247 research outputs found
The microstructure of REM sleep: Why phasic and tonic?
Rapid eye movement (REM) sleep is a peculiar neural state that occupies 20-25% of nighttime sleep in healthy human adults and seems to play critical roles in a variety of functions spanning from basic physiological mechanisms to complex cognitive processes. REM sleep exhibits a plethora of transient neurophysiological features, such as eye movements, muscle twitches, and changes in autonomic activity, however, despite its heterogeneous nature, it is usually conceptualized as a homogeneous sleep state. We propose here that differentiating and exploring the fine microstructure of REM sleep, especially its phasic and tonic constituents would provide a novel framework to examine the mechanisms and putative functions of REM sleep. In this review, we show that phasic and tonic REM periods are remarkably different neural states with respect to environmental alertness, spontaneous and evoked cortical activity, information processing, and seem to contribute differently to the dysfunctions of REM sleep in several neurological and psychiatric disorders. We highlight that a distinctive view on phasic and tonic REM microstates would facilitate the understanding of the mechanisms and functions of REM sleep in healthy and pathological conditions.info:eu-repo/semantics/publishe
Lateralized rhythmic acoustic stimulation during daytime NREM sleep enhances slow waves
Slow wave sleep (SWS) is characterized by the predominance of delta waves and slow oscillations, reflecting the synchronized activity of large cortical neuronal populations. Amongst other functions, SWS plays a crucial role in the restorative capacity of sleep. Rhythmic acoustic stimulation (RAS) during SWS has been shown a cost-effective method to enhance slow wave activity. Slow wave activity can be expressed in a region-specific manner as a function of previous waking activity. However, it is unclear whether slow waves can be enhanced in a region-specific manner using RAS. We investigated the effects of unilaterally presented rhythmic acoustic sound patterns on sleep electroencephalographic (EEG) oscillations. Thirty-five participants received during SWS 12-second long rhythmic bursts of pink noise (at a rate of 1 Hz) that alternated with non-stimulated, silent periods, unilaterally delivered into one of the ears of the participants. As expected, RAS enhanced delta power, especially in its low-frequency components between 0.75 and 2.25 Hz. However, increased slow oscillatory activity was apparent in both hemispheres regardless of the side of the stimulation. The most robust increases in slow oscillatory activity appeared during the first 3-4 seconds of the stimulation period. Furthermore, a short-lasting increase in theta and sigma power was evidenced immediately after the first pulse of the stimulation sequences. Our findings indicate that lateralized RAS has a strong potential to globally enhance slow waves during daytime naps. The lack of localized effects suggests that slow waves are triggered by the ascending reticular system and not directly by specific auditory pathways
Stimulating the sleeping brain: Current approaches to modulating memory-related sleep physiology
Background: One of the most audacious proposals throughout the history of psychology was the potential ability to learn while we sleep. The idea penetrated culture via sci-fi movies and inspired the invention of devices that claimed to teach foreign languages, facts, and even quit smoking by simply listening to audiocassettes or other devices during sleep. However, the promises from this endeavor didn't stand up to experimental scrutiny, and the dream was shunned from the scientific community. Despite the historic evidence that the sleeping brain cannot learn new complex information (i.e., words, images, facts), a new wave of current interventions are demonstrating that sleep can be manipulated to strengthen recent memories. New method: Several recent approaches have been developed that play with the sleeping brain in order to modify ongoing memory processing. Here, we provide an overview of the available techniques to non-invasively modulate memory-related sleep physiology, including sensory, vestibular and electrical stimulation, as well as pharmacological approaches. Results: N/A. Comparison with existing methods: N/A. Conclusions: Although the results are encouraging, suggesting that in general the sleeping brain may be optimized for better memory performance, the road to bring these techniques in free-living conditions is paved with unanswered questions and technical challenges that need to be carefully addressed
While you were sleeping: Evidence for high-level executive processing of an auditory narrative during sleep
During sleep we lack conscious awareness of the external environment. Yet, our internal mental state suggests that high-level cognitive processes persist. The nature and extent to which the external environment is processed during sleep remain largely unexplored. Here, we used an fMRI synchronization-based approach to examine responses to a narrative during wakefulness and sleep. The stimulus elicited the auditory network and a frontoparietal pattern of activity, consistent with high-level narrative plot-following. During REM sleep, the same frontoparietal pattern was observed in one of three participants, and partially in one other, confirming that it is possible to track and follow the moment-to-moment complexities of a narrative during REM sleep. Auditory network recruitment was observed in both non-REM and REM sleep, demonstrating preservation of low-level auditory processing, even in deep sleep. This novel approach investigating cognitive processing at different levels of awareness demonstrates that the brain can meaningfully process the external environment during REM sleep
Shaping memory consolidation via targeted memory reactivation during sleep
Recent studies have shown that the reactivation of specific memories during sleep can be modulated using external stimulation. Specifically, it has been reported that matching a sensory stimulus (e.g., odor or sound cue) with target information (e.g., pairs of words, pictures, and motor sequences) during wakefulness, and then presenting the cue alone during sleep, facilitates memory of the target information. Thus, presenting learned cues while asleep may reactivate related declarative, procedural, and emotional material, and facilitate the neurophysiological processes underpinning memory consolidation in humans. This paradigm, which has been named targeted memory reactivation, has been successfully used to improve visuospatial and verbal memories, strengthen motor skills, modify implicit social biases, and enhance fear extinction. However, these studies also show that results depend on the type of memory investigated, the task employed, the sensory cue used, and the specific sleep stage of stimulation. Here, we present a review of how memory consolidation may be shaped using noninvasive sensory stimulation during sleep
Wake-like activities and electrical silences in the human sleeping brain: functional roles and spatio-temporal dynamics in the thalamo-cortical network
As pointed out by several papers, the less than one hertz oscillations or Sleep
Slow Oscillations (SSOs) are the electrophysiological stigmata of the mammalian
sleep. This cellular behavior, mainly involving thalamus and cortex, consisted of
hyperpolarized phases (lasting 500 msec, down states) followed by depolarized
ones (lasting 500 msec, up states). The electrical silence during down states, on
the one hand, prevents any synaptic and network activity and on the other hand,
creates the ionic conditions for a rebound of neural discharge (huge synaptic and
network activity during up state). The presence of down states clearly marks the
phenomenon of cortical bistability, which in turn reflects a deep hyperpolarization
sustained by the opening of different K+-channels. According to the Integrated
Information Theory of Giulio Tononi, down states prevent the emergence of largescale
neural integrations and thus induce the break down of functional connectivity.
This allows a functional segregation of independent cortical modules, which
represents the condicio sine qua non for sleep unconsciousness. Other functional
roles endowed in the SSO are memory consolidation and synaptic downscaling.
The aim of this thesis is to investigate, via EEG, in human spontaneous and evoked
SSOs: (i) the relationships between wake-like activities and electrical silence; (ii)
the role of the thalamus; (iii) the quenching of sensory processing and thus of
consciousness. Regarding point (i) we have found a positive bump preceding
the down state characterized by an increase of high frequency activities. The
presence of this high frequency activity before down state suggests a cortical
ignition mechanism for the spontaneous SSO. As far as point (ii) is concerned, we
have investigated how the thalamus influences the cortical expression of the SSOs.
To this aim, we have studied SSO features in a case of Fatal Familial Insomnia (FFI)
with a selective thalamic neurodegeneration of nuclei mainly involved in spindle
generation. In the FFI patient, we have found a reduction of SSO event rate, some
morphological alterations of SSO structure, and a significant reduction in grouping
high frequency activity during up state. As for point (iii), we studied K-Complexes
(KCs), namely SSOs evoked by sensory stimulations. The main results of this
study are: a positive wave (P200) precedes the down state (N550); the topology of
P200 latency depends on the sensory modality of stimulation (acoustic, tactile and
visual) with earliest waves in the related primary sensory areas; the P200 travels as
a cortical excitation inducing N550 and P900 (up state) in associative fronto-central
areas; when KCs are not evoked the P200-like excitations have lower amplitude
compared to evoked KC P200; the down state latency topology is affected by the
proneness to bistability, i.e. the amount of K+-channel that favor a synchronized
falling into down state. As a whole the results of the thesis indicate that Slow
Wave Sleep (SWS) is not a mere quiescent state but rather an active state in which
changes of neural dynamics allow a well orchestrated interplay of unconscious
behavior and memory consolidation. The final consequence is the maintenance of
homeostasis. The SSO is the cellular phenomenon capable to coalesce wake-like
activities and electrical silences, synthesizing at microscopic level the macroscopic
complexity of SWS. This thesis allowed exploring thalamo-cortical dynamics by
studying spontaneous and evoked SSOs. In synthesis the human-environment
interaction (including visceral stimuli) during sleep overlaps that of wakefulness,
since thalamus and cortical areas devoted to the first step of sensory processing
are identical. The difference between wake and sleep is only sustained by the
down state. In conclusion the study of SSO clarifies many issues linked to sleep
and in particular to the real efficacy of a good sleep. This opens the door to the
application of SSO study in different preclinical or clinical conditions
Advances in Clinical Neurophysiology
Including some of the newest advances in the field of neurophysiology, this book can be considered as one of the treasures that interested scientists would like to collect. It discusses many disciplines of clinical neurophysiology that are, currently, crucial in the practice as they explain methods and findings of techniques that help to improve diagnosis and to ensure better treatment. While trying to rely on evidence-based facts, this book presents some new ideas to be applied and tested in the clinical practice. Advances in Clinical Neurophysiology is important not only for the neurophysiologists but also for clinicians interested or working in wide range of specialties such as neurology, neurosurgery, intensive care units, pediatrics and so on. Generally, this book is written and designed to all those involved in, interpreting or requesting neurophysiologic tests
Cortical and subcortical speech-evoked responses in young and older adults: Effects of background noise, arousal states, and neural excitability
This thesis investigated how the brain processes speech signals in human adults across a wide age-range in the sensory auditory systems using electroencephalography (EEG). Two types of speech-evoked phase-locked responses were focused on: (i) cortical responses (theta-band phase-locked responses) that reflect processing of low-frequency slowly-varying envelopes of speech; (ii) subcortical/peripheral responses (frequency-following responses; FFRs) that reflect encoding of speech periodicity and temporal fine structure information. The aims are to elucidate how these neural activities are affected by different internal (aging, hearing loss, level of arousal and neural excitability) and external (background noise) factors during our daily life through three studies. Study 1 investigated theta-band phase-locking and FFRs in noisy environments in young and older adults. It investigated how aging and hearing loss affect these activities under quiet and noisy environments, and how these activities are associated with speech-in-noise perception. The results showed that ageing and hearing loss affect speech-evoked phase-locked responses through different mechanisms, and the effects of aging on cortical and subcortical activities take different roles in speech-in-noise perception. Study 2 investigated how level of arousal, or consciousness, affects phase-locked responses in young and older adults. The results showed that both theta-band phase-locking and FFRs decreases following decreases in the level of arousal. It was further found that neuro-regulatory role of sleep spindles on theta-band phase-locking is distinct between young and older adults, indicating that the mechanisms of neuro-regulation for phase-locked responses in different arousal states are age-dependent. Study 3 established a causal relationship between the auditory cortical excitability and FFRs using combined transcranial direct current stimulation (tDCS) and EEG. FFRs were measured before and after tDCS was applied over the auditory cortices. The results showed that changes in neural excitability of the right auditory cortex can alter FFR magnitudes along the contralateral pathway. This shows important theoretical and clinical implications that causally link functions of auditory cortex with neural encoding of speech periodicity. Taken together, findings of this thesis will advance our understanding of how speech signals are processed via neural phase-locking in our everyday life across the lifespan
Sleep-based interventions in Alzheimer’s disease: promising approaches from prevention to treatment along the disease trajectory
The multifactorial nature of Alzheimer’s disease (AD) has led scientific researchers to focus
on the modifiable and treatable risk factors of AD. Sleep fits into this context, given the bidirectional
relationship with AD confirmed by several studies over the last years. Sleep disorders appear at
an early stage of AD and continue throughout the entire course of the pathology. Specifically, sleep
abnormalities, such as more fragmented sleep, increase in time of awakenings, worsening of sleep
quality and primary sleep disorders raise with the severity and progression of AD. Intervening on
sleep, therefore, means acting both with prevention strategies in the pre-clinical phase and with
treatments during the course of the disease. This review explores sleep disturbances in the different
stages of AD, starting from the pre-clinical stage. Particular attention is given to the empirical
evidence investigating obstructive sleep apnea (OSA) disorder and the mechanisms overlapping
and sharing with AD. Next, we discuss sleep-based intervention strategies in the healthy elderly
population, mild cognitive impairment (MCI), and AD patients. We mention interventions related to
behavioral strategies, combination therapies, and bright light therapy, leaving extensive space for
new and raising evidence on continuous positive air pressure (CPAP) treatment effectiveness. Finally,
we clarify the role of NREM sleep across the AD trajectory and consider the most recent studies based
on the promising results of NREM sleep enhancement, which use innovative experimental designs
and techniques
Effects of auditory sleep modulation approaches on brain oscillatory and cardiovascular dynamics
Slow waves, the hallmark feature of deep nonrapid eye movement sleep, do potentially drive restorative effects of sleep on brain and body functions. Sleep modulation techniques to elucidate the functional role of slow waves thus have gained large interest. Auditory slow wave stimulation is a promising tool; however, directly comparing auditory stimulation approaches within a night and analyzing induced dynamic brain and cardiovascular effects are yet missing. Here, we tested various auditory stimulation approaches in a windowed, 10 s ON (stimulations) followed by 10 s OFF (no stimulations), within-night stimulation design and compared them to a SHAM control condition. We report the results of three studies and a total of 51 included nights and found a large and global increase in slow-wave activity (SWA) in the stimulation window compared to SHAM. Furthermore, slow-wave dynamics were most pronouncedly increased at the start of the stimulation and declined across the stimulation window. Beyond the changes in brain oscillations, we observed, for some conditions, a significant increase in the mean interval between two heartbeats within a stimulation window, indicating a slowing of the heart rate, and increased heart rate variability derived parasympathetic activity. Those cardiovascular changes were positively correlated with the change in SWA, and thus, our findings provide insight into the potential of auditory slow wave enhancement to modulate cardiovascular restorative conditions during sleep. However, future studies need to investigate whether the potentially increased restorative capacity through slow-wave enhancements translates into a more rested cardiovascular system on a subsequent day
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