Prior findings on direct intracranial electrical stimulation (iES) of the human orbitofrontal cortex (OFC), which includes the orbital and ventromedial prefrontal regions, have been mixed, with several reports lacking replication. We aimed to clarify the effects of iES in the OFC. We analyzed data from 608 stimulations across 277 OFC site pairs (352 sites total) in 49 patients collected over 17 years of our practice. We found 24.4% of sites as responsive to iES, with subjects reporting visual and olfactory sensations. However, post hoc analysis revealed that these responses largely originated from the stimulation of nearby non-OFC optic and olfactory structures. After applying quality controls, stimulation of only 0.6% of OFC sites (2 sites, 2 patients) produced changes in subjective domain, while 99.4% had no reportable effects. Contrary to earlier studies, we found no evidence of valence lateralization or functional organization within the OFC. Our findings suggest that the electrical perturbation of OFC is largely silent and does not lead to reportable change in the subjective state of the individual. Orbitofrontal cortex is a higher transmodal cortical area. The variability and limited replicability of reported effects from prior publications and the inconsistencies in the extant literature about OFC stimulations can be attributed to methodological shortcomings.

Neuroimaging studies of mentalizing (i.e., theory of mind) consistently implicate the default mode network (DMN). Nevertheless, the social cognitive functions of individual DMN regions remain unclear, perhaps due to limited spatiotemporal resolution in neuroimaging. Here we use electrocorticography (ECoG) to directly record neuronal population activity while 16 human participants judge the psychological traits of themselves and others. Self- and other-mentalizing recruit near-identical cortical sites in a common spatiotemporal sequence. Activations begin in the visual cortex, followed by temporoparietal DMN regions, then finally in medial prefrontal regions. Moreover, regions with later activations exhibit stronger functional specificity for mentalizing, stronger associations with behavioral responses, and stronger self/other differentiation. Specifically, other-mentalizing evokes slower and longer activations than self-mentalizing across successive DMN regions, implying lengthier processing at higher levels of representation. Our results suggest a common neurocognitive pathway for self- and other-mentalizing that follows a complex spatiotemporal gradient of functional specialization across DMN and beyond.

Intracranial electrical stimulation (iES) has been used to map and perturb brain function in neurosurgical patients for nearly 150 years [[1]Borchers S. et al.Direct electrical stimulation of human cortex--the gold standard for mapping brain functions?.Nat Rev Neurosci. 2012; 13: 63Crossref Scopus (247) Google Scholar]. During so-called ‘functional mapping’ sessions with iES, brief pulses of electrical stimulation are followed by careful observation of elicited effects on both the body and mind. IES has great clinical utility when used to identify brain tissue involved in speech and other motor functions that should be spared during neurosurgical resection [[2]Desmurget M. et al.Re-establishing the merits of electrical brain stimulation.Trends Cognit Sci. 2013; 17: 442-449Abstract Full Text Full Text PDF PubMed Scopus (68) Google Scholar]. Over the past few decades, the clinical application of iES has moved beyond the relatively straightforward mapping of objectively-observable outcomes like motor movements, and into the modulation of subtle and subjective experiences like mood, anxiety, and obsessive-compulsive urges [[3]Holtzheimer P.E. Mayberg H.S. Deep brain stimulation for psychiatric disorders.Annu Rev Neurosci. 2011; 34: 289-307Crossref PubMed Scopus (199) Google Scholar,[4]Harmsen I.E. et al.Clinical trials for deep brain stimulation: current state of affairs.Brain Stimul. 2020; 13: 378-385Abstract Full Text Full Text PDF PubMed Scopus (38) Google Scholar]. In parallel, there has been a rapid proliferation of non-invasive neuromodulation methodologies designed with similar aims in mind [[5]Brunoni A.R. et al.Clinical research with transcranial direct current stimulation (tDCS): challenges and future directions.Brain Stimul. 2012; 5: 175-195Abstract Full Text Full Text PDF PubMed Scopus (911) Google Scholar,[6]Rossini P.M. et al.Non-invasive electrical and magnetic stimulation of the brain, spinal cord, roots and peripheral nerves: basic principles and procedures for routine clinical and research application. An updated report from an IFCN Committee.Clin Neurophysiol. 2015; 126: 1071-1107Crossref PubMed Scopus (1497) Google Scholar]. All efforts to neuromodulate unverifiable subjective states face shared challenges: the problem of demand characteristics and the potency of the placebo effect underscore the necessity for sham stimulations as a control condition in any such intervention. Although sham stimulation is routinely employed in both iES and other non-invasive neuromodulation interventions, to our knowledge there has been little empirical assessment of how effective this control actually is. For instance, one transcranial direct current stimulation (tDCS) study found that while the rate of reported side effects was significantly higher during veridical stimulation, nonetheless sham stimulation elicited considerable and complex effects, including impaired concentration, fatigue, pain, anxiety, and changes in mood and visual perception [[7]Kessler S.K. et al.Differences in the experience of active and sham transcranial direct current stimulation.Brain Stimul. 2012; 5: 155-162Abstract Full Text Full Text PDF PubMed Scopus (178) Google Scholar]. These findings suggest that patients have considerable difficulty differentiating spontaneous fluctuations in conscious experience from the effects actually elicited by non-invasive neuromodulation. This is especially problematic given that the content of false positive reports (e.g., pain, anxiety, and mood changes [[7]Kessler S.K. et al.Differences in the experience of active and sham transcranial direct current stimulation.Brain Stimul. 2012; 5: 155-162Abstract Full Text Full Text PDF PubMed Scopus (178) Google Scholar]) closely resembles the very symptoms neuromodulation is intended to mitigate. Given these important concerns, more empirical work is needed to quantitatively explore the false positive (Type I error) rate of first-person reports following sham stimulation with invasive neuromodulation methods and to identify possible triggering factors for such false reports. The present study sought to address these aims by exploring iES functional mapping sessions accumulated from more than a decade of neurosurgical inpatients undergoing intracranial electroencephalography (iEEG) monitoring for intractable epilepsy at a U.S. tertiary care medical center. Our specific aim was to assess the fidelity of first-person reports following sham iES in a large sample of patients, with the broader goal of informing estimates of the reliability of neuromodulation of subjective experience in general. Veridical iES was performed using standard procedures and parameters: bipolar stimulation was delivered using an alternating square wave current applied across two adjacent electrodes at 50 Hz, 2–10 mA current, and square-wave pulse width of 200–300 μs for a stimulation duration of 1–2 s. Following each iES pulse or sham stimulation, patients were asked standardized, open-ended questions about any experiences evoked (e.g., “Did you notice anything?” or “Any change?“, as in our recent work [[8]Fox K.C.R. et al.Changes in subjective experience elicited by direct stimulation of the human orbitofrontal cortex.Neurology. 2018; 91: e1519-e1527Crossref PubMed Scopus (22) Google Scholar,[9]Fox K.C.R. et al.Intrinsic network architecture predicts the effects elicited by intracranial electrical stimulation of the human brain.Nat. Human Behav. 2020; 4: 1039-1052Crossref PubMed Scopus (43) Google Scholar]), with occasional follow-up questions, as needed, to further clarify the character of effects. Further details of stimulation methods and parameters are described extensively in our prior work [[8]Fox K.C.R. et al.Changes in subjective experience elicited by direct stimulation of the human orbitofrontal cortex.Neurology. 2018; 91: e1519-e1527Crossref PubMed Scopus (22) Google Scholar,[9]Fox K.C.R. et al.Intrinsic network architecture predicts the effects elicited by intracranial electrical stimulation of the human brain.Nat. Human Behav. 2020; 4: 1039-1052Crossref PubMed Scopus (43) Google Scholar]. For each patient, we stimulated as many electrodes as possible, subject to practical (e.g., time constraints) and clinical (e.g., seizure onset) considerations. Stimulation was administered across all electrodes sequentially, in a single session typically lasting 45–90 minutes. The inter-stimulation interval was typically on the order of 10–20 seconds but varied with the duration and complexity of patients' reports of the effects elicited by iES. For instance, when the patient reported ‘no change’ in their experience (null effects), the subsequent stimulation, at higher amplitude, was typically administered within a few seconds. However, when a complex effect was elicited, 1–2 minutes might elapse before the subsequent stimulation (either veridical or sham), in order to allow the patient to explain the effect in detail and allow the experimenter(s) to ask clarifying follow-up questions. During sham stimulation, the experimenter behaved exactly as during veridical stimulation, adjusting settings on the stimulator and pressing the same buttons, followed by the same standardized questions about any changes in the patient's experience – the only difference being that no current was actually delivered. We identified 159 sham stimulations administered to 44 patients (M ±SD = 3.6 ±3.3 per patient). A total of 11 false positive reports were identified, for an overall Type I error rate of 6.9% (Fig. 1). Notably, the overwhelming majority of our sample (75%) never committed a single Type I error (Fig. 1); false positives were restricted to only 11 patients (25% of our sample), and no patient committed more than a single Type I error, even after numerous sham stimulations (for details of all false positive reports, see Table S1). As detailed in Table S1, we did not observe any cases of outright confabulation or otherwise complex reports in response to sham stimulation, nor any motor/behavioral effects that might be suggestive of a psychogenic movement disorder. Moreover, many false positives appear to have been lingering effects from recently applied veridical stimulation (Table S1). Patients with refractory epilepsy often have mild to severe cognitive impairment; it is therefore possible that memory deficits, low intelligence, or misunderstanding of instructions could explain some false positive effects. To control for this possibility, we correlated patients’ IQ scores (in a subsample for which neuropsychological testing data were available; n = 32, Table S1) with their false positive rates, but found no significant relationship (r(30) = .18, p = .328). To summarize, we found that neurosurgical patients were highly resilient to false positive reports following sham stimulation. The few false positive reports we did observe (Table S1) were often analogous to preceding veridical effects (e.g., Patients 19 and 20, after veridical olfactory effects had been elicited a moment before by stimulation to the orbitofrontal cortex, reported similar, lingering smells). Moreover, one Type I error directly followed veridical stimulation leading to induction of a mild seizure with accompanying aura experiences (see Patient 38, Table S1). All of these reports could well be a result of lingering effects from previous veridical stimulation or seizure auras, and hence could reasonably be discounted if a less conservative approach were adopted. Another patient reported the urge to urinate (Patient 43, Table S1), which might well have been a spontaneous urge that the patient mistakenly assumed to be the result of stimulation. This false positive, too, could reasonably be excluded under less stringent criteria. Overall, the high fidelity of patients’ first-person reports following iES is remarkable given that all patients had recently undergone neurosurgery (for electrode implantation), all patients had severe neurological disease (intractable epilepsy), and nearly all patients were on some form of analgesic medication at the time of testing (to control pain from the neurosurgery). In conclusion, our findings strongly support the validity of prior research exploring first-person experiences elicited by iES and deep brain stimulation. Overall, patient reports of whether or not brain stimulation is modulating their subjective experience are highly reliable. Many iES neuromodulation interventions aimed at improving subjective well-being face ongoing challenges and controversy, however [[10]Morishita T. et al.Deep brain stimulation for treatment-resistant depression: systematic review of clinical outcomes.Neurotherapeutics. 2014; 11: 475-484Crossref PubMed Scopus (156) Google Scholar]. Further research assessing the fidelity of first-person reports following sham stimulation in larger samples of patients, and with non-invasive methods like tDCS, could further minimize the confounding effects of false positives and increase the overall effectiveness of neuromodulation interventions. We wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome. We further confirm that any aspect of the work covered in this manuscript that has involved human patients has been conducted with the ethical approval of all relevant bodies and that such approvals are acknowledged within the manuscript. We are grateful to the many patients who participated, without whom this research would have been impossible. KCRF was generously supported by a Medical Scholars Research Fellowship from the Stanford University School of Medicine. JP is supported by the National Science Foundation (BCS1850938). The following is the Supplementary data to this article: Download .docx (.03 MB) Help with docx files Multimedia component 1

Hundreds of neuroimaging studies show that mentalizing (i.e., theory of mind) recruits default mode network (DMN) regions with remarkable consistency. Nevertheless, the social-cognitive functions of individual DMN regions remain unclear, perhaps due to the limited spatiotemporal resolution of neuroimaging. We used electrocorticography (ECoG) to record neuronal population activity while 16 human subjects judged the psychological traits of themselves and others. Self- and other-mentalizing recruited near-identical neuronal populations in a common spatiotemporal sequence: activations were earliest in visual cortex, followed by temporoparietal DMN regions, and finally medial prefrontal cortex. Critically, regions with later activations showed greater functional specificity for mentalizing, greater self/other differentiation, and stronger associations with behavioral response times. Moreover, other-mentalizing evoked slower and lengthier activations than self-mentalizing across successive DMN regions, suggesting temporally extended demands on higher-level processes. Our results reveal a common neurocognitive pathway for self- and other-mentalizing that follows a hierarchy of functional specialization across DMN regions.

Reply to commentary by Naccache and colleagues (https://osf.io/zrqp8/) on our paper (Raccah et al., 2021) -https://www.jneurosci.org/content/41/10/2076.abstract

A central debate in philosophy and neuroscience pertains to whether PFC activity plays an essential role in the neural basis of consciousness. Neuroimaging and electrophysiology studies have revealed that the contents of conscious perceptual experience can be successfully decoded from PFC activity, but these findings might be confounded by postperceptual cognitive processes, such as thinking, reasoning, and decision-making, that are not necessary for consciousness. To clarify the involvement of the PFC in consciousness, we present a synthesis of research that has used intracranial electrical stimulation (iES) for the causal modulation of neural activity in the human PFC. This research provides compelling evidence that iES of only certain prefrontal regions (i.e., orbitofrontal cortex and anterior cingulate cortex) reliably perturbs conscious experience. Conversely, stimulation of anterolateral prefrontal sites, often considered crucial in higher-order and global workspace theories of consciousness, seldom elicits any reportable alterations in consciousness. Furthermore, the wide variety of iES-elicited effects in the PFC (e.g., emotions, thoughts, and olfactory and visual hallucinations) exhibits no clear relation to the immediate environment. Therefore, there is no evidence for the kinds of alterations in ongoing perceptual experience that would be predicted by higher-order or global workspace theories. Nevertheless, effects in the orbitofrontal and anterior cingulate cortices suggest a specific role for these PFC subregions in supporting emotional aspects of conscious experience. Overall, this evidence presents a challenge for higher-order and global workspace theories, which commonly point to the PFC as the basis for conscious perception based on correlative and possibly confounded information.

Intracranial electrical stimulation (iES) of the human brain has long been known to elicit a remarkable variety of perceptual, motor and cognitive effects, but the functional–anatomical basis of this heterogeneity remains poorly understood. We conducted a whole-brain mapping of iES-elicited effects, collecting first-person reports following iES at 1,537 cortical sites in 67 participants implanted with intracranial electrodes. We found that intrinsic network membership and the principal gradient of functional connectivity strongly predicted the type and frequency of iES-elicited effects in a given brain region. While iES in unimodal brain networks at the base of the cortical hierarchy elicited frequent and simple effects, effects became increasingly rare, heterogeneous and complex in heteromodal and transmodal networks higher in the hierarchy. Our study provides a comprehensive exploration of the relationship between the hierarchical organization of intrinsic functional networks and the causal modulation of human behaviour and experience with iES. Intracranial brain stimulation in humans elicits a large variety of perceptual, motor and cognitive effects. Fox et al. show strong links between the distribution and content of these responses and the brain’s intrinsic network architecture.

. . . in what is presumably an effort to explain how meditation has been shown to influence emotion regulation, correlated with alterations in amygdala activity (e.g., Goldin & Gross, 2010), [Hanson] has stated, “In terms of amygdala activity, people seem to belong to one of three groups . . . the ones with a joyful amygdala—are more focused on promoting the good than on preventing the bad” (Hanson, 2013, pp. 43–44). As a result of such oversimplifications, meditative benefits may be exaggerated and undue societal urgency to undertake mindfulness practices may be encouraged (e.g., Farias & Wikholm, 2015).

Though separated from human beings by tens of millions of years of independent evolution, whales and dolphins are highly intelligent and social animals, recognizably like human beings, if only in ways that we can sense but cannot quite measure. Kieran Fox reviews historical attitudes toward cetaceans, studies of cetacean communication, and links between brain size and sociability.

In recent years, several studies have indicated that healthy older adults exhibit a reduction in task-unrelated thoughts compared to young adults. However, much less is known regarding age-related differences in time spent engaging in stimulus-independent thoughts or in their neural correlates in the absence of an ongoing task. In the current study, we collected functional magnetic resonance imaging (fMRI) data while 29 young (mean age = 22y) and 22 older (mean age = 70y) adults underwent experience sampling in the absence of an ongoing task (i.e., at "rest"). Although both age groups reported spending a similar amount of time engaged in stimulus-independent thoughts, older adults rated their thoughts as more present-oriented (rather than atemporal) and more novel. Moreover, controlling for these age-related differences in content, we found that experiencing stimulus-independent thoughts was associated with increased posterior cingulate and left angular gyrus activation across age groups compared to exhibiting an external focus of attention. When experiencing stimulus-independent thoughts, younger adults engaged medial and left lateral prefrontal cortex as well as left superior temporal gyrus to a greater degree than older adults. Taken together, our results suggest that, in the absence of an ongoing task, although young and older adults spend a similar amount of time engaging in stimulus-independent thoughts, the content and neural correlates of these thoughts differ with age.

The subjective and behavioral effects of intracranial electrical stimulation (iES) have been studied for decades, but there is a knowledge gap regarding the relationship between the magnitude of electric current and the type, intensity and valence of evoked subjective experiences. We report on rare iES data from 18 neurosurgical patients with implanted intracranial electrodes in the orbitofrontal cortex (OFC), the insula (INS) and the anterior portion of cingulate cortex (ACC). ACC stimulation elicited somatic and visceral sensations, whereas OFC stimulation predominantly elicited olfactory and gustatory responses, and INS stimulation elicited a mix of effects involving somatic and visceral sensations, olfaction and gustation. Further, we found striking evidence that the magnitude of electric current delivered intracranially correlated positively with the perceived intensity of subjective experience and the evoked emotional state, a relationship observed across all three regions. Finally, we observed that the majority of reported experiences were negatively valenced and unpleasant, especially those elicited by ACC stimulation. The present study provides novel case studies from the human brain confirming that these structures contribute causally to the creation of affective states and demonstrates a direct relationship between the magnitude of electrical stimulation of these structures and the qualia of elicited subjective experience. Summary: This study provides critical knowledge about the effect of electrical charge magnitude on the intensity of human subjective experiences and emotional states. We shed light on the fundamental relationship between the electrical (physical) state of cortical tissue and the modality and intensity of human (subjective) experience. As electroceutical interventions are increasingly employed to treat neurological and psychiatric disorders, these findings highlight the importance of electrical stimulation magnitude for eliciting specific changes in human subjective experience.

Despite increasing scientific interest in self-generated thought—mental content largely independent of the immediate environment—there has yet to be any comprehensive synthesis of the subjective experience and neural correlates of affect in these forms of thinking. Here, we aim to develop an integrated affective neuroscience encompassing many forms of self-generated thought—normal and pathological, moderate and excessive, in waking and in sleep. In synthesizing existing literature on this topic, we reveal consistent findings pertaining to the prevalence, valence, and variability of emotion in self-generated thought, and highlight how these factors might interact with self-generated thought to influence general well-being. We integrate these psychological findings with recent neuroimaging research, bringing attention to the neural correlates of affect in self-generated thought. We show that affect in self-generated thought is prevalent, positively biased, highly variable (both within and across individuals), and consistently recruits many brain areas implicated in emotional processing, including the orbitofrontal cortex, amygdala, insula, and medial prefrontal cortex. Many factors modulate these typical psychological and neural patterns, however; the emerging affective neuroscience of self-generated thought must endeavor to link brain function and subjective experience in both everyday self-generated thought as well as its dysfunctions in mental illness.

Despite increasing scientific interest in self‐generated thought—mental content largely independent of the immediate environment—there has yet to be any comprehensive synthesis of the subjective experience and neural correlates of affect in these forms of thinking. Here, we aim to develop an integrated affective neuroscience encompassing many forms of self‐generated thought—normal and pathological, moderate and excessive, in waking and in sleep. In synthesizing existing literature on this topic, we reveal consistent findings pertaining to the prevalence, valence, and variability of emotion in self‐generated thought, and highlight how these factors might interact with self‐generated thought to influence general well‐being. We integrate these psychological findings with recent neuroimaging research, bringing attention to the neural correlates of affect in self‐generated thought. We show that affect in self‐generated thought is prevalent, positively biased, highly variable (both within and across individuals), and consistently recruits many brain areas implicated in emotional processing, including the orbitofrontal cortex, amygdala, insula, and medial prefrontal cortex. Many factors modulate these typical psychological and neural patterns, however; the emerging affective neuroscience of self‐generated thought must endeavor to link brain function and subjective experience in both everyday self‐generated thought as well as its dysfunctions in mental illness.

An often-overlooked characteristic of the human mind is its propensity to wander. Despite growing interest in the science of mind-wandering, most studies operationalize mind-wandering by its task-unrelated contents, which may be orthogonal to the processes constraining how thoughts are evoked and unfold over time. In this chapter, we emphasize the importance of incorporating such processes into current definitions of mind-wandering, and proposing that mind-wandering and other forms of spontaneous thought (such as dreaming and creativity) are mental states that arise and transition relatively freely due to an absence of constraints on cognition. We review existing psychological, philosophical, and neuroscientific research on spontaneous thought through the lens of this framework, and call for additional research into the dynamic properties of the mind and brain.

The recent surge of scientific research into mind-wandering has occurred amidst a definitional haze. ‘Mind-wandering’ has been used to refer to a wide range of mental phenomena, from attentional lapses to purposeful, task-unrelated planning; from free-flowing thought and creative idea generation to highly constrained, perseverative rumination. Should we continue to group these disparate phenomena under the umbrella of ‘mind-wandering’ despite the lack of scientific consensus on what mind-wandering is and what it is not? Or should we treat ‘mind-wandering’ as a scientific concept in need of a rigorous theoretical definition that distinguishes it from other types of thought?

Humans have employed an incredible variety of plant-derived substances over the millennia in order to alter consciousness and perception. Among the innumerable narcotics, analgesics, 'ordeal' drugs, and other psychoactive substances discovered and used in ritualistic contexts by cultures around the world, one class in particular stands out not only for its radical psychological effects, but also for the highly charged political and legal atmosphere that has surrounded it since its widespread adoption about 50 years ago: so-called psychedelic substances. We review functional neuroimaging investigations of the neural correlates of the psychedelic experience, and highlight relationships with the psychological and neural bases of creativity, daydreaming, and dreaming.

The aim of this chapter is to provide an accessible introduction to the neuroscience of meditation. First, we review studies examining the relationship between meditation and alterations in the structure of the brain’s grey and white matter (so-called morphometric neuroimaging). Next, we discuss findings from functional neuroimaging methods, such as functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) scans, and what they have taught us about the brain’s patterns of activity during different forms of meditation, how meditation alters the brain’s response to various tasks and experiences, and how the expertise of long-term meditators might be harnessed to help us explore subtle aspects of human cognition. Third, we review electrophysiological methods of measuring brain activity during meditation, such as electroencephalography (EEG), and how these findings relate to what we have learned from morphometric and functional neuroimaging. Finally, we discuss the implications of this research and of meditation more generally for brain health and psychological well-being. Specifically, we focus on how meditation might ameliorate the deficits related to cognitive aging, as well as help ameliorate the symptoms and underlying neural substrates associated with neurodegenerative and psychiatric disease.

Functional magnetic resonance imaging (fMRI) has begun to narrow down the neural correlates of self-generated forms of thought, with current evidence pointing toward central roles for the default, frontoparietal, and visual networks. Recent work has linked the arising of thoughts more specifically to default network activity, but the limited temporal resolution of fMRI has precluded more detailed conclusions about where in the brain self-created mental content is generated and how this is achieved. Here I argue that the unparalleled spatiotemporal resolution of intracranial electrophysiology (iEEG) in human epilepsy patients can begin to provide answers to questions about the specific neural origins of self-generated thought. I review the extensive body of literature from iEEG studies over the past few decades and show that many studies involving passive recording or direct electrical stimulation throughout the brain all point to the medial temporal lobe as a key site of thought-generation. At the same time, null effects from other brain regions suggest that various other default network hubs, such as the posterior cingulate cortex and inferior parietal lobule, might have only a marginal role (if any) in the self-generation or initiation of mental content like dreams, visual imagery, memories, and prospective simulations. Ultimately, combining a variety of neuroscientific methods that compensate for each other's weaknesses and complement each other's strengths may prove to be the most effective way to understand the brain's remarkable ability to decouple from the immediate environment and generate its own experiences.

Humans have been aware for thousands of years that sleep comes in many forms, accompanied by different kinds of mental content. We review the first-person report literature on the frequency and type of content experienced in various stages of sleep, showing that different sleep stages are dissociable at the subjective level. We then relate these subjective differences to the growing literature differentiating the various sleep stages at the neurophysiological level, including evidence from electrophysiology, neurochemistry, and functional neuroimaging. We suggest that there is emerging evidence for relationships between sleep stage, neurophysiological activity, and subjective experiences. Specifically, we emphasize that functional neuroimaging work suggests a parallel between activation and deactivation of default network and visual network brain areas and the varying frequency and intensity of imagery and dream mentation across sleep stages; additionally, frontoparietal control network activity across sleep stages may parallel levels of cognitive control and meta-awareness. Together these findings suggest intriguing brain-mind isomorphisms and may serve as a first step toward a comprehensive understanding of the relationship between neurophysiology and psychology in sleep and dreaming.

Limited by space, here we describe just a few examples of many. Multiple CFS studies have examined faces (see [9] for a review). Studies find that upright faces (and human bodies) break suppression faster than inverted faces although inversion leaves low-level visual features intact [6]. Considering that the inversion effect is considered a signature of integrative processing, observing it under CFS indicates integration without awareness. Likewise, emotional expressions (as well as other social information conveyed by faces), which clearly require integration, can be processed under CFS: some emotional expressions have an advantage in suppression breaking, which depends on observers’ characteristics (e.g., anxious individuals show a fearful-face advantage while psychopathic traits have the opposite effect). Moreover, fully suppressed emotional expressions can bias the processing of concurrent faces and symbols [5].