Infraslow (<0.1 Hz) oscillations in thalamic relay nuclei basic mechanisms and significance to health and disease states

Serotonin modulates infraslow oscillation in the dentate gyrus during Non-REM sleep

Synchronous neuronal activity is organized into neuronal oscillations with various frequency and time domains across different brain areas and brain states. For example, hippocampal theta, gamma and sharp wave oscillations are critical for memory formation and communication between hippocampal subareas and the cortex. In this study, we investigated the neuronal activity of the dentate gyrus (DG) with electrophysiological and optical imaging tools during sleep-wake cycles. We found that the activity of major glutamatergic cell populations in the DG is organized into in-fraslow oscillations (0.01 - 0.03 Hz) during NREM sleep. Although the DG is considered a sparsely active network during wakefulness, we found that 50% of granule cells and about 25% of mossy cells exhibit increased activity during NREM sleep. Further experiments revealed that the infraslow oscillation in the DG is modulated by rhythmic serotonin release during sleep, which oscillates at the same frequency but in an opposite phase. Genetic manipulation of 5-HT receptors revealed that this neuromodulatory regulation is mediated by 5-HT1a receptors and the knockdown of these receptors leads to memory impairment. Together, our results provide novel mechanistic insights into how the 5-HT system can influence hippocampal activity patterns during sleep.

Recent developments and challenges in positron emission tomography imaging of gliosis in chronic neuropathic pain

ABSTRACT

Understanding the mechanisms that underpin the transition from acute to chronic pain is critical for the development of more effective and targeted treatments. There is growing interest in the contribution of glial cells to this process, with cross-sectional preclinical studies demonstrating specific changes in these cell types capturing targeted timepoints from the acute phase and the chronic phase. In vivo longitudinal assessment of the development and evolution of these changes in experimental animals and humans has presented a significant challenge. Recent technological advances in preclinical and clinical positron emission tomography, including the development of specific radiotracers for gliosis, offer great promise for the field. These advances now permit tracking of glial changes over time and provide the ability to relate these changes to pain-relevant symptomology, comorbid psychiatric conditions, and treatment outcomes at both a group and an individual level. In this article, we summarize evidence for gliosis in the transition from acute to chronic pain and provide an overview of the specific radiotracers available to measure this process, highlighting their potential, particularly when combined with ex vivo/in vitro techniques, to understand the pathophysiology of chronic neuropathic pain. These complementary investigations can be used to bridge the existing gap in the field concerning the contribution of gliosis to neuropathic pain and identify potential targets for interventions.

Infra-slow fluctuations in cortical potentials and respiration drive fast cortical EEG rhythms in sleeping and waking states

OBJECTIVE

Infra-slow fluctuations (ISF, 0.008-0.1 Hz) characterize hemodynamic and electric potential signals of human brain. ISFs correlate with the amplitude dynamics of fast (>1 Hz) neuronal oscillations, and may arise from permeability fluctuations of the blood-brain barrier (BBB). It is unclear if physiological rhythms like respiration drive or track fast cortical oscillations, and the role of sleep in this coupling is unknown.

METHODS

We used high-density full-band electroencephalography (EEG) in healthy human volunteers (N = 21) to measure concurrently the ISFs, respiratory pulsations, and fast neuronal oscillations during periods of wakefulness and sleep, and to assess the strength and direction of their phase-amplitude coupling.

RESULTS

The phases of ISFs and respiration were both coupled with the amplitude of fast neuronal oscillations, with stronger ISF coupling being evident during sleep. Phases of ISF and respiration drove the amplitude dynamics of fast oscillations in sleeping and waking states, with different contributions.

CONCLUSIONS

ISFs in slow cortical potentials and respiration together significantly determine the dynamics of fast cortical oscillations.

SIGNIFICANCE

We propose that these slow physiological phases play a significant role in coordinating cortical excitability, which is a fundamental aspect of brain function.

A dietary polyphenol metabolite alters CA1 excitability ex vivo and mildly affects cortico-hippocampal field potential generators in anesthetized animals

Dietary polyphenols have beneficial effects in situations of impaired cognition in acute models of neurodegeneration. The possibility that they may have a direct effect on the electrical activity of neuronal populations has not been tested. We explored the electrophysiological action of protocatechuic acid (PCA) on CA1 pyramidal cells ex vivo and network activity in anesthetized female rats using pathway-specific field potential (FP) generators obtained from laminar FPs in cortex and hippocampus. Whole-cell recordings from CA1 pyramidal cells revealed increased synaptic potentials, particularly in response to basal dendritic excitation, while the associated evoked firing was significantly reduced. This counterintuitive result was attributed to a marked increase of the rheobase and voltage threshold, indicating a decreased ability to generate spikes in response to depolarizing current. Systemic administration of PCA only slightly altered the ongoing activity of some FP generators, although it produced a striking disengagement of infraslow activities between the cortex and hippocampus on a scale of minutes. To our knowledge, this is the first report showing the direct action of a dietary polyphenol on electrical activity, performing neuromodulatory roles at both the cellular and network levels.

Infraslow closed-loop brain training for anxiety and depression (ISAD): a protocol for a randomized, double-blind, sham-controlled pilot trial in adult females with internalizing disorders

BACKGROUND

The core intrinsic connectivity networks (core-ICNs), encompassing the default-mode network (DMN), salience network (SN) and central executive network (CEN), have been shown to be dysfunctional in individuals with internalizing disorders (IDs, e.g. major depressive disorder, MDD; generalized anxiety disorder, GAD; social anxiety disorder, SOC). As such, source-localized, closed-loop brain training of electrophysiological signals, also known as standardized low-resolution electromagnetic tomography (sLORETA) neurofeedback (NFB), targeting key cortical nodes within these networks has the potential to reduce symptoms associated with IDs and restore normal core ICN function. We intend to conduct a randomized, double-blind (participant and assessor), sham-controlled, parallel-group (3-arm) trial of sLORETA infraslow (<0.1 Hz) fluctuation neurofeedback (sLORETA ISF-NFB) 3 times per week over 4 weeks in participants (n=60) with IDs. Our primary objectives will be to examine patient-reported outcomes (PROs) and neurophysiological measures to (1) compare the potential effects of sham ISF-NFB to either genuine 1-region ISF-NFB or genuine 2-region ISF-NFB, and (2) assess for potential associations between changes in PRO scores and modifications of electroencephalographic (EEG) activity/connectivity within/between the trained regions of interest (ROIs). As part of an exploratory analysis, we will investigate the effects of additional training sessions and the potential for the potentiation of the effects over time.

METHODS

We will randomly assign participants who meet the criteria for MDD, GAD, and/or SOC per the MINI (Mini International Neuropsychiatric Interview for DSM-5) to one of three groups: (1) 12 sessions of posterior cingulate cortex (PCC) ISF-NFB up-training (n=15), (2) 12 sessions of concurrent PCC ISF up-training and dorsal anterior cingulate cortex (dACC) ISF-NFB down-training (n=15), or (3) 6 sessions of yoked-sham training followed by 6 sessions genuine ISF-NFB (n=30). Transdiagnostic PROs (Hospital Anxiety and Depression Scale, HADS; Inventory of Depression and Anxiety Symptoms - Second Version, IDAS-II; Multidimensional Emotional Disorder Inventory, MEDI; Intolerance of Uncertainty Scale - Short Form, IUS-12; Repetitive Thinking Questionnaire, RTQ-10) as well as resting-state neurophysiological measures (full-band EEG and ECG) will be collected from all subjects during two baseline sessions (approximately 1 week apart) then at post 6 sessions, post 12 sessions, and follow-up (1 month later). We will employ Bayesian methods in R and advanced source-localisation software (i.e. exact low-resolution brain electromagnetic tomography; eLORETA) in our analysis.

DISCUSSION

This protocol will outline the rationale and research methodology for a clinical pilot trial of sLORETA ISF-NFB targeting key nodes within the core-ICNs in a female ID population with the primary aims being to assess its potential efficacy via transdiagnostic PROs and relevant neurophysiological measures.

TRIAL REGISTRATION

Our study was prospectively registered with the Australia New Zealand Clinical Trials Registry (ANZCTR; Trial ID: ACTRN12619001428156). Registered on October 15, 2019.

Electrical signaling in cochlear efferents is driven by an intrinsic neuronal oscillator

Efferent neurons are believed to play essential roles in maintaining auditory function. The lateral olivocochlear (LOC) neurons-which project from the brainstem to the inner ear, where they release multiple transmitters including peptides, catecholamines, and acetylcholine-are the most numerous yet least understood elements of efferent control of the cochlea. Using in vitro calcium imaging and patch-clamp recordings, we found that LOC neurons in juvenile and young adult mice exhibited extremely slow waves of activity (∼0.1 Hz). These seconds-long bursts of Na⁺ spikes were driven by an intrinsic oscillator dependent on L-type Ca²⁺ channels and were not observed in prehearing mice, suggesting an age-dependent mechanism underlying the intrinsic oscillator. Using optogenetic approaches, we identified both ascending (T-stellate cells of the cochlear nucleus) and descending (auditory cortex) sources of synaptic excitation, as well as the synaptic receptors used for such excitation. Additionally, we identified potent inhibition originating in the glycinergic medial nucleus of trapezoid body (MNTB). Conductance-clamp experiments revealed an unusual mechanism of electrical signaling in LOC neurons, in which synaptic excitation and inhibition served to switch on and off the intrinsically generated spike burst mechanism, allowing for prolonged periods of activity or silence controlled by brief synaptic events. Protracted bursts of action potentials may be essential for effective exocytosis of the diverse transmitters released by LOC fibers in the cochlea.

Animal Models in Epileptic Spasms and the Development of Novel Treatment Options

SUMMARY

The infantile spasms (IS) syndrome is a catastrophic developmental epileptic encephalopathy syndrome characterized by an age-specific expression of epileptic spasms that are associated with extremely abnormal, oftentimes described as chaotic, interictal EEG pattern known as hypsarrhythmia. Patients with IS generally have poor neurodevelopmental outcomes, in large part because of the frequent epileptic spasms and interictal EEG abnormalities. Current first-line treatments such as adrenocorticotropic hormone or vigabatrin are often ineffective and are associated with major toxic side effects. There is therefore a need for better and safer treatments for patients with IS, especially for the intractable population. Hope is on the horizon as, over the past 10 years, there has been robust progress in the development of etiology-specific animal models of IS. These models have been used to identify potential new treatments for IS and are beginning to provide some important insights into the pathophysiological substrates for this disease. In this review, we will highlight strengths and weaknesses of the currently available animal models of IS in addition to new insights into the pathophysiology and treatment options derived from these models.

The enigma of infra-slow fluctuations in the human EEG

Spontaneous Infra-Slow Fluctuations (ISFs) of the human EEG (EEG-ISFs) were discovered 60 years ago when appropriate amplifiers for their recordings were designed. To avoid skin-related artifacts the recording of EEG-ISFs required puncturing the skin under the electrode. In the beginning of the 21st century the interest in EEG-ISFs was renewed with the appearance of commercially available DC-coupled amplified and by observation of ISFs of the blood oxygen level–dependent (BOLD) functional magnetic resonance imaging signal at a similar frequency. The independent components of irregular EEG-ISFs were shown to correlate with BOLD signals which in turn were driven by changes in arousal level measured by galvanic skin response (GSR), pupil size and HRV. There is no consensus regarding the temporal organization of EEG-ISFs: some studies emphasize the absence of peaks on EEG-ISFs spectra, some studies report prominent oscillations with frequency around 0.1 or 0.02 Hz, while some emphasize multiple discrete infraslow oscillations. No studies used parameters of EEG-ISFs as neuromarkers to discriminate psychiatric patients from healthy controls. Finally, a set of working hypotheses is suggested that must be tested in future research to solve the enigma of EEG-ISFs.

Source localized infraslow neurofeedback training in people with chronic painful knee osteoarthritis: A randomized, double-blind, sham-controlled feasibility clinical trial

Persistent pain is a key symptom in people living with knee osteoarthritis (KOA). Infra-slow Neurofeedback (ISF-NF) training is a recent development focusing on modulating cortical slow-wave activity to improve pain outcomes. A parallel, two-armed double-blinded, randomized sham-controlled, feasibility clinical trial aimed to determine the feasibility and safety of a novel electroencephalography-based infraslow fluctuation neurofeedback (EEG ISF-NF) training in people with KOA and determine the variability of clinical outcomes and EEG changes following NF training. Eligible participants attended nine 30-min ISF-NF training sessions involving three cortical regions linked to pain. Feasibility measures were monitored during the trial period. Pain and functional outcomes were measured at baseline, post-intervention, and follow-up after 2 weeks. Resting-state EEG was recorded at baseline and immediate post-intervention. Participants were middle-aged (61.7 ± 7.6 years), New Zealand European (90.5%), and mostly females (62%) with an average knee pain duration of 4 ± 3.4 years. The study achieved a retention rate of 91%, with 20/22 participants completing all the sessions. Participants rated high levels of acceptance and “moderate to high levels of perceived effectiveness of the training.” No serious adverse events were reported during the trial. Mean difference (95% CI) for clinical pain and function measures are as follows for pain severity [active: 0.89 ± 1.7 (−0.27 to 2.0); sham: 0.98 ± 1.1 (0.22–1.7)], pain interference [active: 0.75 ± 2.3 (−0.82 to 2.3); Sham: 0.89 ± 2.1 (−0.60 to 2.4)], pain unpleasantness [active: 2.6 ± 3.7 (0.17–5.1); sham: 2.8 ± 3 (0.62–5.0)] and physical function [active: 6.2 ± 13 (−2.6 to 15); sham: 1.6 ± 12 (−6.8 to 10)]. EEG sources demonstrated frequency-specific neuronal activity, functional connectivity, and ISF ratio changes following NF training. The findings of the study indicated that the ISF-NF training is a feasible, safe, and acceptable intervention for pain management in people with KOA, with high levels of perceived effectiveness. The study also reports the variability in clinical, brain activity, and connectivity changes following training.

Unconscious mind activates central cardiovascular network and promotes adaptation to microgravity possibly anti-aging during 1-year-long spaceflight

The intrinsic cardiovascular regulatory system (β, 0.00013–0.02 Hz) did not adapt to microgravity after a 6-month spaceflight. The infraslow oscillation (ISO, 0.01–0.10 Hz) coordinating brain dynamics via thalamic astrocytes plays a key role in the adaptation to novel environments. We investigate the adaptive process of a healthy astronaut during a 12-month-long spaceflight by analyzing heart rate variability (HRV) in the LF (0.01–0.05 Hz) and MF1 (0.05–0.10 Hz) bands for two consecutive days on four occasions: before launch, at 1-month (ISS01) and 11-month (ISS02) in space, and after return to Earth. Alteration of β during ISS01 improved during ISS02 (P = 0.0167). During ISS01, LF and MF1 bands, reflecting default mode network (DMN) activity, started to increase at night (by 43.1% and 32.0%, respectively), when suprachiasmatic astrocytes are most active, followed by a 25.9% increase in MF1-band throughout the entire day during ISS02, larger at night (47.4%) than during daytime. Magnetic declination correlated positively with β during ISS01 (r = 0.6706, P < 0.0001) and ISS02 (r = 0.3958, P = 0.0095). Magnetic fluctuations may affect suprachiasmatic astrocytes, and the DMN involving ISOs and thalamic astrocytes may then be activated, first at night, then during the entire day, a mechanism that could perhaps promote an anti-aging effect noted in other investigations.

Time-Frequency Representations of Brain Oscillations: Which One Is Better?

Brain oscillations are thought to subserve important functions by organizing the dynamical landscape of neural circuits. The expression of such oscillations in neural signals is usually evaluated using time-frequency representations (TFR), which resolve oscillatory processes in both time and frequency. While a vast number of methods exist to compute TFRs, there is often no objective criterion to decide which one is better. In feature-rich data, such as that recorded from the brain, sources of noise and unrelated processes abound and contaminate results. The impact of these distractor sources is especially problematic, such that TFRs that are more robust to contaminants are expected to provide more useful representations. In addition, the minutiae of the techniques themselves impart better or worse time and frequency resolutions, which also influence the usefulness of the TFRs. Here, we introduce a methodology to evaluate the "quality" of TFRs of neural signals by quantifying how much information they retain about the experimental condition during visual stimulation and recognition tasks, in mice and humans, respectively. We used machine learning to discriminate between various experimental conditions based on TFRs computed with different methods. We found that various methods provide more or less informative TFRs depending on the characteristics of the data. In general, however, more advanced techniques, such as the superlet transform, seem to provide better results for complex time-frequency landscapes, such as those extracted from electroencephalography signals. Finally, we introduce a method based on feature perturbation that is able to quantify how much time-frequency components contribute to the correct discrimination among experimental conditions. The methodology introduced in the present study may be extended to other analyses of neural data, enabling the discovery of data features that are modulated by the experimental manipulation.

The Infraslow Frequency Oscillatory Transcranial Direct Current Stimulation Over the Left Dorsolateral Prefrontal Cortex Enhances Sustained Attention

Background

The vigilance fluctuation and decrement of sustained attention have large detrimental consequences to most tasks in daily life, especially among the elderly. Non-invasive brain stimulations (e.g., transcranial direct current stimulation, tDCS) have been widely applied to improve sustained attention, however, with mixed results.

Objective

An infraslow frequency oscillatory tDCS approach was designed to improve sustained attention.

Methods

The infraslow frequency oscillatory tDCS (O-tDCS) over the left dorsolateral prefrontal cortex at 0.05 Hz was designed and compared with conventional tDCS (C-tDCS) to test whether this new protocol improves sustained attention more effectively. The sustained attention was evaluated by reaction time and accuracy.

Results

Compared with the C-tDCS and sham, the O-tDCS significantly enhanced sustained attention by increasing response accuracy, reducing response time, and its variability. These effects were predicted by the evoked oscillation of response time at the stimulation frequency.

Conclusion

Similar to previous studies, the modulation effect of C-tDCS on sustained attention is weak and unstable. In contrast, the O-tDCS effectively and systematically enhances sustained attention by optimizing vigilance fluctuation. The modulation effect of O-tDCS is probably driven by neural oscillations at the infraslow frequency range.

Increased fMRI connectivity upon chemogenetic inhibition of the mouse prefrontal cortex

While shaped and constrained by axonal connections, fMRI-based functional connectivity reorganizes in response to varying interareal input or pathological perturbations. However, the causal contribution of regional brain activity to whole-brain fMRI network organization remains unclear. Here we combine neural manipulations, resting-state fMRI and in vivo electrophysiology to probe how inactivation of a cortical node causally affects brain-wide fMRI coupling in the mouse. We find that chronic inhibition of the medial prefrontal cortex (PFC) via overexpression of a potassium channel increases fMRI connectivity between the inhibited area and its direct thalamo-cortical targets. Acute chemogenetic inhibition of the PFC produces analogous patterns of fMRI overconnectivity. Using in vivo electrophysiology, we find that chemogenetic inhibition of the PFC enhances low frequency (0.1–4 Hz) oscillatory power via suppression of neural firing not phase-locked to slow rhythms, resulting in increased slow and δ band coherence between areas that exhibit fMRI overconnectivity. These results provide causal evidence that cortical inactivation can counterintuitively increase fMRI connectivity via enhanced, less-localized slow oscillatory processes.

Pathological perturbation affects whole brain network activity. Here the authors show in mice that cortical inactivation unexpectedly results in increased fMRI connectivity between the manipulated regions and its direct axonal targets.

Functional ultrasound imaging: A useful tool for functional connectomics?

Functional ultrasound (fUS) is a hemodynamic-based functional neuroimaging technique, primarily used in animal models, that combines a high spatiotemporal resolution, a large field of view, and compatibility with behavior. These assets make fUS especially suited to interrogating brain activity at the systems level. In this review, we describe the technical capabilities offered by fUS and discuss how this technique can contribute to the field of functional connectomics. First, fUS can be used to study intrinsic functional connectivity, namely patterns of correlated activity between brain regions. In this area, fUS has made the most impact by following connectivity changes in disease models, across behavioral states, or dynamically. Second, fUS can also be used to map brain-wide pathways associated with an external event. For example, fUS has helped obtain finer descriptions of several sensory systems, and uncover new pathways implicated in specific behaviors. Additionally, combining fUS with direct circuit manipulations such as optogenetics is an attractive way to map the brain-wide connections of defined neuronal populations. Finally, technological improvements and the application of new analytical tools promise to boost fUS capabilities. As brain coverage and the range of behavioral contexts that can be addressed with fUS keep on increasing, we believe that fUS-guided connectomics will only expand in the future. In this regard, we consider the incorporation of fUS into multimodal studies combining diverse techniques and behavioral tasks to be the most promising research avenue.

Noradrenergic circuit control of non-REM sleep substates

To understand what makes sleep vulnerable in disease, it is useful to look at how wake-promoting mechanisms affect healthy sleep. Wake-promoting neuronal activity is inhibited during non-rapid-eye-movement sleep (NREMS). However, sensory vigilance persists in NREMS in animals and humans, suggesting that wake promotion could remain functional. Here, we demonstrate that consolidated mouse NREMS is a brain state with recurrent fluctuations of the wake-promoting neurotransmitter noradrenaline on the ∼50-s timescale in the thalamus. These fluctuations occurred around mean noradrenaline levels greater than the ones of quiet wakefulness, while noradrenaline (NA) levels declined steeply in REMS. They coincided with a clustering of sleep spindle rhythms in the forebrain and with heart-rate variations, both of which are correlates of sensory arousability. We addressed the origins of these fluctuations by using closed-loop optogenetic locus coeruleus (LC) activation or inhibition timed to moments of low and high spindle activity during NREMS. We could suppress, lock, or entrain sleep-spindle clustering and heart-rate variations, suggesting that both fore- and hindbrain-projecting LC neurons show coordinated infraslow activity variations in natural NREMS. Noradrenergic modulation of thalamic, but not cortical, circuits was required for sleep-spindle clustering and involved NA release into primary sensory and reticular thalamic nuclei that activated both α1- and β-adrenergic receptors to cause slowly decaying membrane depolarizations. Noradrenergic signaling by LC constitutes a vigilance-promoting mechanism that renders mammalian NREMS vulnerable to disruption on the close-to-minute timescale through sustaining thalamocortical and autonomic sensory arousability. VIDEO ABSTRACT.

Active Dendrites and Local Field Potentials: Biophysical Mechanisms and Computational Explorations

Neurons and glial cells are endowed with membranes that express a rich repertoire of ion channels, transporters, and receptors. The constant flux of ions across the neuronal and glial membranes results in voltage fluctuations that can be recorded from the extracellular matrix. The high frequency components of this voltage signal contain information about the spiking activity, reflecting the output from the neurons surrounding the recording location. The low frequency components of the signal, referred to as the local field potential (LFP), have been traditionally thought to provide information about the synaptic inputs that impinge on the large dendritic trees of various neurons. In this review, we discuss recent computational and experimental studies pointing to a critical role of several active dendritic mechanisms that can influence the genesis and the location-dependent spectro-temporal dynamics of LFPs, spanning different brain regions. We strongly emphasize the need to account for the several fast and slow dendritic events and associated active mechanisms - including gradients in their expression profiles, inter- and intra-cellular spatio-temporal interactions spanning neurons and glia, heterogeneities and degeneracy across scales, neuromodulatory influences, and activitydependent plasticity - towards gaining important insights about the origins of LFP under different behavioral states in health and disease. We provide simple but essential guidelines on how to model LFPs taking into account these dendritic mechanisms, with detailed methodology on how to account for various heterogeneities and electrophysiological properties of neurons and synapses while studying LFPs.

Brainstem functional oscillations across the migraine cycle: A longitudinal investigation

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Individual migraineurs’ brainstem function alters through the migraine cycle.

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Functional changes occurred in the 24-hour period immediately prior to a migraine.

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Greater resting activity variability was found in the SpV, pons and PAG.

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Increased infra-slow oscillations and regional homogeneity in the SpV and pons.

Although the mechanisms responsible for migraine initiation remain unknown, recent evidence shows that brain function is different immediately preceding a migraine. This is consistent with the idea that altered brain function, particularly in brainstem sites, may either trigger a migraine or facilitate a peripheral trigger that activates the brain, resulting in pain. The aim of this longitudinal study is therefore to expand on the above findings, and to determine if brainstem function oscillates over a migraine cycle in individual subjects. We performed resting state functional magnetic resonance imaging in three migraineurs and five controls each weekday for four weeks. We found that although resting activity variability was similar in controls and interictal migraineurs, brainstem variability increased dramatically during the 24-hour period preceding a migraine. This increase occurred in brainstem areas in which orofacial afferents terminate: the spinal trigeminal nucleus and dorsal pons. These increases were characterized by increased power at infra-slow frequencies, principally between 0.03 and 0.06 Hz. Furthermore, these power increases were associated with increased regional homogeneity, a measure of local signal coherence. The results show within-individual alterations in brain activity immediately preceding migraine onset and support the hypothesis that altered regional brainstem function before a migraine attack is involved in underlying migraine neurobiology.

Infra-slow modulation of fast beta/gamma oscillations in the mouse visual system

KEY POINTS

Neurophysiological activity in the subcortical visual system fluctuates in both infra-slow and fast oscillatory ranges, but the level of co-occurrence and potential functional interaction of these rhythms is unknown. Analysing dark-adapted spontaneous activity in the mouse subcortical visual system, we find that these two types of oscillation interact uniquely through a population of neurons expressing both rhythms. Genetic ablation of rod/cone signalling potentiates infra-slow and abolishes fast beta/gamma oscillations while genetic ablation of melanopsin substantially diminishes the interaction between these two rhythms. Our results indicate that in an intact visual system the phase of infra-slow modulates fast beta/gamma oscillations. Thus one possible impact of infra-slow oscillations in vision is to guide visual processing by interacting with fast narrowband oscillations.

ABSTRACT

Infra-slow (<0.02 Hz) and fast beta/gamma (20-100 Hz) oscillations in neurophysiological activity have been widely found in the subcortical visual system. While it is well established that fast beta/gamma oscillations are involved in visual processing, the role (if any) of infra-slow oscillations is currently unknown. One possibility is that infra-slow oscillations exert influence by modulating the amplitude of fast oscillations, yet the extent to which these different oscillations arise independently and interact remains unknown. We addressed these questions by recording in vivo spontaneous activity from the subcortical visual system of visually intact mice, and animals whose retinal network was disrupted by advanced rod/cone degeneration (rd/rd cl) or melanopsin loss (Opn4^-/- ). We found many neurons expressing only one type of oscillation, and indeed fast oscillations were absent in rd/rd cl. Conversely, neurons co-expressing the two oscillations were also common, and were encountered more often than expected by chance in visually intact but not Opn4^-/- mice. Finally, where they co-occurred we found that beta/gamma amplitude was modulated by the infra-slow rhythm. Our data thus reveal that: (1) infra-slow and beta-gamma oscillations are separable phenomena; and (2) that they actively co-occur in a subset of neurones in which the phase of infra-slow oscillations defines beta-gamma oscillations amplitude. These findings suggest that infra-slow oscillations could influence vision by modulating beta-gamma oscillations, and raise the possibility that disruptions in these oscillatory behaviours contribute to vision dysfunction in retinal dystrophy.

Scale-free amplitude modulation of low-frequency fluctuations in episodic migraine

Arrhythmic fluctuations in neural activity occur at many levels of the nervous system. Such activity does not have a characteristic temporal periodicity but can exhibit statistical similarities, most commonly power-law scaling behavior, which is indicative of scale-free dynamics. The recurrence of scaling laws across many different systems and its manifestation in behavior has prompted a search for unifying principles in human brain function. With this in mind, a focused search for abnormities in scale-free dynamics is of considerable clinical relevance to migraine and other clinical pain disorders. Here, we examined the scale-free properties of the resting-state functional magnetic resonance imaging (fMRI) signal in the broadband frequency range known to be related to spontaneous neural activity (0.01-0.1 Hz). In a large cohort of episodic migraine patients (N = 40), we observed that the strength of long-range temporal correlations in the fMRI signal (captured by the scaling exponent α) was significantly higher in the sensorimotor network compared with healthy controls. Increases in the scaling exponent were positively correlated with fMRI signal variance and negatively correlated with the patient's self-reported headache intensity. These changes in the fMRI signal suggest that the temporal structure of amplitude fluctuations carries valuable information about the dynamic state of the underlying neuronal networks and ensuing sensory impairments in migraine. The demonstrated scaling laws pose a novel quantitative approach for examining clinically relevant interindividual variability in migraine and other pain disorders.

Effects of the glial modulator palmitoylethanolamide on chronic pain intensity and brain function

Background: Chronic neuropathic pain (NP) is a complex disease that results from damage or presumed damage to the somatosensory nervous system. Current treatment regimens are often ineffective. The major impediment in developing effective treatments is our limited understanding of the underlying mechanisms. Preclinical evidence suggests that glial changes are crucial for the development of NP and a recent study reported oscillatory activity differences within the ascending pain pathway at frequencies similar to that of cyclic gliotransmission in NP. Furthermore, there is evidence that glial modifying medications may be effective in treating NP. The aim of this Phase I open-label clinical trial is to determine whether glial modifying medication palmitoylethanolamide (PEA) will reduce NP and whether this is associated with reductions in oscillatory activity within the pain pathway.

Methods: We investigated whether 6 weeks of PEA treatment would reduce pain and infra-slow oscillatory activity within the ascending trigeminal pathway in 22 individuals (17 females) with chronic orofacial NP.

Results: PEA reduced pain in 16 (73%) of the 22 subjects, 11 subjects showed pain reduction of over 20%. Whilst both the responders and non-responders showed reductions in infra-slow oscillatory activity where orofacial nociceptor afferents terminate in the brainstem, only responders displayed reductions in the thalamus. Furthermore, functional connections between the brainstem and thalamus were altered only in responders.

Conclusion: PEA is effective at relieving NP. This reduction is coupled to a reduction in resting oscillations along the ascending pain pathway that are likely driven by rhythmic astrocytic gliotransmission.

Separability of calcium slow waves and functional connectivity during wake, sleep, and anesthesia

Modulation of brain state, e.g., by anesthesia, alters the correlation structure of spontaneous activity, especially in the delta band. This effect has largely been attributed to the ∼ 1    Hz slow oscillation that is characteristic of anesthesia and nonrapid eye movement (NREM) sleep. However, the effect of the slow oscillation on correlation structures and the spectral content of spontaneous activity across brain states (including NREM) has not been comprehensively examined. Further, discrepancies between activity dynamics observed with hemoglobin versus calcium (GCaMP6) imaging have not been reconciled. Lastly, whether the slow oscillation replaces functional connectivity (FC) patterns typical of the alert state, or superimposes on them, remains unclear. Here, we use wide-field calcium imaging to study spontaneous cortical activity in awake, anesthetized, and naturally sleeping mice. We find modest brain state-dependent changes in infraslow correlations but larger changes in GCaMP6 delta correlations. Principal component analysis of GCaMP6 sleep/anesthesia data in the delta band revealed that the slow oscillation is largely confined to the first three components. Removal of these components revealed a correlation structure strikingly similar to that observed during wake. These results indicate that, during NREM sleep/anesthesia, the slow oscillation superimposes onto a canonical FC architecture.

Infraslow activity as a potential modulator of corticomotor excitability

Fluctuations in cortical excitability are a candidate mechanism involved in the trial-to-trial variation of motor evoked potentials (MEPs) to transcranial magnetic stimulation (TMS). We explore whether infraslow EEG activity (<0.1 Hz) modulates corticomotor excitability by evaluating the presence of temporal and phase clustering of TMS-induced MEPs. In addition, we evaluate the dependence of MEP amplitude on the phase of the infraslow activity. Twenty-three subjects were stimulated at an intensity above the resting motor threshold (rMT) and ten at the rMT. We evaluated whether temporal and phase clustering of MEP size and MEP generation were present, using 1,000 surrogates with a similar amplitude or occurrence distribution. To evaluate the MEP amplitude dependence, we used the least-square method to approximate the linear circular data by fitting a sine function. We observed significant temporal clustering at a group level, in all individual subjects stimulated at rMT and in the majority of those stimulated above rMT, suggesting underlying determinism of corticomotor excitability instead of randomly generated fluctuations. The majority of subjects showed significant phase clustering for MEP size and for MEP occurrence, and significant phase clustering was found at the group level. Furthermore, in approximately one-quarter to one-half of the subjects we found a significant correlation and dependence of MEP amplitude on the phase of infraslow activity, respectively. Although other mechanisms very likely contribute as well, our findings seem to suggest that infraslow activity is involved in the variability of cortical excitability and TMS-induced responses. NEW & NOTEWORTHY Cortical excitability measures are highly variable during transcranial magnetic stimulation. Although ongoing brain oscillations are assumed to modulate excitability, no consistent associations are found for the traditional frequency bands. We focus on the role of infraslow EEG activity, defined as rhythms with frequencies < 0.1 Hz. We provide experimental evidence suggesting that infraslow activity most likely modulates corticomotor excitability and that response variation could be reduced when stimulation is targeted at a specific infraslow phase.

A Magnetoencephalography Study of Pediatric Interictal Neuromagnetic Activity Changes and Brain Network Alterations Caused by Epilepsy in the High Frequency (80-1000 Hz)

More and more studies propose that high frequency brain signals are promising biomarkers of epileptogenic zone. In this paper, our aim is to investigate the neuromagnetic changes and brain network topological alterations during an interictal period at high frequency ranges (80-1000 Hz) between healthy controls and epileptic patients with Magnetoencephalography. We analyzed neuromagnetic activities with accumulated source imaging, and constructed brain network based on graph theory. Neuromagnetic activity changes and brain network alterations between two groups were analyzed in three frequency bands: ripple (80-250 Hz), fast ripples (FRs, 250-500 Hz), and very high frequency oscillations (VHFO, 500-1000 Hz). We found that epileptic patients showed significantly altered patterns of neuromagnetic source localization and altered brain network patterns. And, we also found that mean functional connectivity and the number of modules from epileptic patients significantly increased in the ripple and FRs bands, and mean clustering coefficient from epileptic patients significantly decreased in the ripple and FRs bands. We also found that the mean functional connectivity was positively correlated with duration of epilepsy in the ripple and VHFO bands, and the number of modules was positively correlated with the duration of epilepsy in the ripple, FRs, and VHFO bands. Our results indicate that epilepsy can alter patients' neuromagnetic activities and brain networks in the high-frequency ranges, and these alterations become more pathological as the duration of epilepsy grows longer.

Glutamate triggers intracellular Ca2+ oscillations and nitric oxide release by inducing NAADP- and InsP3 -dependent Ca2+ release in mouse brain endothelial cells

The neurotransmitter glutamate increases cerebral blood flow by activating postsynaptic neurons and presynaptic glial cells within the neurovascular unit. Glutamate does so by causing an increase in intracellular Ca²⁺ concentration ([Ca²⁺ ]_i ) in the target cells, which activates the Ca²⁺ /Calmodulin-dependent nitric oxide (NO) synthase to release NO. It is unclear whether brain endothelial cells also sense glutamate through an elevation in [Ca²⁺ ]_i and NO production. The current study assessed whether and how glutamate drives Ca²⁺ -dependent NO release in bEND5 cells, an established model of brain endothelial cells. We found that glutamate induced a dose-dependent oscillatory increase in [Ca²⁺ ]_i , which was maximally activated at 200 μM and inhibited by α-methyl-4-carboxyphenylglycine, a selective blocker of Group 1 metabotropic glutamate receptors. Glutamate-induced intracellular Ca²⁺ oscillations were triggered by rhythmic endogenous Ca²⁺ mobilization and maintained over time by extracellular Ca²⁺ entry. Pharmacological manipulation revealed that glutamate-induced endogenous Ca²⁺ release was mediated by InsP₃ -sensitive receptors and nicotinic acid adenine dinucleotide phosphate (NAADP) gated two-pore channel 1. Constitutive store-operated Ca²⁺ entry mediated Ca²⁺ entry during ongoing Ca²⁺ oscillations. Finally, glutamate evoked a robust, although delayed increase in NO levels, which was blocked by pharmacologically inhibition of the accompanying intracellular Ca²⁺ signals. Of note, glutamate induced Ca²⁺ -dependent NO release also in hCMEC/D3 cells, an established model of human brain microvascular endothelial cells. This investigation demonstrates for the first time that metabotropic glutamate-induced intracellular Ca²⁺ oscillations and NO release have the potential to impact on neurovascular coupling in the brain.

Cognitive and Physiologic Impacts of the Infraslow Oscillation

Brain states are traditionally recognized via sleep-wake cycles, but modern neuroscience is beginning to identify many sub-states within these larger arousal types. Multiple lines of converging evidence now point to the infraslow oscillation (ISO) as a mediator of brain sub-states, with impacts ranging from the creation of resting state networks (RSNs) in awake subjects to interruptions in neural activity during sleep. This review will explore first the basic characteristics of the ISO in human subjects before reviewing findings in sleep and in animals. Networks of consistently correlated brain regions known as RSNs seen in human functional neuroimaging studies oscillate together at infraslow frequencies. The infraslow rhythm subdivides nonREM in a manner that may correlate with plasticity. The mechanism of this oscillation may be found in the thalamus and may ultimately come from glial cells. Finally, I review the functional impacts of ISOs on brain phenomena ranging from higher frequency oscillations, to brain networks, to information representation and cognitive performance. ISOs represent a relatively understudied phenomenon with wide effects on the brain and behavior.

Cortico-thalamic hypo- and hyperconnectivity extend consistently to basal ganglia in schizophrenia

Schizophrenia is characterized by hypoconnectivity or decreased intrinsic functional connectivity (iFC) between prefrontal-limbic cortices and thalamic nuclei, as well as hyperconnectivity or increased iFC between primary-sensorimotor cortices and thalamic nuclei. However, cortico-thalamic iFC overlaps with larger, structurally defined cortico-striato-pallido-thalamo-cortical (CSPTC) circuits. If such an overlap is relevant for intrinsic hypo-/hyperconnectivity, it suggests (i) that patterns of cortico-subcortical hypo-/hyperconnectivity extend consistently from thalamus to basal ganglia nuclei; and (ii) such consistent hypo-/hyperconnectivity might link distinctively but consonant with different symptom dimensions, namely cognitive and psychotic impairments. To test this hypothesis, 57 patients with schizophrenia and 61 healthy controls were assessed by resting-state functional magnetic resonance imaging (fMRI) and clinical-behavioral testing. IFC from intrinsic cortical networks into thalamus, striatum, and pallidum was estimated by partial correlations between fMRI time courses. In patients, the salience network covering prefrontal-limbic cortices was hypoconnected with the mediodorsal thalamus and ventral parts of striatum and pallidum; these iFC-hypoconnectivity patterns were correlated both among each other and specifically with patients' impaired cognition. In contrast, the auditory-sensorimotor network covering primary-sensorimotor cortices was hyperconnected with the anterior ventral nucleus of the thalamus and dorsal parts of striatum and pallidum; these iFC-hyperconnectivity patterns were likewise correlated among each other and specifically with patients' psychotic symptoms. The results demonstrate that prefrontal-limbic hypoconnectivity and primary-sensorimotor hyperconnectivity extend consistently across subcortical nuclei and specifically across distinct symptom dimensions. Data support the model of consistent cortico-subcortical hypo-/hyperconnectivity within CSPTC circuits in schizophrenia.

Synchronous tonic-to-bursting transitions in a neuronal hub motif

We study a heterogeneous neuronal network motif where a central node (hub neuron) is connected via electrical synapses to other nodes (peripheral neurons). Our numerical simulations show that the networked neurons synchronize in three different states: (i) robust tonic, (ii) robust bursting, and (iii) tonic initially evolving to bursting through a period-doubling cascade and chaos transition. This third case displays interesting features, including the carrying on of a characteristic firing rate found in the single neuron tonic-to-bursting transition.

Seizure susceptibility and infraslow modulatory activity in the intracranial electroencephalogram

OBJECTIVE

Studies of infraslow amplitude modulations (<0.15 Hz) of band power time series suggest that these envelope correlations may form a basis for distant spatial coupling in the brain. In this study, we sought to determine how infraslow relationships are affected by antiepileptic drug (AED) taper, time of day, and seizure.

METHODS

We studied intracranial electroencephalographic (icEEG) data collected from 13 medically refractory adult epilepsy patients who underwent monitoring at Yale-New Haven Hospital. We estimated the magnitude-squared coherence (MSC) at <0.15 Hz of traditional EEG frequency band power time series for all electrode contact pairs to quantify infraslow envelope correlations between them. We studied, first, hour-long background icEEG epochs before and after AED taper to understand the effect of taper. Second, we analyzed the entire record for each patient to study the effect of time of day. Finally, for each patient, we reviewed the clinical record to find all seizures that were at least 6 hours removed from other seizures and analyzed infraslow envelope MSC before and after them.

RESULTS

Infraslow envelope MSC increased slightly, but significantly, after AED taper, and increased on average during the night and decreased during the day. It was also increased significantly in all frequency bands up to 3 hours preseizure and 1 hour postseizure as compared to background icEEG (61 seizures studied). These changes occurred for both daytime and nighttime seizures (28 daytime, 33 nighttime). Interestingly, there was significant spatial variability to these changes, with the seizure onset area peaking at 3 hours preseizure, then showing progressive desynchronization from 3 hours preseizure to 1 hour postseizure.

SIGNIFICANCE

Infraslow envelope analysis may be used to understand long-term changes over the course of icEEG monitoring, provide unique insight into interictal electrophysiological changes related to ictogenesis, and contribute to the development of novel seizure forecasting algorithms.

Synchronization of infra-slow oscillations of brain potentials with respiration

We study the synchronization of infra-slow oscillations in human scalp electroencephalogram signal with the respiratory signal. For the cases of paced respiration with a fixed frequency and linearly increasing frequency, we reveal the phase and frequency locking of infra-slow oscillations of brain potentials by respiration. It is shown that for different brain areas, the infra-slow oscillations and respiration can exhibit synchronous regimes of different orders.

Deep in the brain: Changes in subcortical function immediately preceding a migraine attack

The neural mechanism responsible for migraine remains unclear. While the role of an external trigger in migraine initiation remains vigorously debated, it is generally assumed that migraineurs display altered brain function between attacks. This idea stems from relatively few brain imaging studies with even fewer studies exploring changes in the 24 h period immediately prior to a migraine attack. Using functional magnetic resonance imaging, we measured infra-slow oscillatory activity, regional homogeneity, and connectivity strengths of resting activity in migraineurs directly before (n = 8), after (n = 11), and between migraine attacks (n = 26) and in healthy control subjects (n = 78). Comparisons between controls and each migraine group and between migraine groups were made for each of these measures. Directly prior to a migraine, increased infra-slow oscillatory activity occurred in brainstem and hypothalamic regions that also display altered activity during a migraine itself, that is, the spinal trigeminal nucleus, dorsal pons, and hypothalamus. Furthermore, these midbrain and hypothalamic sites displayed increased connectivity strengths and regional homogeneity directly prior to a migraine. Remarkably, these resting oscillatory and connectivity changes did not occur directly after or between migraine attacks and were significantly different to control subjects. These data provide evidence of altered brainstem and hypothalamic function in the period immediately before a migraine and raise the prospect that such changes contribute to the expression of a migraine attack.

Pupil Size Coupling to Cortical States Protects the Stability of Deep Sleep via Parasympathetic Modulation

During wakefulness, pupil diameter can reflect changes in attention, vigilance, and cortical states. How pupil size relates to cortical activity during sleep, however, remains unknown. Pupillometry during natural sleep is inherently challenging since the eyelids are usually closed. Here, we present a novel head-fixed sleep paradigm in combination with infrared back-illumination pupillometry (iBip) allowing robust tracking of pupil diameter in sleeping mice. We found that pupil size can be used as a reliable indicator of sleep states and that cortical activity becomes tightly coupled to pupil size fluctuations during non-rapid eye movement (NREM) sleep. Pharmacological blocking experiments indicate that the observed pupil size changes during sleep are mediated via the parasympathetic system. We furthermore found that constrictions of the pupil during NREM episodes might play a protective role for stability of sleep depth. These findings reveal a fundamental relationship between cortical activity and pupil size, which has so far been hidden behind closed eyelids.

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Infrared back-illumination allows accurate pupillometry in sleeping mice

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Brain activity and pupil diameter are tightly coupled during sleep

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The parasympathetic system is the main driver of pupillary changes during NREM sleep

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Pupillary constrictions might have a protective function to stabilize deep sleep

The role of neuron-glia interactions in the emergence of ultra-slow oscillations

Ultra-slow cortical oscillatory activity of 1-100 mHz has been recorded in human by electroencephalography and in dissociated cultures of cortical rat neurons, but the underlying mechanisms remain to be elucidated. This study presents a computational model of ultra-slow oscillatory activity based on the interaction between neurons and astrocytes. We predict that the frequency of these oscillations closely depends on activation of astrocytes in the network, which is reflected by oscillations of their intracellular calcium concentrations with periods between tens of seconds and minutes. An increase of intracellular calcium in astrocytes triggers the release of adenosine triphosphate from these cells which may alter transmission at nearby synapses by increasing or decreasing neurotransmitter release. These results provide theoretical support for the emerging awareness of astrocytes as active players in the regulation of neural activity and identify neuron-astrocyte interactions as a potential primary mechanism for the emergence of ultra-slow cortical oscillations.

Disruption of default mode network dynamics in acute and chronic pain states

It has been proposed that pain competes with other attention-demanding stimuli for cognitive resources, and many chronic pain patients display significant attention and mental flexibility deficits. These alterations may result from disruptions in the functioning of the default mode network (DMN) which plays a critical role in attention, memory, prospection and self-processing, and recent investigations have found alterations in DMN function in multiple chronic pain conditions. Whilst it has been proposed that these DMN alterations are a characteristic of pain that is chronic in nature, we recently reported altered oscillatory activity in the DMN during an acute, 5  minute noxious stimulus in healthy control subjects. We therefore hypothesize that altered DMN activity patterns will not be restricted to those in chronic pain but instead will also occur in healthy individuals during tonic noxious stimuli. We used functional magnetic resonance imaging to measure resting state infra-slow oscillatory activity and functional connectivity in patients with chronic orofacial pain at rest and in healthy controls during a 20-minute tonic pain stimulus. We found decreases in oscillatory activity in key regions of the DMN in patients with chronic pain, as well as in healthy controls during tonic pain in addition to changes in functional connectivity between the posterior cingulate cortex and areas of the DMN in both groups. The results show that similar alterations in DMN function occur in healthy individuals during acute noxious stimuli as well as in individuals with chronic pain. These DMN changes may reflect the presence of pain per se and may underlie alterations in attentional processes that occur in the presence of pain.

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Default mode network dynamics were measured in chronic and acute pain.

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Altered infra-slow activity and connectivity occurred in chronic and acute pain.

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Default mode network changes characterize pain per se.

Increased Amplitude of Thalamocortical Low-Frequency Oscillations in Patients with Migraine

For many years, neurobiological theories have emphasized the importance of neuronal oscillations in the emergence of brain function. At the same time, clinical studies have shown that disturbances or irregularities in brain rhythms may relate to various common neurological conditions, including migraine. Increasing evidence suggests that the CNS plays a fundamental role in the predisposition to develop different forms of headache. Here, we present human imaging data that strongly support the presence of abnormal low-frequency oscillations (LFOs) in thalamocortical networks of patients in the interictal phase of migraine. Our results show that the main source of arrhythmic activity was localized to the higher-order thalamic relays of the medial dorsal nucleus. In addition, spontaneous LFOs in the thalamus were selectively associated with the headache attack frequency, meaning that the varying amplitude of dysrhythmia could predispose patients to recurrent attacks. Rhythmic cortical feedback to the thalamus is a major factor in the amplification of thalamocortical oscillations, making it a strong candidate for influencing neuronal excitability. We further speculate that the intrinsic dynamics of thalamocortical network oscillations are crucial for early sensory processing and therefore could underlie important pathophysiological processes involved in multisensory integration.

SIGNIFICANCE STATEMENT

In many cases, migraine attacks are thought to begin centrally. A major obstacle to studying intrinsic brain activity has been the identification of the precise anatomical structures and functional networks that are involved in migraine. Here, we present imaging data that strongly support the presence of abnormal low-frequency oscillations in thalamocortical networks of patients in the interictal phase of migraine. This arrhythmic activity was localized to the higher-order thalamic relays of the medial dorsal nucleus and was selectively associated with headache attack frequency. Rhythmic cortical feedback to the thalamus is a major factor in the amplification of thalamocortical oscillations, making it a strong candidate for influencing neuronal excitability and higher-level processes involved in multisensory integration.

Reproducing Infra-Slow Oscillations with Dopaminergic Modulation

In the human brain, billions of neurons construct a neural network via synaptic connections. Neuronal excitation and inhibition are transmitted to other neurons through synapses via neurotransmitters. Dopamine is one of these neurotransmitters that plays a number of important roles. There are a variety of rhythms in the brain, such as alpha rhythm, beta rhythm, and so on. Infra-slow oscillation, ISO, is one of the rhythms observed in the brain, and ranges below 0.1 Hz. One of the key roles of dopamine is the generation of ISO in neural networks. Although the mechanism underlying the generation of ISO remains unknown, ISO can be generated by activation of the D1-type dopamine receptor. The D1-type receptor regulates spike timing-dependent plasticity (STDP), which is a learning rule of the change in synaptic weights. In this paper, to reproduce ISO in neural networks, we show that dopaminergic modulation of STDP is essential. More specifically, we discovered a close relationship between two dopaminergic effects: modulation of the STDP function and generation of ISO. We therefore, numerically investigated the relationship in detail and proposed a possible mechanism by which ISO is generated.

Imaging Acute and Chronic Pain in the Human Brainstem and Spinal Cord

While acute pain serves as a protective mechanism designed to warn an individual of potential or actual damaging stimuli, chronic pain provides no benefit and is now considered a disease in its own right. Since the advent of human brain imaging techniques, many investigations that have explored the central representation of acute and chronic pain have focused on changes in higher order brain regions. In contrast, far fewer have explored brainstem and spinal cord function, mainly due to significant technical difficulties. In this review, we present some of the recent human brain imaging studies that have specifically explored brainstem and spinal cord function during acute noxious stimuli and in individuals with chronic pain. We focus particularly on investigations that explore changes in areas that receive nociceptor afferents and compare humans and experimental animal data in an attempt to describe both microscopic and macroscopic changes associated with acute and chronic pain.

Altered brainstem anatomy in migraine

Background

The exact mechanisms responsible for migraine remain unknown, although it has been proposed that changes in brainstem anatomy and function, even between attacks, may contribute to the initiation and maintenance of headache during migraine attacks. The aim of this investigation is to use brainstem-specific analyses of anatomical and diffusion weighted images to determine if the trigeminal system displays altered structure in individuals with migraine.

Methods

Voxel-based morphometry of T1-weighted anatomical images (57 controls, 24 migraineurs) and diffusion tensor images (22 controls, 24 migraineurs) were used to assess brainstem anatomy in individuals with migraine compared with controls.

Results

We found grey matter volume decreases in migraineurs in the spinal trigeminal nucleus and dorsomedial pons. In addition, reduced grey matter volume and increased free water diffusivity occurred in areas of the descending pain modulatory system, including midbrain periaqueductal gray matter, dorsolateral pons, and medullary raphe. These changes were not correlated to migraine frequency, duration, intensity or time to next migraine.

Conclusion

Brainstem anatomy changes may underlie changes in activity that result in activation of the ascending trigeminal pathway and the perception of head pain during a migraine attack.

Homeostatic dynamics, hysteresis and synchronization in a low-dimensional model of burst suppression

Burst suppression, a pattern of the electroencephalogram characterized by quasi-periodic alternation of high-voltage activity (burst) and isoelectric silence (suppression), is typically associated with states of unconsciousness, such as in deep general anesthesia and certain etiologies of coma. Recent computational models for burst suppression have attributed the slow (up to tens of seconds) time-scale of burst termination and re-initiation to cycling in supportive physiological process, such as cerebral metabolism. That is, activity-dependent substrate ('energy') depletion during bursts, followed by substrate recovery during suppression. Such a model falls into the category of a fast-slow dynamical system, commonly used to describe neuronal bursting more generally. Here, following this basic paradigm, we develop a low dimensional mean field model for burst suppression that adds several new features and capabilities to previous models. Most notably, this new model includes explicit homeostatic interactions wherein the rates of substrate recovery are tied to neuronal activity in a supply demand loop, creating a physiologically consistent, reciprocal interaction between the neural and substrate processes. We develop formal analysis of the model dynamics, showing, in particular, the capability of the model to produce burst-like activity as a consequence of neuronal downregulation only, without any direct perturbation to the substrate dynamics. Further, we use a synchronization analysis to contrast different mechanisms for spatially local versus global bursting. The analysis performed generates characterizations that are consistent with experimental observations of spatiotemporal features such as burst onset, duration, and spatial organization and, moreover, generates predictions regarding the presence of bistability and hysteresis in the underlying system. Thus, the model provides new dynamical insight into the mechanisms of burst suppression and, moreover, a tractable platform for more detailed future characterizations.

Chronic Neuropathic Pain: It's about the Rhythm

The neural mechanisms underlying the development and maintenance of chronic neuropathic pain remain unclear. Evidence from human investigations suggests that neuropathic pain is associated with altered thalamic burst firing and thalamocortical dysrhythmia. Additionally, experimental animal investigations show that neuropathic pain is associated with altered infra-slow (<0.1 Hz) frequency oscillations within the dorsal horn and somatosensory thalamus. The aim of this investigation was to determine whether, in humans, neuropathic pain was also associated with altered infra-slow oscillations within the ascending "pain" pathway. Using resting-state functional magnetic resonance imaging, we found that individuals with orofacial neuropathic pain have increased infra-slow oscillatory activity throughout the ascending pain pathway, including within the spinal trigeminal nucleus, somatosensory thalamus, thalamic reticular nucleus, and primary somatosensory cortex. Furthermore, these infra-slow oscillations were temporally coupled across these multiple sites and occurred at frequencies similar to calcium waves in activated astrocytes. The region encompassing the spinal trigeminal nucleus also displayed increased regional homogeneity, consistent with a local spread of neural activity by astrocyte activation. In contrast, no increase in oscillatory behavior within the ascending pain pathway occurred during acute noxious stimuli in healthy individuals. These data reveal increased oscillatory activity within the ascending pain pathway that likely underpins increased thalamocortical oscillatory activity, a self-sustaining thalamocortical dysrhythmia, and the constant perception of pain. Significance statement: Chronic neuropathic pain is associated with altered thalamic firing and thalamocortical dysrhythmia. The mechanisms responsible for these changes remain unknown. In this study, we report in individuals with neuropathic pain increased oscillatory neural activity within the ascending pain pathway with evidence that these changes result from altered neural-astrocyte coupling. We propose a series of neural and glial events after nerve injury that result in the generation of altered thalamocortical activity and a persistent neuropathic pain state. Defining the underlying mechanisms responsible for neuropathic pain is critical if we are to develop more effective treatment regimens.

How do neuroanatomical changes in individuals with chronic pain result in the constant perception of pain?

Since the advent of anatomical brain imaging analysis techniques, numerous reports have shown altered regional brain anatomy in individuals with various chronic pain conditions. While early reports of increased regional brain volumes in taxi drivers and pianists were simply interpreted as responses to excessive use, the mechanisms responsible for anatomical changes associated with chronic pain are not so straightforward. The main aim of this paper is to explore the potential underlying cellular changes responsible for change in gross brain anatomy in individuals with chronic pain, in particular pain following nervous system damage. Determining the basis of these changes may provide a platform for development of targeted, personalized and ultimately more effective treatment regimens.

The contribution of inner and outer retinal photoreceptors to infra-slow oscillations in the rat olivary pretectal nucleus

A subpopulation of olivary pretectal nucleus (OPN) neurons discharges action potentials in an oscillatory manner, with a period of approximately two minutes. This 'infra-slow' oscillatory activity depends on synaptic excitation originating in the retina. Signals from rod-cone photoreceptors reach the OPN via the axons of either classic retinal ganglion cells or intrinsically photosensitive retinal ganglion cells (ipRGCs), which use melanopsin for photon capturing. Although both cell types convey light information, their physiological functions differ considerably. The aim of the present study was to disentangle how rod-cone and melanopsin photoresponses contribute to generation of oscillatory activity. Pharmacological manipulations of specific phototransduction cascades were used whilst recording extracellular single-unit activity in the OPN of anaesthetized rats. The results show that under photopic conditions (bright light), ipRGCs play a major role in driving infra-slow oscillations, as blocking melanopsin phototransmission abolishes or transiently disturbs oscillatory firing of the OPN neurons. On the other hand, blocking rod-cone phototransmission does not change firing patterns in photopic conditions. However, under mesopic conditions (moderate light), when melanopsin phototransmission is absent, blocking rod-cone signalling causes disturbances or even the disappearance of oscillations implying that classic photoreceptors are of greater importance under moderate light. Evidence is provided that all photoreceptors are required for the generation of oscillations in the OPN, although their roles in driving the rhythm are determined by the lighting conditions, consistent with their relative sensitivities. The results further suggest that maintained retinal activity is crucial to observe infra-slow oscillatory activity in the OPN.

Neuromagnetic high-frequency oscillations correlate with seizure severity in absence epilepsy

OBJECTIVE

This study quantified the clinical correlation of interictal and ictal neuromagnetic activities from low- to very-high-frequency ranges in childhood absence epilepsy (CAE).

METHODS

Twelve patients with clinically diagnosed drug-naïve CAE were studied using a 275-channel whole-head magnetoencephalography (MEG) system. MEG data were digitized at 6000 Hz and analyzed at both sensor and source levels with multi-frequency analyses.

RESULTS

Neuromagnetic changes from interictal to ictal periods predominantly occurred in medial prefrontal cortex and parieto-occipito-temporal junction in absence seizures. The changes were statistically significant in low-frequency bands only (<30 Hz, p<0.0001). There was a significant correlation between the source strength of ictal high-frequency oscillations (HFOs) in 200-1000 Hz and the number of daily seizures (r=0.734, p<0.01).

CONCLUSIONS

CAE has focal neuromagnetic sources. The transition from interictal to ictal periods is associated with the elevation of low-frequency brain activities. The strength of HFOs reflects the severity of absence seizures.

SIGNIFICANCE

Low- and high-frequency MEG signals reveal distinct brain activities in CAE. HFOs is a new biomarker for the study of absence seizures.

Infraslow status epilepticus: A new form of subclinical status epilepticus recorded in a child with Sturge-Weber syndrome

BACKGROUND

Analysis of infraslow EEG activity (ISA) has shown potential in the evaluation of patients with epilepsy and in the differentiation between focal and generalized epilepsies. Infraslow EEG activity analysis may also provide insights into the pathophysiology of refractory clinical and subclinical status epilepticus. The purpose of this report is to describe a girl with Sturge-Weber syndrome (SWS) who presented with a 96-h refractory encephalopathy and nonischemic hemiparesis and who was identified to have infraslow status epilepticus (ISSE), which successfully resolved after midazolam administration.

METHODS

The continuous EEG recording of a 5-year-old girl with known structural epilepsy due to Sturge-Weber syndrome is presented. The patient presented to the ED with acute confusion, eye deviation, and right hemiparesis similar to two previous admissions. Despite administration of lorazepam, fosphenytoin, phenobarbital, and valproic loads, the patient showed no improvement in the clinical condition after 48 h. The continuous video-EEG monitoring (VEM) showed continuous severe diffuse nonrhythmic asymmetric slowing but no apparent ictal activity on continuous conventional EEG recording settings. As brain CT, CTA, CTV, and complete MRI scans including DWI obtained within 72 h of presentation failed to demonstrate any ischemic changes, analysis of the EEG infraslow (ISA) activity was undertaken using LFF: 0.01 Hz and HFF: of 0.1 Hz, respectively.

RESULTS

Continuous subclinical unilateral rhythmic ictal ISA was identified. This was only evident on the left hemisphere which correlated with the structural changes due to SWS. A trial of continuous 120 to 240 μg/kg/h of IV midazolam resulted in immediate resolution of the contralateral hemiparesis and encephalopathy.

CONCLUSION

Continuous prolonged rhythmic ictal infraslow activity (ISA) can cause super-refractory subclinical focal status epilepticus. This has not been previously reported, and we propose that this be called infraslow status epilepticus (ISSE). Infraslow EEG activity analysis should be performed in all patients with unexplained subclinical status epilepticus. This article is part of a Special Issue entitled "Status Epilepticus".

Infraslow EEG changes in infantile spasms

PURPOSE

Infantile spasms (IS) are a devastating epileptic encephalopathy syndrome of infancy. Analysis of infraslow EEG activity (ISA) has shown potential in the presurgical evaluation of patients with epilepsy and in differentiating between focal and generalized epilepsy syndromes. Infraslow EEG activity analysis may provide insights into the pathophysiology of some difficult-to-treat epilepsy syndromes, such as IS. To our knowledge, there are no published reports describing ISA in patients with IS. The purpose of this study was to describe ictal patterns of ISA in patients with IS and to correlate with clinical data.

METHODS

EEG recordings of all cases of IS in the past 10 years at the Alberta Children's Hospital were reviewed. Inclusion criteria were a technically adequate video EEG recording that captured at least one spasm. For each patient, the first 10 confirmed spasms were examined. Spasms were evaluated for changes in ISA, which were either generalized, lateralized, or absent ISA (g-ISA, l-ISA, or n-ISA, respectively). Results were correlated with treatments, clinical course, and information pertinent to likely etiology of the IS.

RESULTS

A total of 77% of spasms were associated with ISA; 57% with g-ISA, 20% l-ISA, and 21% n-ISA. All patients with exclusively g-ISA showed at least a partial response to initial therapy, while this was the case in 66.7% of those with at least some l-ISA and 50% of those with exclusively n-ISA. Other seizure types occurred in 60% of patients with exclusively g-ISA versus 83% with some l-ISA and all patients with exclusively n-ISA.

CONCLUSIONS

Ictal ISA was observed in the majority of IS. Trends were observed suggesting that the presence of exclusive g-ISA changes may be a positive prognostic factor in IS.