Differential modulation of global and local neural oscillations in REM sleep by homeostatic sleep regulation

Simultaneous two-photon imaging and wireless EEG recording in mice

Background

In vivo two-photon imaging is a reliable method with high spatial resolution that allows observation of individual neuron and dendritic activity longitudinally. Neurons in local brain regions can be influenced by global brain states such as levels of arousal and attention that change over relatively short time scales, such as minutes. As such, the scientific rigor of investigating regional neuronal activities could be enhanced by considering the global brain state.

New method

In order to assess the global brain state during in vivo two-photon imaging, CBRAIN (collective brain research platform aided by illuminating neural activity), a wireless EEG collecting and labeling device, was controlled by the same computer of two-photon microscope. In an experiment to explore neuronal responses to isoflurane anesthesia through two-photon imaging, we investigated whether the response of individual cells correlated with concurrent EEG changes induced by anesthesia.

Results

In two-photon imaging, calcium activities of the excitatory neurons in the primary somatosensory cortex disappeared in about 30s after to the initiation of isoflurane anesthesia. The simultaneously recorded EEG showed various transitional activity for about 7 min from the initiation of anesthesia and continued with burst and suppression alternating pattern thereafter. As such, there was a dissociation between excitatory neuron activity of the primary somatosensory cortex and the global brain activity under anesthesia.

Comparison with existing methods

Existing methods to combine two-photon and EEG recording used wired EEG recording. In this study, wireless EEG was used in conjunction with two-photon imaging, facilitated by CBRAIN. More importantly, built-in algorithms of the CBRAIN can automatically detect brain state such as sleep. The codes used for EEG classification are easy to use, with no prior experience required.

Conclusion

Simultaneous recording of wireless EEG and two-photon imaging provides a practical way to capture individual neuronal activities with respect to global brain state in an experimental set-up.

[Chronic sleep restriction in rats leads to a weakening of compensatory reactions in response to acute sleep deprivation]

OBJECTIVE

To identify features in the compensatory mechanisms of sleep regulation in response to acute sleep deprivation after chronic sleep restriction in rats.

MATERIAL AND METHODS

Male Wistar rats 7-8 months old underwent 5-day sleep restriction: 3 h of sleep deprivation and 1 h of sleep opportunity repeating throughout each day. Six-hour acute total sleep deprivation was performed at the beginning of daylight hours on the 3rd day after sleep restriction. Polysomnogramms were recorded throughout the day before chronic sleep restriction, on the 2nd recovery day after chronic sleep restriction and after acute sleep deprivation. The control group was not subjected to chronic sleep restriction.

RESULTS

The animals after chronic sleep restriction had the compensatory increase in total sleep time in response to acute sleep deprivation weaker than in control animals. Animals after sleep restriction had the compensatory increase in the time of slow-wave sleep (SWS) only in the first 6 hours after acute sleep deprivation, whereas in control animals the period of compensation of SWS lasted 12 hours. A compensatory increase in slow-wave activity (SWA) was observed in both groups of animals, but in animals experiencing chronic sleep restriction the amplitude of SWA after acute sleep deprivation was less than in control animals. A compensatory increase in REM sleep in sleep restricted animals occurred immediately after acute sleep deprivation and coincides with a compensatory increase in SWS and SWA, whereas in control conditions these processes are spaced in time.

CONCLUSION

Compensatory reactions in response to acute sleep deprivation (sleep homeostasis) are weakened in animals subjected to chronic sleep restriction, as the reaction time and amplitude are reduced.

Nine-day continuous recording of EEG and 2-hour of high-density EEG under chronic sleep restriction in mice

This work provides an EEG dataset collected from nine mice during the sleep deprivation (SD) paradigm for the sleep science community. It includes 9-day of continuous recording of the frontal and parietal EEG, accelerometer, and 2-hour of high-density EEG (HD-EEG) under SD and SD-free conditions. Eighteen hours of SD were conducted on 5 consecutive days. The HD-EEG data were saved in the EEGLAB format and stored as the brain imaging data structure (BIDS). These datasets can be used to (i) compare mouse HD-EEG to human HD-EEG, (ii) track oscillatory activities of the sleep EEG (e.g., slow waves, spindles) across the cortical regions under different conditions of sleep pressure, and (iii) investigate the cortical traveling waves in the mouse brain. We also provided Python code for basic analyses of this dataset, including the detection of slow waves and sleep spindles. We hope that our dataset will reveal hidden activities during sleep and lead to a better understanding of the functions and mechanisms of sleep.

Relevance of sleep and associated structural changes in GBA1 mouse to human rapid eye movement behavior disorder

Rapid eye movement (REM) sleep behaviour disorder (RBD) is a REM parasomnia that often predicts the later occurrence of alpha-synucleinopathies. Variants in the gene encoding for the lysosomal enzyme glucocerebrosidase, GBA, strongly increase the risk of RBD. In a GBA1-mouse model recently shown to mimic prodromal stages of α-synucleinopathy, we now demonstrate striking REM and NREM electroencephalographic sleep abnormalities accompanied by distinct structural changes in the more widespread sleep neurocircuitry.

A review on pathophysiological aspects of Sleep Deprivation

Sleep deprivation (SD) (also referred as insomnia) is a condition in which peoples fails to get enough sleep due to excessive yawning, facing difficulty to learn new concepts, forgetfulness as well as depressed mood. This could be occurs due to several possible reasons including medications, stress (caused by shift work). Despite the fact that sleep is important for the normal physiology, it currently affects millions of people around the world US (70 million) and Europe (45 million). Due to increase work demand nowadays lots of peoples experiencing sleep deprivation hence, this could be the reason for several car accident followed by death and morbidity. This review highlighted the impact of SD on neurotransmitter release and functions, theories (Flip-flop theory, oxidative stress theory, neuroinflammation theory, neurotransmitter theory, and hormonal theory) associated with SD pathogenesis apart from this it also demonstrate the molecular pathways underlying SD (PI3K and Akt , NF-κB, Nrf2, and adenosine pathway. However, this study also elaborates the SD induced changes in the level of neurotransmitters, hormonal, and mitochondrial functions. Along with this, it also covers several molecular aspects associated with SD as well. Through this study a link is made between SD and associated causes, which will further help to develop potential therapeutic strategy against SD.

Reduced Cross-Frequency Coupling and Daytime Sleepiness in Obstructive Sleep Apnea Patients

Obstructive sleep apnea (OSA) is associated with sleep-stage- and respiratory-event-specific sensorimotor cortico-muscular disconnection. The modulation of phase−amplitude cross-frequency coupling (PACFC) may influence information processing throughout the brain. We investigated whether sleep-stage-specific PACFC is impaired at the sensorimotor areas in OSA patients. C3 and C4 electrode EEG polysomnography recordings of 170 participants were evaluated. Different frequency band combinations were used to compute CFC modulation index (MI) to assess if MI differs between OSA and non-significant OSA patients in distinct sleep stages. We tested if the CFC-MI could predict daytime sleepiness in OSA. Theta−gamma CFC-MI at cortical sensorimotor areas was significantly reduced during all sleep stages; the delta−alpha CFC-MI was significantly reduced during REM and N1 while increasing during N2 in patients with respiratory disturbance index (RDI) > 15/h compared to those with RDI ≤ 15/h. A sleep stage classification using MI values was achieved in both patient groups. Theta−gamma MI during N2 and N3 could predict RDI and Epworth Sleepiness Scale, while delta−alpha MI during REM predicted RDI. This increase in disconnection at the cortical sensorimotor areas with increasing respiratory distress during sleep supports a cortical motor dysfunction in OSA patients. The MI provides an objective marker to quantify subjective sleepiness and respiratory distress in OSA.

Bile Acids and the Microbiome: Making Sense of This Dynamic Relationship in Their Role and Management in Crohn's Disease

BACKGROUND

Bile acids help maintain the physiological balance of the gut microbiome and the integrity of the intestinal epithelial barrier. Similarly, intestinal bacteria play a major role in bile acid metabolism as they are involved in crucial biotransformation steps in the enterohepatic circulation pathway. Understanding the relationship between bile acid signalling and the gut microbiome in Crohn's disease can help target new and innovative treatment strategies.

AIMS

This review summarises the relationship between bile acids and the microbiome in Crohn's disease and discusses potential novel therapeutic options.

METHODS

We performed a literature review on bile acid signalling, its effect on the gut microbiome, and therapeutic applications in Crohn's disease.

RESULTS

Current research suggests that there is a strong interplay between the dysregulated microbiota, bile acid metabolism, and the mucosal immune system that can result in a changed immunological function, triggering the inflammatory response in Crohn's disease. Recent studies have demonstrated an association with altering the enterohepatic circulation and activating the farnesoid X receptor signalling pathway with the use of probiotics and faecal microbial transplantation, respectively. Bile acid sequestrants have been shown to have anti-inflammatory, cytoprotective, and anti-apoptotic properties with the potential to alter the intestinal microbial composition, suggesting a possible role in inducing and maintaining Crohn's disease.

CONCLUSIONS

Active Crohn's disease has been correlated with changes in bacterial concentrations, which may be associated with changes in bile acid modification. Further research should focus on targeting these areas for future therapeutic options.

A heuristic perspective on non-variational free energy modulation at the sleep-like edge

BACKGROUND

The variational Free Energy Principle (FEP) establishes that a neural system minimizes a free energy function of their internal state through environmental sensing entailing beliefs about hidden states in their environment.

PROBLEM

Because sensations are drastically reduced during sleep, it is still unclear how a self-organizing neural network can modulate free energy during sleep transitions.

GOAL

To address this issue, we study how network's state-dependent changes in energy, entropy and free energy connect with changes at the synaptic level in the absence of sensing during a sleep-like transition.

APPROACH

We use simulations of a physically plausible, environmentally isolated neuronal network that self-organize after inducing a thalamic input to show that the reduction of non-variational free energy depends sensitively upon thalamic input at a slow, rhythmic Poisson (delta) frequency due to spike timing dependent plasticity.

METHODS

We define a non-variational free energy in terms of the relative difference between the energy and entropy of the network from the initial distribution (prior to activity dependent plasticity) to the nonequilibrium steady-state distribution (after plasticity). We repeated the analysis under different levels of thalamic drive - as defined by the number of cortical neurons in receipt of thalamic input.

RESULTS

Entraining slow activity with thalamic input induces a transition from a gamma (awake-like state) to a delta (sleep-like state) mode of activity, which can be characterized through a modulation of network's energy and entropy (non-variational free energy) of the ensuing dynamics. The self-organizing response to low and high thalamic drive also showed characteristic differences in the spectrum of frequency content due to spike timing dependent plasticity.

CONCLUSIONS

The modulation of this non-variational free energy in a network that self-organizes, seems to be an organizational network principle. This could open a window to new empirically testable hypotheses about state changes in a neural network.

Ramelteon modulates gamma oscillations in the rat primary motor cortex during non-REM sleep

Sleep disorders adversely affect daily activities and cause physiological and psychiatric problems. The shortcomings of benzodiazepine hypnotics have led to the development of ramelteon, a melatonin MT1 and MT2 agonist. Although the sleep-promoting effects of ramelteon have been documented, few studies have precisely investigated the structure of sleep and neural oscillatory activities. In this study, we recorded electrocorticograms in the primary motor cortex, the primary somatosensory cortex and the olfactory bulb as well as electromyograms in unrestrained rats treated with either ramelteon or vehicle. A neural-oscillation-based algorithm was used to classify the behavior of the rats into three vigilance states (e.g., awake, rapid eye movement (REM) sleep, and non-REM (NREM) sleep). Moreover, we investigated the region-, frequency- and state-specific modulation of extracellular oscillations in the ramelteon-treated rats. We demonstrated that in contrast to benzodiazepine treatment, ramelteon treatment promoted NREM sleep and enhanced fast gamma power in the primary motor cortex during NREM sleep, while REM sleep was unaffected. Gamma oscillations locally coordinate neuronal firing, and thus, ramelteon modulates neural oscillations in sleep states in a unique manner and may contribute to off-line information processing during sleep.

Differential modulation of NREM sleep regulation and EEG topography by chronic sleep restriction in mice

Compensatory elevation in NREM sleep EEG delta power has been typically observed following prolonged wakefulness and widely used as a sleep homeostasis indicator. However, recent evidence in human and rodent chronic sleep restriction (CSR) studies suggests that NREM delta power is not progressively increased despite of accumulated sleep loss over days. In addition, there has been little progress in understanding how sleep EEG in different brain regions responds to CSR. Using novel high-density EEG electrode arrays in the mouse model of CSR where mice underwent 18-h sleep deprivation per day for 5 consecutive days, we performed an extensive analysis of topographical NREM sleep EEG responses to the CSR condition, including period-amplitude analysis of individual slow waves. As previously reported in our analysis of REM sleep responses, we found different patterns of changes: (i) progressive decrease in NREM sleep duration and consolidation, (ii) persistent enhancement in NREM delta power especially in the frontal and parietal regions, and (iii) progressive increases in individual slow wave slope and frontal fast oscillation power. These results suggest that multiple sleep-wake regulatory systems exist in a brain region-specific manner, which can be modulated independently, especially in the CSR condition.

Sex differences in brain oscillatory activity during sleep and wakefulness in obstructive sleep apnea

Epidemiological studies consistently show a male predominance in obstructive sleep apnea (OSA). Hormonal differences, breathing control, upper airway anatomy and fat distribution have been proposed as causes of gender differences in OSA. Clinical manifestations are accentuated in men, although white matter structural integrity is affected in women. To the best of our knowledge, no previous studies have explored gender differences in the electrical brain activity features of OSA. Polysomnography was performed on 43 patients with untreated OSA (21 women, 22 men), and power spectral density (1-50 Hz) was compared between groups across sleep and wakefulness at two levels of OSA severity. Severe versus moderate OSA showed decreased power for fast frequencies (25-29 Hz) during wakefulness. OSA men displayed decreased power of a large frequency range (sigma, beta and gamma) during sleep compared with women. Comparisons of men with severe versus moderate OSA presented significantly decreased sigma power during non-rapid eye movement (NREM) sleep, but significantly increased delta activity during REM sleep. Meanwhile, women with severe versus moderate OSA showed no significant power differences in any condition. These findings indicated a different evolution of brain oscillations between OSA men and women with significant impairment of brain activity related to cognitive processes. Our study emphasizes the importance of understanding the differential effects of sleep disorders on men and women in order to develop more precise diagnostic criteria according to gender, including quantitative electroencephalogram (EEG) analysis tools.

Sleep-Wake Cycle in Young and Older Mice

Sleep plays a key role in multiple cognitive functions and sleep pattern changes with aging. Human studies revealed that aging decreases sleep efficiency and reduces the total sleep time, the time spent in slow-wave sleep (SWS), and the delta power (1–4 Hz) during sleep; however, some studies of sleep and aging in mice reported opposing results. The aim of our work is to estimate how features of sleep–wake state in mice during aging could correspond to age-dependent changes observed in human. In this study, we investigated the sleep/wake cycle in young (3 months old) and older (12 months old) C57BL/6 mice using local-field potentials (LFPs). We found that older adult mice sleep more than young ones but only during the dark phase of sleep-wake cycle. Sleep fragmentation and sleep during the active phase (dark phase of cycle), homologous to naps, were higher in older mice. Older mice show a higher delta power in frontal cortex, which was accompanied with similar trend for age differences in slow wave density. We also investigated regional specificity of sleep–wake electrographic activities and found that globally posterior regions of the cortex show more rapid eye movement (REM) sleep whereas somatosensory cortex displays more often SWS patterns. Our results indicate that the effects of aging on the sleep–wake activities in mice occur mainly during the dark phase and the electrode location strongly influence the state detection. Despite some differences in sleep–wake cycle during aging between human and mice, some features of mice sleep share similarity with human sleep during aging.

Regional Delta Waves In Human Rapid Eye Movement Sleep

Although the EEG slow wave of sleep is typically considered to be a hallmark of nonrapid eye movement (NREM) sleep, recent work in mice has shown that slow waves can also occur in REM sleep. Here, we investigated the presence and cortical distribution of negative delta (1-4 Hz) waves in human REM sleep by analyzing high-density EEG sleep recordings obtained in 28 healthy subjects. We identified two clusters of delta waves with distinctive properties: (1) a frontal-central cluster characterized by ∼2.5-3.0 Hz, relatively large, notched delta waves (so-called "sawtooth waves") that tended to occur in bursts, were associated with increased gamma activity and rapid eye movements (EMs), and upon source modeling displayed an occipital-temporal and a frontal-central component and (2) a medial-occipital cluster characterized by more isolated, slower (<2 Hz), and smaller waves that were not associated with rapid EMs, displayed a negative correlation with gamma activity, and were also found in NREM sleep. Therefore, delta waves are an integral part of REM sleep in humans and the two identified subtypes (sawtooth and medial-occipital slow waves) may reflect distinct generation mechanisms and functional roles. Sawtooth waves, which are exclusive to REM sleep, share many characteristics with ponto-geniculo-occipital waves described in animals and may represent the human equivalent or a closely related event, whereas medial-occipital slow waves appear similar to NREM sleep slow waves.SIGNIFICANCE STATEMENT The EEG slow wave is typically considered a hallmark of nonrapid eye movement (NREM) sleep, but recent work in mice has shown that it can also occur in REM sleep. By analyzing high-density EEG recordings collected in healthy adult individuals, we show that REM sleep is characterized by prominent delta waves also in humans. In particular, we identified two distinctive clusters of delta waves with different properties: a frontal-central cluster characterized by faster, activating "sawtooth waves" that share many characteristics with ponto-geniculo-occipital waves described in animals and a medial-occipital cluster containing slow waves that are more similar to NREM sleep slow waves. These findings indicate that REM sleep is a spatially and temporally heterogeneous state and may contribute to explaining its known functional and phenomenological properties.

Remembering to Forget: A Dual Role for Sleep Oscillations in Memory Consolidation and Forgetting

It has been known since the time of patient H. M. and Karl Lashley's equipotentiality studies that the hippocampus and cortex serve mnestic functions. Current memory models maintain that these two brain structures accomplish unique, but interactive, memory functions. Specifically, most modeling suggests that memories are rapidly acquired during waking experience by the hippocampus, before being later consolidated into the cortex for long-term storage. Sleep has been shown to be critical for the transfer and consolidation of memories in the cortex. Like memory consolidation, a role for sleep in adaptive forgetting has both historical precedent, as Francis Crick suggested in 1983 that sleep was for "reverse-learning," and recent empirical support. In this article I review the evidence indicating that the same brain activity involved in sleep replay associated memory consolidation is responsible for sleep-dependent forgetting. In reviewing the literature, it became clear that both a cellular mechanism for systems consolidation and an agreed upon general, as well as cellular, mechanism for sleep-dependent forgetting is seldom discussed or is lacking. I advocate here for a candidate cellular systems consolidation mechanism wherein changes in calcium kinetics and the activation of consolidative signaling cascades arise from the triple phase locking of non-rapid eye movement sleep (NREMS) slow oscillation, sleep spindle and sharp-wave ripple rhythms. I go on to speculatively consider several sleep stage specific forgetting mechanisms and conclude by discussing a notional function of NREM-rapid eye movement sleep (REMS) cycling. The discussed model argues that the cyclical organization of sleep functions to first lay down and edit and then stabilize and integrate engrams. All things considered, it is increasingly clear that hallmark sleep stage rhythms, including several NREMS oscillations and the REMS hippocampal theta rhythm, serve the dual function of enabling simultaneous memory consolidation and adaptive forgetting. Specifically, the same sleep rhythms that consolidate new memories, in the cortex and hippocampus, simultaneously organize the adaptive forgetting of older memories in these brain regions.

Reusable Multielectrode Array Technique for Electroencephalography in Awake Freely Moving Mice

Translational comparison of rodent models of neurological and neuropsychiatric diseases to human electroencephalography (EEG) biomarkers in these conditions will require multisite rodent EEG on the skull surface, rather than local area electrocorticography (ECoG) or multisite local field potential (LFP) recording. We have developed a technique for planar multielectrode array (MEA) implantation on the mouse skull surface, which enables multisite EEG in awake and freely moving mice and reusability of the MEA probes. With this method, we reliably obtain 30-channel low-noise EEG from awake mice. Baseline and stimulus-evoked EEG recordings can be readily obtained and analyzed. For example, we have demonstrated EEG responses to auditory stimuli. Broadband noise elicits reliable 30-channel auditory event-related potentials (ERPs), and chirp stimuli induce phase-locked EEG responses just as in human sound presentation paradigms. This method is unique in achieving chronic implantation of novel MEA technology onto the mouse skull surface for chronic multisite EEG recordings. Furthermore, we demonstrate a reliable method for reusing MEA probes for multiple serial implantations without loss of EEG quality. This skull surface MEA methodology can be used to obtain simultaneous multisite EEG recordings and to test EEG biomarkers in diverse mouse models of human neurological and neuropsychiatric diseases. Reusability of the MEA probes makes it more cost-effective to deploy this system for various studies.

Dynamic coupling between the central and autonomic nervous systems during sleep: A review

Sleep is characterized by coordinated cortical and cardiac oscillations reflecting communication between the central (CNS) and autonomic (ANS) nervous systems. Here, we review fluctuations in ANS activity in association with CNS-defined sleep stages and cycles, and with phasic cortical events during sleep (e.g., arousals, K-complexes). Recent novel analytic methods reveal a dynamic organization of integrated physiological networks during sleep and indicate how multiple factors (e.g., sleep structure, age, sleep disorders) affect "CNS-ANS coupling". However, these data are mostly correlational and there is a lack of clarity of the underlying physiology, making it challenging to interpret causality and direction of coupling. Experimental manipulations (e.g., evoking K-complexes or arousals) provide information on the precise temporal sequence of cortical-cardiac activity, and are useful for investigating physiological pathways underlying CNS-ANS coupling. With the emergence of new analytical approaches and a renewed interest in ANS and CNS communication during sleep, future work may reveal novel insights into sleep and cardiovascular interactions during health and disease, in which coupling could be adversely impacted.

Oscillatory Encoding of Visual Stimulus Familiarity

Familiarity of the environment changes the way we perceive and encode incoming information. However, the neural substrates underlying this phenomenon are poorly understood. Here we describe a new form of experience-dependent low-frequency oscillations in the primary visual cortex (V1) of awake adult male mice. The oscillations emerged in visually evoked potentials and single-unit activity following repeated visual stimulation. The oscillations were sensitive to the spatial frequency content of a visual stimulus and required the mAChRs for their induction and expression. Finally, ongoing visually evoked θ (4-8 Hz) oscillations boost the visually evoked potential amplitude of incoming visual stimuli if the stimuli are presented at the high excitability phase of the oscillations. Our results demonstrate that an oscillatory code can be used to encode familiarity and serves as a gate for oncoming sensory inputs.SIGNIFICANCE STATEMENT Previous experience can influence the processing of incoming sensory information by the brain and alter perception. However, the mechanistic understanding of how this process takes place is lacking. We have discovered that persistent low-frequency oscillations in the primary visual cortex encode information about familiarity and the spatial frequency of the stimulus. These familiarity evoked oscillations influence neuronal responses to the oncoming stimuli in a way that depends on the oscillation phase. Our work demonstrates a new mechanism of visual stimulus feature detection and learning.

Sleep architecture and EEG power spectra in recently detoxified alcohol dependent patients

OBJECTIVES

Persistent sleep abnormalities during abstinence are a harbinger for relapse in patients with chronic alcohol dependence. The present study aimed to compare polysomnography (PSG) data between 'recently detoxified' patients with chronic alcohol dependence and healthy controls.

METHODS

Both conventional sleep architectural and power spectral analyses were conducted. Twenty subjects in each of the groups were enrolled. A 2 nights' sleep (first-habituation and second-experimental) PSG data was collected. Computer assisted scoring supplemented by manual method using the Rechtschaffen and Kales criteria were used for sleep staging. Twenty eight channels were used for the EEG recording. Spectral power across early NREM (Non-rapid-eye-movement), Slow Wave Sleep and REM was computed using the Welch's averaged periodogram method.

RESULTS

Results on conventional sleep staging showed that patients had significantly lesser total sleep time, sleep efficiency and stage shifts and longer sleep onset latency; while duration of each NREM stages were significantly lower, and latency of stage 2 NREM was significantly longer in patients. After controlling for multiple comparisons, spectral power analysis revealed significant differences only during REM sleep and specifically in high frequency (beta and gamma) bands.

CONCLUSIONS

Stating the mutually complementary role of conventional and spectral analyses of polysomnography EEG data, we conclude that sleep abnormalities are fairly evident in recently detoxified alcohol dependent patients.

A Novel Visualization Method for Sleep Spindles Based on Source Localization of High Density EEG

Equivalent dipole source localization is a well-established approach to localizing the electrical activity in electroencephalogram (EEG). So far, source localization has been used primarily in localizing the epileptic source in human epileptic patients. Currently, source localization techniques have been applied to account for localizing epileptic source among the epileptic patients. Here, we present the first application of source localization in the field of sleep spindle in mouse brain. The spatial distribution of cortical potential was obtained by high density EEG and then the anterior and posterior sleep spindles were classified based on the K-mean clustering algorithm. To solve the forward problem, a realistic geometry brain model was produced based on boundary element method (BEM) using mouse MRI. Then, we applied four different source estimation algorithms (minimum norm, eLORETA, sLORETA, and LORETA) to estimate the spatial location of equivalent dipole source of sleep spindles. The estimated sources of anterior and posterior spindles were plotted in a cine-mode that revealed different topographic patterns of spindle propagation. The characterization of sleep spindles may be better be distinguished by our novel visualization method.

Reciprocal Interactions between Medial Septum and Hippocampus in Theta Generation: Granger Causality Decomposition of Mixed Spike-Field Recordings

The medial septum (MS) plays an essential role in rhythmogenesis in the hippocampus (HIPP); theta-rhythmic bursts of MS neurons are believed to drive theta oscillations in rats’ HIPP. The MS theta pacemaker hypothesis has solid foundation but the MS-hippocampal interactions during different behavioral states are poorly understood. The MS and the HIPP have reciprocal connections and it is not clear in particular what role, if any, the strong HIPP to MS projection plays in theta generation. To study the functional interactions between MS and HIPP during different behavioral states, this study investigated the relationship between MS single-unit activity and HIPP field potential oscillations during theta states of active waking and REM sleep and non-theta states of slow wave sleep (SWS) and quiet waking (QW), i.e., sleep-wake states that comprise the full behavioral repertoire of undisturbed, freely moving rats. We used non-parametric Granger causality (GC) to decompose the MS-HIPP synchrony into its directional components, MS→HIPP and HIPP→MS, and to examine the causal interactions between them within the theta frequency band. We found a significant unidirectional MS→HIPP influence in non-theta states which switches to bidirectional theta drive during theta states with MS→HIPP and HIPP→MS GC being of equal magnitude. In non-theta states, unidirectional MS→HIPP influence was accompanied by significant MS-HIPP coherence, but no signs of theta oscillations in the HIPP. In theta states of active waking and REM sleep, sharp theta coherence and strong theta power in both structures was associated with a rise in HIPP→MS to the level of the MS→HIPP drive. Thus, striking differences between waking and REM sleep theta states and non-theta states of SWS and QW were primarily observed in activation of theta influence carried by the descending HIPP→MS pathway associated with more regular rhythmic bursts in the MS and sharper MS→HIPP GC spectra without a significant increase in MS→HIPP GC magnitude. The results of this study suggest an essential role of descending HIPP to MS projections in theta generation.