Envelope analysis links oscillatory and arrhythmic EEG activities to two types of neuronal synchronization.

Traditionally, EEG is understood as originating from the synchronous activation of neuronal populations that generate rhythmic oscillations in specific frequency bands. Recently, new neuronal dynamics regimes have been identified (e.g. neuronal avalanches) characterized by irregular or arrhythmic activity. In addition, it is starting to be acknowledged that broadband properties of EEG spectrum (following a 1/f law) are tightly linked to brain function. Nevertheless, there is still no theoretical framework accommodating the coexistence of these two EEG phenomenologies: rhythmic/narrowband and arrhythmic/broadband. To address this problem, we present a new framework for EEG analysis based on the relation between the Gaussianity and the envelope of a given signal. EEG Gaussianity is a relevant assessment because if EEG emerges from the superposition of uncorrelated sources, it should exhibit properties of a Gaussian process, otherwise, as in the case of neural synchronization, deviations from Gaussianity should be observed. We use analytical results demonstrating that the coefficient of variation of the envelope (CVE) of Gaussian noise (or any of its filtered sub-bands) is the constant 4π-1≈0.523, thus enabling CVE to be a useful metric to assess EEG Gaussianity. Furthermore, a new and highly informative analysis space (envelope characterization space) is generated by combining the CVE and the envelope average amplitude. We use this space to analyze rat EEG recordings during sleep-wake cycles. Our results show that delta, theta and sigma bands approach Gaussianity at the lowest EEG amplitudes while exhibiting significant deviations at high EEG amplitudes. Deviations to low-CVE appeared prominently during REM sleep, associated with theta rhythm, a regime consistent with the dynamics shown by the synchronization of weakly coupled oscillators. On the other hand, deviations to high-CVE, appearing mostly during NREM sleep associated with EEG phasic activity and high-amplitude Gaussian waves, can be interpreted as the arrhythmic superposition of transient neural synchronization events. These two different manifestations of neural synchrony (low-CVE/high-CVE) explain the well-known spectral differences between REM and NREM sleep, while also illuminating the origin of the EEG 1/f spectrum.

Adolescent social defeat disturbs adult aggression-related impulsivity in wild-type rats.

Adolescence is generally considered as a developmental period during which adverse social experiences may have lasting consequences in terms of an increased vulnerability to affective disorders. This study aimed at determining the individual susceptibility to adolescent social stress using a rat model. We used rats of the Wild-type Groningen strain, which are characterized by a broad variation in adult levels of aggression and impulsivity. We hypothesized that experience of social defeat in adolescence results in heightened aggression and impulsivity levels in adulthood. In contrast to our expectation, adolescent social defeat did not lead to a difference in the average adult level of aggression and impulsivity, but the significant correlation between offensive aggression and impulsivity found in control animals was not present in animals defeated during adolescence.

Remote long-term registrations of sleep-wake rhythms, core body temperature and activity in marmoset monkeys.

Initial studies in the day active marmoset monkey (Callithrix jacchus) indicate that the sleep-wake cycle of these non-human primates resembles that of humans and therefore conceivably represent an appropriate model for human sleep. The methods currently employed for sleep studies in marmosets are limited. The objective of this study was to employ and validate the use of specific remote monitoring system technologies that enable accurate long-term recordings of sleep-wake rhythms and the closely related rhythms of core body temperature (CBT) and locomotor activity in unrestrained group-housed marmosets. Additionally, a pilot sleep deprivation (SD) study was performed to test the recording systems in an applied experimental setup. Our results show that marmosets typically exhibit a monophasic sleep pattern with cyclical alternations between NREM and REM sleep. CBT displays a pronounced daily rhythm and locomotor activity is primarily restricted to the light phase. SD caused an immediate increase in NREM sleep time and EEG slow-wave activity as well as a delayed REM sleep rebound that did not fully compensate for REM sleep that had been lost during SD. In conclusion, the combination of two innovative technical approaches allows for simultaneous measurements of CBT, sleep cycles and activity in multiple subjects. The employment of these systems represents a significant refinement in terms of animal welfare and will enable many future applications and longitudinal studies of circadian rhythms in marmosets.

Telemetric study of sleep architecture and sleep homeostasis in the day-active tree shrew Tupaia belangeri.

STUDY OBJECTIVES: In this study the authors characterized sleep architecture and sleep homeostasis in the tree shrew, Tupaia belangeri, a small, omnivorous, day-active mammal that is closely related to primates. DESIGN: Adult tree shrews were individually housed under a 12-hr light/12-hr dark cycle in large cages containing tree branches and a nest box. The animals were equipped with radio transmitters to allow continuous recording of electroencephalogram (EEG), electromyogram (EMG), and body temperature without restricting their movements. Recordings were performed under baseline conditions and after sleep deprivation (SD) for 6 hr or 12 hr during the dark phase. MEASUREMENTS AND RESULTS: Under baseline conditions, the tree shrews spent a total of 62.4 ± 1.4% of the 24-hr cycle asleep, with 91.2 ± 0.7% of sleep during the dark phase and 33.7 ± 2.8% sleep during the light phase. During the dark phase, all sleep occurred in the nest box; 79.6% of it was non-rapid eye movement (NREM) sleep and 20.4% was rapid eye movement (REM) sleep. In contrast, during the light phase, sleep occurred almost exclusively on the top branches of the cage and only consisted of NREM sleep. SD was followed by an immediate increase in NREM sleep time and an increase in NREM sleep EEG slow-wave activity (SWA), indicating increased sleep intensity. The cumulative increase in NREM sleep time and intensity almost made up for the NREM sleep that had been lost during 6-hr SD, but did not fully make up for the NREM sleep lost during 12-hr SD. Also, only a small fraction of the REM sleep that was lost was recovered, which mainly occurred on the second recovery night. CONCLUSIONS: The day-active tree shrew shares most of the characteristics of sleep structure and sleep homeostasis that have been reported for other mammalian species, with some peculiarities. Because the tree shrew is an established laboratory animal in neurobiological research, it may be a valuable model species for studies of sleep regulation and sleep function, with the added advantage that it is a day-active species closely related to primates.