborating with a select group of 12 long-term mindfulness meditation (MM) practitioners in a neurophenomenological setup, utilizing first-person data to guide the neural analyses. We hypothesized that the underlying neural activity accompanying alterations in the sense of time and space would be related to alterations in bodily processing. The participants were asked to volitionally bring about distinct states of “Timelessness” (outside time) and “Spacelessness” (outside space) while their brain activity was recorded by MEG. In order to rule out the involvement of attention, memory, or imagination, we used control states of “Then” (past) and “There” (another place). MEG sensors evidencing alterations in power values were identified, and the brain regions underlying these changes were estimated via spatial filtering (beamforming). Particularly, we searched for similar neural activity hypothesized to underlie both the state of “Timelessness” and “Spacelessness.” The results were mostly confined to the theta band, and showed that: (1) the “Then”/“There” overlap yielded activity in regions related to autobiographic memory and imagery (right posterior parietal lobule (PPL), right precentral/middle frontal gyrus (MFG), bilateral precuneus); (2) “Timelessness”/“Spacelessness” conditions overlapped in a different network, related to alterations in the sense of the body (posterior cingulate, right temporoparietal junction (TPJ), cerebellum); and (3) phenomenologically-guided neural analyses enabled us to dissociate different levels of alterations in the sense of the body. This study illustrates the utility of employing experienced contemplative practitioners within a neurophenomenological setup for scientifically characterizing a self-induced altered sense of time, space and body, as well as the importance of theta activity in relation with these altered states.