Biomedical subjects

Mercedes Atienza

Publications and source records attributed to Mercedes Atienza.

13 recordsLinked to original sources

PubMed2005

The role of neural synchronization in the emergence of cognition across the wake-sleep cycle.

Searching for the neural code underlying consciousness and cognition is one of the most important activities in contemporary neuroscience. Research with neuronal oscillations at the level of single-neuron, local cell assemblies, and network system have provided invaluable insights into different mechanisms of synaptic interactions involved in the emergence of cognitive acts. A cognitive neuroscience of conscious experience is gradually emerging from behavioral and neuroimaging studies, which can be successfully complemented with the quantitative EEG findings discussed here. This review is an attempt to highlight the value of state-dependent changes in human neurophysiology for a better understanding of the neurobiological substrate underlying those aspects of cognition drastically affected by sleep states. Recent advances related to synchronization mechanisms potentially involved in brain integration processes are discussed, emphasizing the value of scalp and intracranial EEG recordings at determining local and large-scale dynamics in the human brain. Evidence supporting the critical role of state-dependent synchrony in brain integration comes mainly from studies on the theta and gamma oscillations across the wake-sleep continuum, as revealed by human intracranial recordings. This review blends results from different levels of analysis with the firm conviction that state-dependent brain dynamics at different levels of neural integration can provide a deeper understanding of neurobiological correlates of consciousness and sleep functions.

Animals↗

PubMed2004

Gamma EEG dynamics in neocortex and hippocampus during human wakefulness and sleep.

Little is known about the neurophysiological mechanisms underlying the human sleep-wake cycle. Using intracranial electrodes in humans, we investigated changes in topographic distribution of gamma power and local- and long-range gamma EEG coherence in neocortex and hippocampus during different cerebral states. We report significantly greater variability in gamma power across cortical regions during wakefulness than during either slow wave or rapid eye movement (REM) sleep. In addition, local (within cortical regions) and long-range (between cortical regions) gamma coherence was significantly higher during wakefulness than during sleep, and functional gamma-range coupling between the neocortex and hippocampus was seen during wakefulness, but not during sleep. These findings demonstrate a functional link between different stages of conscious awareness and the level of coupling of gamma-band oscillations in the human brain.

Adolescent↗

PubMed2004

Eyelid movements measured by Nightcap predict slow eye movements during quiet wakefulness in humans.

A precipitous decline in eyelid movements (ELMs) has been shown to be a highly reliable indicator of sleep onset. While ELMs correlate well with eye movements during waking and rapid eye movement (REM) sleep, the eye sensor remains silent during the period of slow eye movements (SEMs) typical of sleep onset. If the ELM density (e.g. ELMs per minute) dropped simultaneously with the appearance of SEMs prior to sleep onset, it could be a promising tool for identifying decreases in alertness prior to overt sleep onset. The present study was designed to determine whether the presence of SEMs in the transitional period preceding stage 1 sleep is reflected in decreases in ELM density. ELM densities were computed for 2.5-s epochs with and without SEMs, as well as for 15-s epochs. Decreases in ELM density not only were an excellent correlate of the appearance of SEMs during wakefulness with closed eyes, but also a good predictor of their occurrence (c. 82% accuracy) at a time resolution of 2.5 s. Based on these results, we conclude that ELM density reliably predicts moderate changes in the level of alertness during quiet wakefulness.

Adult↗

PubMed2003

Sleep-dependent theta oscillations in the human hippocampus and neocortex.

Hippocampal theta waves recorded during rapid eye movement (REM) sleep are thought to play a critical role in memory consolidation in lower mammals, but previous attempts to detect similar theta oscillations in the human hippocampus have been unsuccessful. Using subdural and depth recordings from epileptic patients, we now report the first evidence of state-dependent hippocampal theta waves (4-7 Hz) in humans. Unlike the continuous theta in rodents, however, these oscillations were consistently observed during REM sleep in short (approximately 1 sec) bursts and during transitions to wake in longer epochs. Theta waves were also observed in the basal temporal lobe and frontal cortex during transitions from sleep to wake and in quiet wakefulness but not in REM, and they were not coherent with hippocampal theta oscillations. The absence of functional coupling between neocortex and hippocampus during theta periods indicates that multiple theta generators exist in the human brain, and that they are dynamically regulated by brain state. Gamma oscillations were also present during REM theta bursts, but the fluctuations in gamma power were not associated with theta phase, pointing out another significant difference between rodent and human theta properties. Together, these findings suggest that the generation mechanisms of theta oscillations in humans might have evolved from tonic to phasic in hippocampus during REM sleep and extended from hippocampus to cortex, where they appear in certain wakefulness-related states.

Adolescent↗

PubMed2003

Effects of temporal encoding on auditory object formation: a mismatch negativity study.

Analysis of auditory scene relies on the appropriate division of the sensory input into meaningful events. The auditory system uses a sequential grouping strategy to segregate different acoustic events in the time domain. As a result of grouping, the organization of sound is expected to include acoustic events spanning different scales of time. It is unclear, however, at which moment the central auditory system begins to introduce changes in the neural representation of the auditory scene as a result of grouping different information units into a larger acoustic event. The present set of experiments examines this issue by using the mismatch negativity (MMN) component of event-related potentials. The MMN is elicited in response to changes in the previously registered auditory regularity whether or not the subject's attention is focused on the auditory stimuli. By presenting three consecutive pairs of alternating tones in serial stimulus trains, we found that across-train repetitions of the tone elicited the MMN with inter-train intervals up to 240 ms. Beyond this interval, only within-train repetitions of the tone elicited the MMN, suggesting that pairs of tones were bound together into a single acoustic event (i.e., train of six tones) within a temporal window not much longer than 240 ms. Furthermore, a different pattern of responses was seen depending on the position of the deviant event within the train and the frequency distance between the alternating tones. These results suggest that grouping discrete auditory inputs may introduce changes in the neural representation of the auditory scene by applying rules to sounds spanning larger temporal scales.

Acoustic Stimulation↗

PubMed2002

Effects of prolonged waking-auditory stimulation on electroencephalogram synchronization and cortical coherence during subsequent slow-wave sleep.

Evidence suggests that sleep homeostasis is not only dependent on duration of previous wakefulness but also on experience- and/or use-dependent processes. Such homeostatic mechanisms are reflected by selective increases in the duration of a sleep stage, modifications to electrophysiological-metabolic brain patterns in specific sleep states, and/or reactivation to neuronal ensembles in subsequent sleep periods. Use-dependent sleep changes, apparently different from those changes caused by memory consolidation processes, are thought to reflect neuronal restoration processes after the sustained exposure to stimulation during the preceding wakefulness. In the present study, we investigated changes in the brain electrical activity pattern during human sleep after 6 hr of continuous auditory stimulation during previous wakefulness. Poststimulation nights showed a widespread increase of spectral power within the alpha (8-12 Hz) and sleep spindle (12-15 Hz) frequency range during slow-wave sleep (SWS) compared with the baseline night. This effect was mainly attributable to an enhanced EEG amplitude rather than an increase of oscillations, except for temporal (within alpha and sleep spindles) and parietal regions (within sleep spindles) in which both parameters contributed equally to the increase of spectral energy. Power increments were accompanied by a strengthening of the coherence between fronto-temporal cortical regions within a broad frequency range during SWS but to the detriment of the coherence between temporal and parieto-occipital areas, suggesting underlying compensatory mechanisms between temporal and other cortical regions. In both cases, coherence was built up progressively across the night, although no changes were observed within each SWS period. No electrophysiological changes were found in rapid eye movement sleep. These results point to SWS as a critical brain period for correcting the cortical synaptic imbalance produced by the predominant use of specific neuronal populations during the preceding wakefulness, as well as for synaptic reorganization after prolonged exposure to a novel sensory experience.

Acoustic Stimulation↗

PubMed2002

Nightcap: a reliable system for determining sleep onset latency.

Previous studies have demonstrated that the Nightcap home-based sleep-monitoring system can differentiate waking, NREM and REM sleep based on eyelid movements (ELMs) and head movement behavior. The present study aims at determining the reliability of the Nightcap in determining the human sleep onset latency (SOL) as revealed by standard polysomnography (PSG). Four naps were recorded in each of ten normal subjects using both PSG and the Nightcap simultaneously. The Nightcap algorithm scored sleep onset as the first of 4 consecutive 30-sec epochs with less than 5 ELMs. The mean percentage of agreement between the Nightcap and PSG was 93% (k = 0.79), and the average absolute difference was 45 sec (13.3% of SOL(PSG)). SOL(NC) differed by less than 1 min in 85% of onsets. Recordings of EEG activity from 90 sec before and after PSG-identified sleep onsets were subjected to spectral analysis. Changes in spectral power in the theta (4-7 Hz) and alpha (8-12 Hz) frequency bands during the transition into light sleep correlated well with eyelid behavior. However, changes in ELM density predicted sleep onset better than did changes in theta and alpha spectral power. These results suggest that the Nightcap may be a potential alternative to the PSG technique in the assessment of SOL in normal subjects.

Adult↗

PubMed2002

Effects of waking-auditory stimulation on human sleep architecture.

Evidence suggests that sleep architecture is affected by endogenous homeostatic mechanisms as well as by behavioral and sensory demands during the prior wakefulness. Regarding the auditory system, sensory deprivation has shown to drastically modify the sleep structure, stressing the relevance of such sensory system for sleep organization. Changes in sleep architecture following prolonged auditory stimulation during prior wakefulness would provide additional support to this hypothesis. In the present study, auditory stimulation was administered over a 6 h period prior to sleep. Sleep parameters obtained from visual scoring were quantified across the total sleep period, for each sleep cycle, and for the two halves of the night, separately. Results showed that 6 h of waking-auditory stimulation were followed by an increase in the duration of slow wave sleep, a shortening of the latency between slow wave sleep periods, and a longer sleep onset latency as compared with the baseline night. In contrast, REM sleep parameters were unaffected by the pre-sleep auditory stimulation. These results indicate that sleep architecture depends on auditory demands during the prior wakefulness, suggesting that the local neural activation underlying auditory stimulation may trigger brain control mechanisms selectively involved in both the slow wave sleep maintenance and organization.

Acoustic Stimulation↗

PubMed2002

Mismatch negativity (MMN): an objective measure of sensory memory and long-lasting memories during sleep.

Sleep, unlike wakefulness, facilitates the internal stimulus generation and hinders the processing of external stimulation. Nevertheless, evidence yielded by physiological studies in animals and event-related potential (ERP) studies in humans suggest that basic functions of the central auditory system are still preserved during sleep. This review is focused on the automatic change-detection function of the auditory system as revealed by a negative ERP component called mismatch negativity (MMN). MMN mainly originates in the auditory cortex, although it also receives an important contribution from subcortical areas (especially at thalamic level), as well as frontal areas. We discuss recent experiments supporting the use of MMN as an objective measure of sensory memory and long-lasting memories not only during wakefulness, but also during sleep. The outcome of the activation of MMN generating system during sleep highly differs from that in waking, especially when there is no previous information about the stimulus sequence in the neuronal network as a result of learning. We discuss these differences in MMN generation in terms of a dynamicist view of the brain that emphasizes the importance of the integration between bottom-up and top-down influences on sensory processing, independently of the processing level in the auditory hierarchy.

Acoustic Stimulation↗

PubMed2002

Human alpha oscillations in wakefulness, drowsiness period, and REM sleep: different electroencephalographic phenomena within the alpha band.

Cortical oscillations in the range of alpha activity (8-13 Hz) are one of the fundamental electrophysiological phenomena of the human electroencephalogram (EEG). Evidence from quantitative EEG data has shown that their electrophysiological features, cortical generation mechanisms, and therefore, their functional correlates vary along the sleep-wake continuum. Specifically, spectral microstructure and EEG coherence levels between anterior and posterior cortical regions permit to differentiate among alpha activity spontaneously appearing in relaxed wakefulness with eyes closed, drowsiness period, and REM sleep, by reflecting distinct properties of neural networks involved in its cortical generation as well as a different interplay between cortical generators, respectively. Besides, the dissimilar spatiotemporal features of brain electrical microstates within the alpha range reveals a different geometry of active neural structures underlying each alpha variant or, simply, changes in the stability level of neural networks during each brain state. Studies reviewed in this paper support the hypothesis that two different alpha variants occur during human REM sleep: 'background responsive alpha activity', blocked over occipital regions when rapid eye movements are present, and 'REM-alpha bursts', non modulated by the alteration of tonic and phasic periods. Altogether, evidence suggests that electrophysiological features of human cortical oscillations in the alpha frequency range vary across different behavioural states, as well as within state, reflecting different cerebral phenomena with probably dissimilar functional meaning.

Alpha Rhythm↗

PubMed2002

The restorative effect of naps on perceptual deterioration.

Human performance on visual texture discrimination tasks improves slowly (over days) in the absence of additional training. This 'slow learning' requires nocturnal sleep after training and is limited to the region of visual space in which training occurred. Here, we tested human subjects four times in one day and found that with repeated, within-day testing, perceptual thresholds actually increased progressively across the four test sessions. This performance deterioration was prevented either by shifting the target stimuli to an untrained region of visual space or by having the subjects take a mid-day nap between the second and third sessions.

Humans↗

PubMedDate not supplied

The time course of neural changes underlying auditory perceptual learning.

Improvement in perception takes place within the training session and from one session to the next. The present study aims at determining the time course of perceptual learning as revealed by changes in auditory event-related potentials (ERPs) reflecting preattentive processes. Subjects were trained to discriminate two complex auditory patterns in a single session. ERPs were recorded just before and after training, while subjects read a book and ignored stimulation. ERPs showed a negative wave called mismatch negativity (MMN)-which indexes automatic detection of a change in a homogeneous auditory sequence-just after subjects learned to consciously discriminate the two patterns. ERPs were recorded again 12, 24, 36, and 48 h later, just before testing performance on the discrimination task. Additional behavioral and neurophysiological changes were found several hours after the training session: an enhanced P2 at 24 h followed by shorter reaction times, and an enhanced MMN at 36 h. These results indicate that gains in performance on the discrimination of two complex auditory patterns are accompanied by different learning-dependent neurophysiological events evolving within different time frames, supporting the hypothesis that fast and slow neural changes underlie the acquisition of improved perception.

Adolescent↗

PubMedDate not supplied

Posttraining sleep enhances automaticity in perceptual discrimination.

Perceptual learning can develop over extended periods, with slow, at times sleep-dependent, improvement seen several days after training. As a result, performance can become more automatic, that is, less dependent on voluntary attention. This study investigates whether the brain correlates of this enhancement of automaticity are sleep-dependent. Event-related potentials produced in response to complex auditory stimuli were recorded while subjects' attention was focused elsewhere. We report here that following training on an auditory discrimination task, performance continued to improve, without significant further training, for 72 hr. At the same time, several event-related potential components became evident 48-72 hr after training. Posttraining sleep deprivation prevented neither the continued performance improvement nor the slow development of cortical dynamics related to an enhanced familiarity with the task. However, those brain responses associated with the automatic shift of attention to unexpected stimuli failed to develop. Thus, in this auditory learning paradigm, posttraining sleep appears to reduce the voluntary attentional effort required for successful perceptual discrimination by facilitating the intrusion of a potentially meaningful stimulus into one's focus of attention for further evaluation.

Adult↗