No full-text available
To read the full-text of this research,
you can request a copy directly from the author.
Exploring the brain bases of dreaming. Commentary on "Beyond the neuropsychology of dreaming: Insights into the neural basis of dreaming with new techniques of sleep recording and analysis." Sleep Medicine Reviews
Jean Lhermitte (1877-1959) was one of the pioneers of behavioral neurology, including the field of hallucinations. This article focuses on his work concerning the relationship between hallucinations, sleep, and dreams. From 1910, Lhermitte became interested in sleep and its disorders, particularly narcolepsy and its accompanying symptoms. He also reported on sleep disorders and hallucinations occurring in people with lesions of the diencephalic region ("infundibular syndrome"), and later encephalitis lethargica. In 1922, he described a syndrome of complex, predominantly visual hallucinations in patients with vascular damage to the midbrain, known as peduncular hallucinosis. Twelve historical cases of peduncular hallucinosis, including 10 from Lhermitte, are reviewed. He gave a precise phenomenological description of peduncular hallucinosis, and put forward the hypothesis that the lesion disrupted the anatomy and connections of a center regulating wakefulness and sleep, thus enabling a dissociation of the mechanisms of dream and waking states. Although the pathophysiology of peduncular hallucinosis remains to this day partly obscure, the model of a limited subcortical lesion acting through complex mechanisms and ultimately involving the cortex remains valid. Lhermitte was also a pioneer in characterizing what contemporary sleep specialists call dissociation of states.
The common knowledge of a uniqueness of REM sleep as a privileged scenario of dreaming still persists, although consolidated empirical evidence shows that the assumption that dreaming is just an epiphenomenon of REM sleep is no longer tenable. However, the brain mechanisms underlying dream generation and its encoding in memory during NREM sleep are still mostly unknown. In fact, only few studies have investigated on the mechanisms of dream phenomenology related to NREM sleep. For this reason, our study is specifically aimed to elucidate the electrophysiological (EEG) correlates of dream recall (DR) upon NREM sleep awakenings. Under the assumption that EEG activity predicts the presence/absence of DR also during NREM sleep, we have investigated whether DR from stage 2 NREM sleep shares similar brain mechanisms to those involved in the encoding of episodic memory during wakefulness, or it depends on the specific electrophysiological milieu of the sleep period along the desynchronized/synchronized EEG continuum. We collected DR from a multiple nap protocol in a within-subjects design. We found that DR is predicted by an extensive reduction of delta activity during the last segment of sleep, encompassing left frontal and temporo-parietal areas. The results could represent an update on the mechanisms underlying the sleep mentation during NREM sleep. In particular, they support the hypothesis that an increased cortical EEG activation is a prerequisite for DR, and they are not necessarily in conflict with the hypothesis of common wake-sleep mechanisms. We also confirmed that EEG correlates of DR depend on a state-like relationship.
Significance
A major challenge in neuroscience is our limited ability to image neural signals noninvasively in humans. Oscillations in brain activity are important for perception, attention, and awareness, and progress in cognitive neuroscience depends on localizing these patterns. fMRI is thought to be too slow to measure brain oscillations because it depends on slow changes in blood flow. Here, we use recently developed imaging techniques to show that fMRI can measure faster neural oscillations than previously thought, and responses are 10 times larger than expected. With computational modeling and simultaneous electroencephalography we show that vascular responses are surprisingly fast when brain activity fluctuates rapidly. These results suggest that fMRI can be used to track oscillating brain activity directly during human cognition.
Auditory-verbal hallucinations (AVHs) are frequent and disabling symptoms, which can be refractory to conventional psychopharmacological treatment in more than 25% of the cases. Recent advances in brain imaging allow for a better understanding of the neural underpinnings of AVHs. These findings strengthened transdiagnostic neurocognitive models that characterize these frequent and disabling experiences. At the same time, technical improvements in real-time functional magnetic resonance imaging (fMRI) enabled the development of innovative and non-invasive methods with the potential to relieve psychiatric symptoms, such as fMRI-based neurofeedback (fMRI-NF). During fMRI-NF, brain activity is measured and fed back in real time to the participant in order to help subjects to progressively achieve voluntary control over their own neural activity. Precisely defining the target brain area/network(s) appears critical in fMRI-NF protocols. After reviewing the available neurocognitive models for AVHs, we elaborate on how recent findings in the field may help to develop strong a priori strategies for fMRI-NF target localization. The first approach relies on imaging-based “trait markers” (i.e., persistent traits or vulnerability markers that can also be detected in the presymptomatic and remitted phases of AVHs). The goal of such strategies is to target areas that show aberrant activations during AVHs or are known to be involved in compensatory activation (or resilience processes). Brain regions, from which the NF signal is derived, can be based on structural MRI and neurocognitive knowledge, or functional MRI information collected during specific cognitive tasks. Because hallucinations are acute and intrusive symptoms, a second strategy focuses more on “state markers.” In this case, the signal of interest relies on fMRI capture of the neural networks exhibiting increased activity during AVHs occurrences, by means of multivariate pattern recognition methods. The fine-grained activity patterns concomitant to hallucinations can then be fed back to the patients for therapeutic purpose. Considering the potential cost necessary to implement fMRI-NF, proof-of-concept studies are urgently required to define the optimal strategy for application in patients with AVHs. This technique has the potential to establish a new brain imaging-guided psychotherapy for patients that do not respond to conventional treatments and take functional neuroimaging to therapeutic applications.
The <1Hz slow oscillation (SO) and spindles are hallmarks of mammalian non-rapid eye movement and slow wave sleep. Spindle activity occurring phase-locked to the SO is considered a candidate mediator of memory consolidation during sleep. We used source localization of magnetoencephalographic (MEG) and electroencephalographic (EEG) recordings from 11 sleeping human subjects for an in-depth analysis of the temporal and spatial properties of sleep spindles co-occurring with SOs. Slow oscillations and spindles were identified in the EEG and related to the MEG signal, providing enhanced spatial resolution. In the temporal domain, we confirmed a phase-locking of classical 12-15Hz fast spindle activity to the depolarizing SO up-state and of 9-12Hz slow spindle activity to the up-to-down-state transition of the SO. In the spatial domain, we show a broad spread of spindle activity, with less distinct anterior-posterior separation of fast and slow spindles than commonly seen in the EEG. We further tested a prediction of current memory consolidation models, namely the existence of a spatial bias of SOs over sleep spindles as a mechanism to promote localized neuronal synchronization and plasticity. In contrast to that prediction, a comparison of SOs dominating over the left vs. right hemisphere did not reveal any signs of a concurrent lateralization of spindle activity co-occurring with these SOs. Our data are consistent with the concept of the neocortical SO exerting a top-down control over thalamic spindle generation. However, they call into question the notion that SOs locally coordinate spindles and thereby inform spindle-related memory processing.
Are rapid eye movements (REMs) in sleep associated with visual-like activity, as during wakefulness? Here we examine single-unit activities (n ¼ 2,057) and intracranial electroencephalography across the human medial temporal lobe (MTL) and neocortex during sleep and wakefulness, and during visual stimulation with fixation. During sleep and wakefulness, REM onsets are associated with distinct intracranial potentials, reminiscent of ponto-geniculate-occipital waves. Individual neurons, especially in the MTL, exhibit reduced firing rates before REMs as well as transient increases in firing rate immediately after, similar to activity patterns observed upon image presentation during fixation without eye movements. Moreover, the selectivity of individual units is correlated with their response latency, such that units activated after a small number of images or REMs exhibit delayed increases in firing rates. Finally, the phase of theta oscillations is similarly reset following REMs in sleep and wakefulness, and after controlled visual stimulation. Our results suggest that REMs during sleep rearrange discrete epochs of visual-like processing as during wakefulness.
Dreaming represents a peculiar form of cognitive activity during sleep. On the basis of the well-known relationship between sleep and memory, there has been a growing interest in the predictive role of human brain activity during sleep on dream recall. Neuroimaging studies indicate that rapid eye movement (REM) sleep is characterized by limbic activation and prefrontal cortex deactivation. This pattern could explain the presence of emotional contents in dream reports. Furthermore, the morphoanatomical measures of amygdala and hippocampus predict some features of dream contents (bizarreness, vividness, and emotional load). More relevant for a general view of dreaming mechanisms, empirical data from neuropsychological and electroencephalographic (EEG) studies support the hypothesis that there is a sort of continuity between the neurophysiological mechanisms of encoding and retrieval of episodic memories across sleep and wakefulness. A notable overlap between the electrophysiological mechanisms underlying emotional memory formation and some peculiar EEG features of REM sleep has been suggested. In particular, theta (5–8 Hz) EEG oscillations on frontal regions in the pre-awakening sleep are predictive of dream recall, which parallels the predictive relation during wakefulness between theta activity and successful retrieval of episodic memory. Although some observations support an interpretation more in terms of an intraindividual than interindividual mechanism, the existing empirical evidence still precludes from definitely disentangling if this relation is explained by state- or trait-like differences.
Dreaming is still poorly understood. Notably, its cerebral underpinning remains unclear. Neuropsychological studies have shown that lesions in the temporoparietal junction (TPJ) and/or the white matter of the medial prefrontal cortex (MPFC) lead to the global cessation of dream reports, suggesting that these regions of the default mode network have key roles in the dreaming process (forebrain 'dream-on' hypothesis). To test this hypothesis, we measured regional cerebral blood flow (rCBF) using [(15)O]H2O positron emission tomography in healthy subjects with high and low dream recall frequencies (DRFs) during wakefulness (rest) and sleep (rapid eye movement (REM) sleep, N2, and N3). Compared with Low recallers (0.5±0.3 dream recall per week in average), High recallers (5.2±1.4) showed higher rCBF in the TPJ during REM sleep, N3, and wakefulness, and in the MPFC during REM sleep and wakefulness. We demonstrate that the resting states of High recallers and Low recallers differ during sleep and wakefulness. It coheres with previous ERP results and confirms that a high/low DRF is associated with a specific functional organization of the brain. These results support the forebrain 'dream-on' hypothesis and suggest that TPJ and MPFC are not only involved in dream recall during wakefulness but also have a role in dreaming during sleep (production and/or encoding). Increased activity in the TPJ and MPFC might promote the mental imagery and/or memory encoding of dreams. Notably, increased activity in TPJ might facilitate attention orienting toward external stimuli and promote intrasleep wakefulness, facilitating the encoding of the dreams in memory.Neuropsychopharmacology advance online publication, 19 February 2014; doi:10.1038/npp.2014.6.
Isolated reports have long suggested a similarity in content and thought processes across mind wandering (MW) during waking, and dream mentation during sleep. This overlap has encouraged speculation that both “daydreaming” and dreaming may engage similar brain mechanisms. To explore this possibility, we systematically examined published first-person experiential reports of MW and dreaming and found many similarities: in both states, content is largely audiovisual and emotional, follows loose narratives tinged with fantasy, is strongly related to current concerns, draws on long-term memory, and simulates social interactions. Both states are also characterized by a relative lack of meta-awareness. To relate first-person reports to neural evidence, we compared meta-analytic data from numerous functional neuroimaging (PET, fMRI) studies of the default mode network (DMN, with high chances of MW) and rapid eye movement (REM) sleep (with high chances of dreaming). Our findings show large overlaps in activation patterns of cortical regions: similar to MW/DMN activity, dreaming and REM sleep activate regions implicated in self-referential thought and memory, including medial prefrontal cortex (PFC), medial temporal lobe structures, and posterior cingulate. Conversely, in REM sleep numerous PFC executive regions are deactivated, even beyond levels seen during waking MW. We argue that dreaming can be understood as an “intensified” version of waking MW: though the two share many similarities, dreams tend to be longer, more visual and immersive, and to more strongly recruit numerous key hubs of the DMN. Further, whereas MW recruits fewer PFC regions than goal-directed thought, dreaming appears to be characterized by an even deeper quiescence of PFC regions involved in cognitive control and metacognition, with a corresponding lack of insight and meta-awareness. We suggest, then, that dreaming amplifies the same features that distinguish MW from goal-directed waking thought.
Dreams create new stories out of nothing. Although dreams contain themes, concerns, dream figures, objects, etc. that correspond closely to waking life, these are only story elements. The story itself weaves these mnemonic items together in a manner far more novel than a simple assemblage or collage, producing an experience having a life-like timeframe and life-like (if often bizarre and impossible) causality (Pace-Schott, 2007; Hobson, 2009). It is as if one is immersed in another “reality” entirely of one's own non-volitional, making (see Rechtschaffen, 1978). Given this phenomenology, it's not difficult to see why some indigenous animist societies believe dreams represent a separate world parallel with waking life (Nielsen, 1991). But neuroscience offers some other explanations.
Positron emission tomography was used to measure cerebral activity and to evaluate regional interrelationships within visual
cortices and their projections during rapid eye movement (REM) sleep in human subjects. REM sleep was associated with selective
activation of extrastriate visual cortices, particularly within the ventral processing stream, and an unexpected attenuation
of activity in the primary visual cortex; increases in regional cerebral blood flow in extrastriate areas were significantly
correlated with decreases in the striate cortex. Extrastriate activity was also associated with concomitant activation of
limbic and paralimbic regions, but with a marked reduction of activity in frontal association areas including lateral orbital
and dorsolateral prefrontal cortices. This pattern suggests a model for brain mechanisms subserving REM sleep where visual
association cortices and their paralimbic projections may operate as a closed system dissociated from the regions at either
end of the visual hierarchy that mediate interactions with the external world.
The neurophysiological correlates of dreaming remain unclear. According to the "arousal-retrieval" model, dream encoding depends on intrasleep wakefulness. Consistent with this model, subjects with high and low dream recall frequency (DRF) report differences in intrasleep awakenings. This suggests a possible neurophysiological trait difference between the 2 groups. To test this hypothesis, we compared the brain reactivity (evoked potentials) of subjects with high (HR, N = 18) and low (LR, N = 18) DRF during wakefulness and sleep. During data acquisition, the subjects were presented with sounds to be ignored (first names randomly presented among pure tones) while they were watching a silent movie or sleeping. Brain responses to first names dramatically differed between the 2 groups during both sleep and wakefulness. During wakefulness, the attention-orienting brain response (P3a) and a late parietal response were larger in HR than in LR. During sleep, we also observed between-group differences at the latency of the P3a during N2 and at later latencies during all sleep stages. Our results demonstrate differences in the brain reactivity of HR and LR during both sleep and wakefulness. These results suggest that the ability to recall dreaming is associated with a particular cerebral functional organization, regardless of the state of vigilance.
Abstract The function of sleep in humans has been investigated using simultaneous electroencephalography (EEG) and functional magnetic resonance imaging recordings to provide accurate sleep scores with spatial precision. Recent studies have demonstrated that spontaneous brain oscillations and functional connectivity dissociate during nonrapid eye movement (NREM) sleep; this leads to spontaneous cognitive processes, such as memory consolidation and emotional modulation. However, variations in network connectivity across the sleep stages or between sleep/wake transitions require further elucidation. We observed changes in the connectivity of the sensorimotor and default-mode networks (DMN) mediated by midnight sleep among 18 healthy participants. The results indicated that (1) functional connectivity in both networks showed increasing dissociation as NREM sleep deepened, whereas hyperconnectivity occurred during rapid eye movement (REM) sleep; and (2) compared with connectivity before sleep, the DMN presented a comparable connectivity pattern immediately after awakening, whereas the connectivity of the sensorimotor network remained disrupted. These findings showed that connectivity patterns dissociate and reconnect coherently in both cortical networks during NREM and REM sleep, respectively. After the person awakened, the DMN connectivity was re-established before the sensorimotor reconnection. These dynamic sleep-related dissociations and reconnections between sleep/wake conditions might provide the key to understanding cognitive modulations in sleep. If so, connectivity changes might serve as an alternative indicator beyond the EEG signature to unveil the spontaneous processes that occur during sleep.
High density EEG (hdEEG) during sleep combines the superior temporal resolution of EEG recordings with high spatial resolution. Thus, this method allows a topographical analysis of sleep EEG activity and thereby fosters the shift from a global view of sleep to a local one. HdEEG allowed to investigate sleep rhythms in terms of their characteristic behavior (e.g., the traveling of slow waves) and in terms of their relationship to cortical functioning (e.g., consciousness and cognitive abilities). Moreover, recent studies successfully demonstrated that hdEEG can be used to study brain functioning in neurological and neuro-developmental disorders, and to evaluate therapeutic approaches. This review highlights the potential, the problems, and future perspective of hdEEG in sleep research.
The simultaneous recording and analysis of electroencephalography (EEG) and fMRI data in human systems, cognitive and clinical neurosciences is rapidly evolving and has received substantial attention. The significance of multimodal brain imaging is documented by a steadily increasing number of laboratories now using simultaneous EEG-fMRI aiming to achieve both high temporal and spatial resolution of human brain function. Due to recent developments in technical and algorithmic instrumentation, the rate-limiting step in multimodal studies has shifted from data acquisition to analytic aspects. Here, we introduce and compare different methods for data integration and identify the benefits that come with each approach, guiding the reader toward an understanding and informed selection of the integration approach most suitable for addressing a particular research question.
Information processing in the cerebral cortex involves interactions among distributed areas. Anatomical connectivity suggests that certain areas form local hierarchical relations such as within the visual system. Other connectivity patterns, particularly among association areas, suggest the presence of large-scale circuits without clear hierarchical relations. In this study the organization of networks in the human cerebrum was explored using resting-state functional connectivity MRI. Data from 1,000 subjects were registered using surface-based alignment. A clustering approach was employed to identify and replicate networks of functionally coupled regions across the cerebral cortex. The results revealed local networks confined to sensory and motor cortices as well as distributed networks of association regions. Within the sensory and motor cortices, functional connectivity followed topographic representations across adjacent areas. In association cortex, the connectivity patterns often showed abrupt transitions between network boundaries. Focused analyses were performed to better understand properties of network connectivity. A canonical sensory-motor pathway involving primary visual area, putative middle temporal area complex (MT+), lateral intraparietal area, and frontal eye field was analyzed to explore how interactions might arise within and between networks. Results showed that adjacent regions of the MT+ complex demonstrate differential connectivity consistent with a hierarchical pathway that spans networks. The functional connectivity of parietal and prefrontal association cortices was next explored. Distinct connectivity profiles of neighboring regions suggest they participate in distributed networks that, while showing evidence for interactions, are embedded within largely parallel, interdigitated circuits. We conclude by discussing the organization of these large-scale cerebral networks in relation to monkey anatomy and their potential evolutionary expansion in humans to support cognition.
Under the assumption that dream recall is a peculiar form of declarative memory, we have hypothesized that (1) the encoding of dream contents during sleep should share some electrophysiological mechanisms with the encoding of episodic memories of the awake brain and (2) recalling a dream(s) after awakening from non-rapid eye movement (NREM) and rapid eye movement (REM) sleep should be associated with different brain oscillations. Here, we report that cortical brain oscillations of human sleep are predictive of successful dream recall. In particular, after morning awakening from REM sleep, a higher frontal 5-7 Hz (theta) activity was associated with successful dream recall. This finding mirrors the increase in frontal theta activity during successful encoding of episodic memories in wakefulness. Moreover, in keeping with the different EEG background, a different predictive relationship was found after awakening from stage 2 NREM sleep. Specifically, a lower 8-12 Hz (alpha) oscillatory activity of the right temporal area was associated with a successful dream recall. These findings provide the first evidence of univocal cortical electroencephalographic correlates of dream recall, suggesting that the neurophysiological mechanisms underlying the encoding and recall of episodic memories may remain the same across different states of consciousness.
The recent discovery of a circuit of brain regions that is highly active in the absence of overt behavior has led to a quest for revealing the possible function of this so-called default-mode network (DMN). A very recent study, finding similarities in awake humans and anesthetized primates, has suggested that DMN activity might not simply reflect ongoing conscious mentation but rather a more general form of network dynamics typical of complex systems. Here, by performing functional MRI in humans, it is shown that a natural, sleep-induced reduction of consciousness is reflected in altered correlation between DMN network components, most notably a reduced involvement of frontal cortex. This suggests that DMN may play an important role in the sustenance of conscious awareness.
To identify the neural substrate of rapid eye movements (REMs) during REM sleep in humans, we conducted simultaneous functional magnetic resonance imaging (fMRI) and polysomnographic recording during REM sleep. Event-related fMRI analysis time-locked to the occurrence of REMs revealed that the pontine tegmentum, ventroposterior thalamus, primary visual cortex, putamen and limbic areas (the anterior cingulate, parahippocampal gyrus and amygdala) were activated in association with REMs. A control experiment during which subjects made self-paced saccades in total darkness showed no activation in the visual cortex. The REM-related activation of the primary visual cortex without visual input from the retina provides neural evidence for the existence of human ponto-geniculo-occipital waves (PGO waves) and a link between REMs and dreaming. Furthermore, the time-course analysis of blood oxygenation level-dependent responses indicated that the activation of the pontine tegmentum, ventroposterior thalamus and primary visual cortex started before the occurrence of REMs. On the other hand, the activation of the putamen and limbic areas accompanied REMs. The activation of the parahippocampal gyrus and amygdala simultaneously with REMs suggests that REMs and/or their generating mechanism are not merely an epiphenomenon of PGO waves, but may be linked to the triggering activation of these areas.
Rapid-eye-movement (REM) sleep is associated with intense neuronal activity, ocular saccades, muscular atonia and dreaming. The function of REM sleep remains elusive and its neural correlates have not been characterized precisely in man. Here we use positron emission tomography and statistical parametric mapping to study the brain state associated with REM sleep in humans. We report a group study of seven subjects who maintained steady REM sleep during brain scanning and recalled dreams upon awakening. The results show that regional cerebral blood flow is positively correlated with REM sleep in pontine tegmentum, left thalamus, both amygdaloid complexes, anterior cingulate cortex and right parietal operculum. Negative correlations between regional cerebral blood flow and REM sleep are observed bilaterally, in a vast area of dorsolateral prefrontal cortex, in parietal cortex (supramarginal gyrus) as well as in posterior cingulate cortex and precuneus. Given the role of the amygdaloid complexes in the acquisition of emotionally influenced memories, the pattern of activation in the amygdala and the cortical areas provides a biological basis for the processing of some types of memory during REM sleep.
The distribution of regional cerebral blood flow (rCBF) was estimated during sleep and wakefulness by using H 2 ¹⁵ O positron emission tomography (PET) and statistical parametric mapping. A group analysis on 11 good sleepers (8 with steady slow wave sleep, SWS) showed a significant negative correlation between the occurrence of SWS and rCBF in dorsal pons and mesencephalon, thalami, basal ganglia, basal forebrain/hypothalamus, orbitofrontal cortex, anterior cingulate cortex, precuneus, and, on the right side, in a region that follows the medial aspect of the temporal lobe. Given the known decrease in global cerebral blood flow levels during SWS, these negative correlations suggest that rCBF is decreased significantly more in these cerebral areas than in the rest of the brain.
The marked rCBF decreases in the pons, mesencephalon, thalamic nuclei, and basal forebrain reflect their close implication in the generation of SWS rhythms. The influence of these rhythms on the telencephalon usually are thought to be global and homogeneous. In contrast, our results show that rCBF is decreased more in some cortical areas (especially in orbitofrontal cortex) than in the rest of the cortex. We hypothesize that cellular processes taking place during SWS might be modulated differently in these regions.
Given the functions of the ventromedial frontal areas, we surmise that SWS might be particularly critical for the adaptation of behavior to environmental pressures. This hypothesis is supported indirectly by results of sleep deprivation experiments.
To assess dynamic changes in brain function throughout the sleep-wake cycle, CBF was measured with H2(15)O and PET in 37 normal male volunteers: (i) while awake prior to sleep onset; (ii) during Stage 3-4 sleep, i.e. slow wave sleep (SWS); (iii) during rapid eye movement (REM) sleep; and (iv) upon waking following recovery sleep. Subjects were monitored polysomnographically and PET images were acquired throughout the course of a single night. Stage-specific contrasts were performed using statistical parametric mapping. Data were analysed in repeated measures fashion, examining within-subject differences between stages [pre-sleep wakefulness-SWS (n = 20 subjects); SWS-post-sleep wakefulness (n = 14); SWS-REM sleep (n = 7); pre-sleep wakefulness-REM sleep (n = 8); REM sleep-post-sleep wakefulness (n = 7); pre-sleep wakefulness-post-sleep wakefulness (n = 20)]. State dependent changes in the activity of centrencephalic regions, including the brainstem, thalamus and basal forebrain (profound deactivations during SWS and reactivations during REM sleep) are consistent with the idea that these areas are constituents of brain systems which mediate arousal. Shifts in the level of activity of the striatum suggested that the basal ganglia might be more integrally involved in the orchestration of the sleep-wake cycle than previously thought. State-dependent changes in the activity of limbic and paralimbic areas, including the insula, cingulate and mesial temporal cortices, paralleled those observed in centrencephalic structures during both REM sleep and SWS. A functional dissociation between activity in higher order, heteromodal association cortices in the frontal and parietal lobes and unimodal sensory areas of the occipital and temporal lobes appeared to be characteristic of both SWS and REM sleep. SWS was associated with selective deactivation of the heteromodal association areas, while activity in primary and secondary sensory cortices was preserved. SWS may not, as previously thought, represent a generalized decrease in neuronal activity. On the other hand, REM sleep was characterized by selective activation of certain post-rolandic sensory cortices, while activity in the frontoparietal association cortices remained depressed. REM sleep may be characterized by activation of widespread areas of the brain, including the centrencephalic, paralimbic and unimodal sensory regions, with the specific exclusion of areas which normally participate in the highest order analysis and integration of neural information. Deactivation of the heteromodal association areas (the orbital, dorsolateral prefrontal and inferior parietal cortices) constitutes the single feature common to both non-REM and REM sleep states, and may be a defining characteristic of sleep itself. The stages of sleep could also be distinguished by characteristic differences in the relationships between the basal ganglia, thalamic nuclei and neocortical regions of interest.
Despite recent advances in functional neuroimaging, the apparently simple question of how and where we see--the neurobiology of visual consciousness--continues to challenge neuroscientists. Without a method to differentiate neural processing specific to consciousness from unconscious afferent sensory signals, the issue has been difficult to resolve experimentally. Here we use functional magnetic resonance imaging (fMRI) to study patients with the Charles Bonnet syndrome, for whom visual perception and sensory input have become dissociated. We found that hallucinations of color, faces, textures and objects correlate with cerebral activity in ventral extrastriate visual cortex, that the content of the hallucinations reflects the functional specializations of the region and that patients who hallucinate have increased ventral extrastriate activity, which persists between hallucinations.
8 Ss (paid volunteer males, ages 17-27) were placed in the information-gathering sleep situation for 57 (nonconsecutive) nights. Ss were awakened during various stages of sleep (determined by EEG and eye movement activity), and the content of their dream or thoughts at the time of the awakening explored. Mental activity (dreaming, thinking) was reported at all levels of sleep. Reports during periods of rapid eye movement (REM) revealed more statements involving affective, visual, and muscular content with less correspondence to residue of S's waking life, than in non-REM periods. Results are related to Freudian theory of dreams.
Compared with waking state attention, volition and semantic processing play a minor role during sleep. Thus, investigating declarative memory formation during sleep may allow us to isolate mnemonic core processes. The most feasible approach to memory formation during sleep is the analysis of dream memories. Lesion and imaging studies have demonstrated that encoding of declarative memories, i.e. consciously accessible events and facts, depends on operations within the rhinal cortex and the hippocampus, two substructures of the medial temporal lobe. Successful memory formation is accompanied by a transient rhinal-hippocampal interaction. Consequently, the ability to memorize dreams may be related to mediotemporal connectivity. Therefore, we recorded EEG during sleep from rhinal and hippocampal depth electrodes implanted in 12 epilepsy patients (eight women, mean age 41.1 +/- 6.4 years). They were awakened during rapid eye movement sleep (REM) and asked to recall their dream. Via coherence analyses we show that rhinal-hippocampal connectivity values are approximately twice as large for patients with good dream recall versus those patients with poor recall. This suggests that rhinal-hippocampal connectivity is a key factor in determining declarative memory formation.
Objective:
Our aim was to map the in vivo human functional connectivity of several brainstem nuclei with the rest of the brain by using seed-based correlation of ultra-high magnetic field functional magnetic resonance imaging (fMRI) data.
Materials and methods:
We used the recently developed template of 11 brainstem nuclei derived from multi-contrast structural MRI at 7 Tesla as seed regions to determine their connectivity to the rest of the brain. To achieve this, we used the increased contrast-to-noise ratio of 7-Tesla fMRI compared with 3 Tesla and time-efficient simultaneous multi-slice imaging to cover the brain with high spatial resolution (1.1-mm isotropic nominal resolution) while maintaining a short repetition time (2.5 s).
Results:
The delineated Pearson's correlation-based functional connectivity diagrams (connectomes) of 11 brainstem nuclei of the ascending arousal, motor, and autonomic systems from 12 controls are presented and discussed in the context of existing histology and animal work.
Conclusion:
Considering that the investigated brainstem nuclei play a crucial role in several vital functions, the delineated preliminary connectomes might prove useful for future in vivo research and clinical studies of human brainstem function and pathology, including disorders of consciousness, sleep disorders, autonomic disorders, Parkinson's disease, and other motor disorders.
This authoritative guide to sleep medicine is also available as an e-dition, book (ISBN: 1416003207) plus updated online reference! The new edition of this definitive resource has been completely revised and updated to provide all of the latest scientific and clinical advances. Drs. Kryger, Roth, and Dementand over 170 international expertsdiscuss the most recent data, management guidelines, and treatments for a full range of sleep problems. Representing a wide variety of specialties, including pulmonary, neurology, psychiatry, cardiology, internal medicine, otolaryngology, and primary care, this whos who of experts delivers the most compelling, readable, and scientifically accurate source of sleep medicine available today. Includes user-friendly synopses of important background information before all basic science chapters. Provides expert coverage of narcolepsy * movement disorders * breathing disorders * gastrointestinal problems * neurological conditions * psychiatric disturbances * substance abuse * and more. Discusses hot topics such as the genetic mechanisms of circadian rhythms * the relationship between obesity, hormones, and sleep apnea * sleep apnea and arterial hypertension * and more. Includes a new section on Cardiovascular Disorders that examines the links between sleep breathing disorders and cardiovascular abnormalities, as well as the use of sleep related therapies for congestive heart failure. Provides a new section on Womens Health and Sleep Disorders that includes information on the effects of hormonal changes during pregnancy and menopause on sleep. Features the fresh perspectives of 4 new section editors. Employs a more consistent chapter organization for better readability and easier navigation.
We investigated the role of the dopamine system [i.e., subcortical-medial prefrontal cortex (mPFC) network] in dreaming, by studying patients with Parkinson's Disease (PD) as a model of altered dopaminergic transmission. Subcortical volumes and cortical thickness were extracted by 3T-MR images of 27 PD patients and 27 age-matched controls, who were asked to fill out a dream diary upon morning awakening for one week. PD patients do not substantially differ from healthy controls with respect to the sleep, dream, and neuroanatomical measures. Multivariate correlational analyses in PD patients show that dopamine agonist dosage is associated to qualitatively impoverished dreams, as expressed by lower bizarreness and lower emotional load values. Visual vividness (VV) of their dream reports positively correlates with volumes of both the amygdalae and with thickness of the left mPFC. Emotional load also positively correlates with hippocampal volume. Beside the replication of our previous finding on the role of subcortical nuclei in dreaming experience of healthy subjects, this represents the first evidence of a specific role of the amygdala-mPFC dopaminergic network system in dream recall. The association in PD patients between higher dopamine agonist dosages and impoverished dream reports, however, and the significant correlations between VV and mesolimbic regions, however, provide an empirical support to the hypothesis that a dopamine network plays a key role in dream generation. The causal relation is however precluded by the intrinsic limitation of assuming the dopamine agonist dosage as a measure of the hypodopaminergic state in PD. Hum Brain Mapp, 2015.
The bed nucleus of the stria terminalis (BNST), a portion of the "extended amygdala," is implicated in the pathophysiology of anxiety and addiction disorders. Its small size and connection to other small regions prevents standard imaging techniques from easily capturing it and its connectivity with confidence. Seed-based resting state functional connectivity is an established method for mapping functional connections across the brain from a region of interest. We, therefore, mapped the BNST resting state network with high spatial resolution using 7 Tesla fMRI, demonstrating the in vivo reproduction of many human BNST connections previously described only in animal research. We identify strong BNST functional connectivity in amygdala, hippocampus and thalamic subregions, caudate, periaqueductal gray, hypothalamus, and cortical areas such as the medial PFC and precuneus. This work, which demonstrates the power of ultra-high field for mapping functional connections in the human, is an important step toward elucidating cortical and subcortical regions and subregions of the BNST network. Hum Brain Mapp, 2015. © 2015 Wiley Periodicals, Inc.
© 2015 Wiley Periodicals, Inc.
The neural basis and cognitive functions of various spontaneous thought processes, particularly mind-wandering, are increasingly being investigated. Although strong links have been drawn between the occurrence of spontaneous thought processes and activation in brain regions comprising the default mode network (DMN), spontaneous thought also appears to recruit other, non-DMN regions just as consistently. Here we present the first quantitative meta-analysis of neuroimaging studies of spontaneous thought and mind-wandering in order to address the question of their neural correlates. Examining 24 functional neuroimaging studies of spontaneous thought processes, we conducted a meta-analysis using activation likelihood estimation (ALE). A number of key DMN areas showed consistent recruitment across studies, including medial prefrontal cortex, posterior cingulate cortex, medial temporal lobe, and bilateral inferior parietal lobule. Numerous non-DMN regions, however, were also consistently recruited, including rostrolateral prefrontal cortex, dorsal anterior cingulate cortex, insula, temporopolar cortex, secondary somatosensory cortex, and lingual gyrus. These meta-analytic results indicate that DMN activation alone is insufficient to adequately capture the neural basis of spontaneous thought; frontoparietal control network areas, and other non-DMN regions, appear to be equally central. We conclude that further progress in the cognitive and clinical neuroscience of spontaneous thought will therefore require a re-balancing of our view of the contributions of various regions and networks throughout the brain, and beyond the DMN.
Copyright © 2015 The Authors. Published by Elsevier Inc. All rights reserved.
Recent findings link fronto-temporal gamma electroencephalographic (EEG) activity to conscious awareness in dreams, but a causal relationship has not yet been established. We found that current stimulation in the lower gamma band during REM sleep influences ongoing brain activity and induces self-reflective awareness in dreams. Other stimulation frequencies were not effective, suggesting that higher order consciousness is indeed related to synchronous oscillations around 25 and 40 Hz.
Rapid eye movement (REM) sleep is a behavioral state characterized by cerebral cortical activation with dreaming as an associated behavior. The brainstem mechanisms involved in the generation of REM sleep are well-known, but the forebrain mechanisms that might distinguish it from waking are not well understood. We report here a positron emission tomography (PET) study of regional cerebral glucose utilization in the human forebrain during REM sleep in comparison to waking in six healthy adult females using the 18F-deoxyglucose method. In REM sleep, there is relative activation, shown by increased glucose utilization, in phylogenetically old limbic and paralimbic regions which include the lateral hypothalamic area, amygdaloid complex, septal–ventral striatal areas, and infralimbic, prelimbic, orbitofrontal, cingulate, entorhinal and insular cortices. The largest area of activation is a bilateral, confluent paramedian zone which extends from the septal area into ventral striatum, infralimbic, prelimbic, orbitofrontal and anterior cingulate cortex. There are only small and scattered areas of apparent deactivation. These data suggest that an important function of REM sleep is the integration of neocortical function with basal forebrain-hypothalamic motivational and reward mechanisms. This is in accordance with views that alterations in REM sleep in psychiatric disorders, such as depression, may reflect dysregulation in limbic and paralimbic structures.
REM sleep is a unique brain state characterized by frontal deactivation alongside activation of the posterior association and limbic cortices. Human brain lesion studies have found that the loss of dreaming is characterized by damage to the frontal and posterior parieto-temporo-occipital association cortex. Therefore, it is reasonable to assume that the function of these brain regions might encapsulate the neural processes of dreaming. The aim of the following two experiments was to investigate the effect of transcranial direct current stimulation (tDCs), applied simultaneously to the frontal and right posterior parietal cortex during Stage 2 sleep, on dreaming. In Experiment 1, 17 healthy participants received tDCs (cathodal-frontal, anodal-parietal) and low-intensity tDCs as well as no tDCs (blank control) during Stage 2 sleep in a counterbalanced order across the night. Dream reports were collected upon awakening after each of the three conditions. In Experiment 2, 10 participants received tDCs (cathodal-frontal, anodal-parietal), no tDCs (blank control) and two additional control conditions (reversed polarity and other-cephalic tDCs). In both experiments a significantly greater number of imagery reports were found on awakening after tDCs (cathodal-frontal, anodal-parietal), compared to the blank control conditions. However, in Experiment 2 the frequency of imagery reports from the tDCs (cathodal-frontal, anodal-parietal) was not significantly different from the other two tDC conditions, suggesting a non-specific effect of tDCs. Overall, it was concluded that tDCs (cathodal-frontal, anodal-parietal) increased the frequency of dream reports with visual imagery, possibly via a general arousing effect and/or recreating specific cortical neural activity involved in dreaming.
The pressing need to better understand human brain organization is appreciated by all who have labored to explain the uniqueness of human behavior in health and disease. Early work on the cytoarchitectonics of the human brain by Brodmann and others accompanied by several centuries of lesion behavior work, although valuable, has left us far short of what we need. Fortunately, modern brain imaging techniques have, over the past 40 years, substantially changed the situation by permitting the safe appraisal of both anatomical and functional relationships within the living human brain. An unexpected feature of this work is the critical importance of ongoing, intrinsic activity, which accounts for the majority of brain's energy consumption and exhibits a surprising level of organization that emerges with dimensions of both space and time. In this essay, some of the unique features of intrinsic activity are reviewed, as it relates to our understanding of brain organization.
Since the discovery of the close association between rapid eye movement (REM) sleep and dreaming, much effort has been devoted to link physiological signatures of REM sleep to the contents of associated dreams [1-4]. Due to the impossibility of experimentally controlling spontaneous dream activity, however, a direct demonstration of dream contents by neuroimaging methods is lacking. By combining brain imaging with polysomnography and exploiting the state of "lucid dreaming," we show here that a predefined motor task performed during dreaming elicits neuronal activation in the sensorimotor cortex. In lucid dreams, the subject is aware of the dreaming state and capable of performing predefined actions while all standard polysomnographic criteria of REM sleep are fulfilled [5, 6]. Using eye signals as temporal markers, neural activity measured by functional magnetic resonance imaging (fMRI) and near-infrared spectroscopy (NIRS) was related to dreamed hand movements during lucid REM sleep. Though preliminary, we provide first evidence that specific contents of REM-associated dreaming can be visualized by neuroimaging.
Building on the content, developmental, and neurological evidence that there are numerous parallels between waking cognition and dreaming, this article argues that the likely neural substrate that supports dreaming, which was discovered through converging lesion and neuroimaging studies, may be a subsystem of the waking default network, which is active during mind wandering, daydreaming, and simulation. Support for this hypothesis would strengthen the case for a more general neurocognitive theory of dreaming that starts with established findings and concepts derived from studies of waking cognition and neurocognition. If this theory is correct, then dreaming may be the quintessential cognitive simulation because it is often highly complex, often includes a vivid sensory environment, unfolds over a duration of a few minutes to a half hour, and is usually experienced as real while it is happening.
Recent studies have compared default-mode network (DMN) connectivity in different arousal levels to investigate the relationship between consciousness and DMN. The comparison between the DMN in rapid eye movement (REM) sleep with that in non-REM (NREM) sleep is useful for revealing the relationship between arousal level and DMN, because the arousal level is at its lowest during deep NREM, while during REM sleep it is as high as wakefulness. Functional magnetic resonance imaging (fMRI) and polysomnogram data were acquired from participants in REM, deep NREM, and light NREM sleep, and the DMN was compared using functional connectivity analysis. Our analysis revealed that functional connectivity among the DMN core regions - the posterior cingulate cortex, rostral anterior cingulate cortex, and inferior parietal lobule - remained consistent across sleep states. In contrast, connectivity involving the DMN subsystems of REM sleep differs from that of NREM sleep, and the change well accounts for the characteristics of REM sleep. Our results suggest that both the DMN core region and subsystems may not relate to the maintenance of arousal. The DMN core network and subsystems may respectively serve to integrate brain regions and perform function specific to each level of arousal.
Microstructural analyses by MRI brain scans and by DTI analysis of MR images were used to investigate the possible relationship between deep gray matter structures (amygdala and hippocampus) and dreaming in healthy subjects. Thirty-four subjects ranging in age 20s to 70s underwent to a MRI protocol for the assessment of volume and mean diffusivity (MD) in the amygdala and hippocampus and were asked to fill out a dream diary via audiotape recording upon morning awakening for two weeks. Multiple regression analyses evaluated the relationships between anatomical measures and quantitative and qualitative measures of the reported dreams. The main result points to a dissociation between some quantitative and qualitative aspects of dream reports. While the mean number of dreams recalled per day did not show any significant relationship with the neuroanatomical measures, significant associations with some qualitative features of the recalled dreams (emotional load, bizarreness, and vividness) and, to some extent, with the length of dream reports were observed. Particularly, a higher MD of the left amygdala, reflecting a decreased microstructural integrity, was associated with shorter dream reports and lower scores on emotional load. Bizarreness of dream reports was negatively correlated with the left amygdala volume and positively correlated with the right amygdala MD. Some specific, although weaker, relationships were also found between bizarreness and hippocampal measures. These findings indicate some direct relationships between volumetric and ultrastructural measures of the hippocampus-amygdala complex and specific qualitative features of dreaming.
Thesis (Ph. D.)--University of Chicago, Dept. of Psychology, 1960.
Rapid eye movement (REM) sleep is a behavioral state characterized by cerebral cortical activation with dreaming as an associated behavior. The brainstem mechanisms involved in the generation of REM sleep are well-known, but the forebrain mechanisms that might distinguish it from waking are not well understood. We report here a positron emission tomography (PET) study of regional cerebral glucose utilization in the human forebrain during REM sleep in comparison to waking in six healthy adult females using the 18F-deoxyglucose method. In REM sleep, there is relative activation, shown by increased glucose utilization, in phylogenetically old limbic and paralimbic regions which include the lateral hypothalamic area, amygdaloid complex, septal-ventral striatal areas, and infralimbic, prelimbic, orbitofrontal, cingulate, entorhinal and insular cortices. The largest area of activation is a bilateral, confluent paramedian zone which extends from the septal area into ventral striatum, infralimbic, prelimbic, orbitofrontal and anterior cingulate cortex. There are only small and scattered areas of apparent deactivation. These data suggest that an important function of REM sleep is the integration of neocortical function with basal forebrain-hypothalamic motivational and reward mechanisms. This is in accordance with views that alterations in REM sleep in psychiatric disorders, such as depression, may reflect dysregulation in limbic and paralimbic structures.
Numerous studies have replicated the finding of mentation in both rapid eye movement (REM) and nonrapid eye movement (NREM) sleep. However, two different theoretical models have been proposed to account for this finding: (1) a one-generator model, in which mentation is generated by a single set of processes regardless of physiological differences between REM and NREM sleep; and (2) a two-generator model, in which qualitatively different generators produce cognitive activity in the two states. First, research is reviewed demonstrating conclusively that mentation can occur in NREM sleep; global estimates show an average mentation recall rate of about 50% from NREM sleep--a value that has increased substantially over the years. Second, nine different types of research on REM and NREM cognitive activity are examined for evidence supporting or refuting the two models. The evidence largely, but not completely, favors the two-generator model. Finally, in a preliminary attempt to reconcile the two models, an alternative model is proposed that assumes the existence of covert REM sleep processes during NREM sleep. Such covert activity may be responsible for much of the dreamlike cognitive activity occurring in NREM sleep.
The paradigmatic assumption that REM sleep is the physiological equivalent of dreaming is in need of fundamental revision. A mounting body of evidence suggests that dreaming and REM sleep are dissociable states, and that dreaming is controlled by forebrain mechanisms. Recent neuropsychological, radiological, and pharmacological findings suggest that the cholinergic brain stem mechanisms that control the REM state can only generate the psychological phenomena of dreaming through the mediation of a second, probably dopaminergic, forebrain mechanism. The latter mechanism (and thus dreaming itself) can also be activated by a variety of nonREM triggers. Dreaming can be manipulated by dopamine agonists and antagonists with no concomitant change in REM frequency, duration, and density. Dreaming can also be induced by focal forebrain stimulation and by complex partial (forebrain) seizures during nonREM sleep, when the involvement of brainstem REM mechanisms is precluded. Likewise, dreaming is obliterated by focal lesions along a specific (probably dopaminergic) forebrain pathway, and these lesions do not have any appreciable effects on REM frequency, duration, and density. These findings suggest that the forebrain mechanism in question is the final common path to dreaming and that the brainstem oscillator that controls the REM state is just one of the many arousal triggers that can activate this forebrain mechanism. The "REM-on" mechanism (like its various NREM equivalents) therefore stands outside the dream process itself, which is mediated by an independent, forebrain "dream-on" mechanism.
The disturbance of somatosensory perception and bodily experiences, including somatosensory hallucinations, are main features of the coenaesthesia sub-syndrome of schizophrenia. We used functional MRI to study a coenaesthesia patient with rapidly fluctuating painful somatosensory hallucinatory perceptions. Transient brain activations accompanying hallucinations were similar to the pattern elicited in a control experiment (non-painful tactile stimulation). However, an area in the medial parietal cortex, including parts of the precuneus and previously characterised as a supplementary sensory area, was activated significantly stronger during hallucinations than the control condition. This finding demonstrates elevated brain activity in a somatosensory area accompanying painful somatic hallucinations.
The present study aimed to test whether spontaneous eyelid movements (ELMs) during stage 2 and rapid eye movement (REM) sleep are related to more frequent and vivid reports of visual mentation on awakening. Participants were awakened 15 s after an ELM was observed during ongoing REM and stage 2 sleep and immediately asked for a mentation report and to rate the visual vividness of any imagery they could remember. These reports were compared with control reports collected after a period of ELM quiescence before awakening (noELM). Significantly greater frequencies of imagery reports were collected after ELM awakenings compared with noELM awakenings from stage 2, but not REM sleep. When imagery was reported, imagery ratings were not significantly different between ELM and noELM conditions, regardless of sleep stage. The average amount of electroencephalogram (EEG) arousal 15 s after stage 2 awakenings was significantly higher in the ELM compared with noELM conditions. In addition, within the stage 2 ELM condition, EEG arousal was significantly higher when visual imagery was reported compared with reports without imagery; suggesting that the observed increase in imagery reporting from the stage 2 ELM condition could have been mediated by the level of brain arousal. Such arousal possibly provides better conditions to attend and recall previous mental activity from NREM sleep. However, there was no ELM/arousal effect within REM sleep, possibly because this state is already at maximum sleeping levels of arousal, attention and resulting dream recall.