Serotonergic hyperactivity as a potential factor in developmental, acquired and drug-induced synesthesia

Giftedness and atypical sexual differentiation: enhanced perceptual functioning through estrogen deficiency instead of androgen excess

Syndromic autism spectrum conditions (ASC), such as Klinefelter syndrome, also manifest hypogonadism. Compared to the popular Extreme Male Brain theory, the Enhanced Perceptual Functioning model explains the connection between ASC, savant traits, and giftedness more seamlessly, and their co-emergence with atypical sexual differentiation. Overexcitability of primary sensory inputs generates a relative enhancement of local to global processing of stimuli, hindering the abstraction of communication signals, in contrast to the extraordinary local information processing skills in some individuals. Weaker inhibitory function through gamma-aminobutyric acid type A (GABAA) receptors and the atypicality of synapse formation lead to this difference, and the formation of unique neural circuits that process external information. Additionally, deficiency in monitoring inner sensory information leads to alexithymia (inability to distinguish one's own emotions), which can be caused by hypoactivity of estrogen and oxytocin in the interoceptive neural circuits, comprising the anterior insular and cingulate gyri. These areas are also part of the Salience Network, which switches between the Central Executive Network for external tasks and the Default Mode Network for self-referential mind wandering. Exploring the possibility that estrogen deficiency since early development interrupts GABA shift, causing sensory processing atypicality, it helps to evaluate the co-occurrence of ASC with attention deficit hyperactivity disorder, dyslexia, and schizophrenia based on phenotypic and physiological bases. It also provides clues for understanding the common underpinnings of these neurodevelopmental disorders and gifted populations.

Psilocybin-induced changes in cerebral blood flow are associated with acute and baseline inter-individual differences

Research into the use of psilocybin for the treatment of psychiatric disorders is a growing field. Nevertheless, robust brain-behavior relationships linking psilocybin-induced brain changes to subjective drug-induced effects have not been established. Furthermore, it is unclear if the acute neural effects are dependent on individual heterogeneity in baseline characteristics. To address this, we assessed the effects of three oral doses of psilocybin vs. placebo on cerebral blood flow (CBF) using arterial spin labeling in healthy participants (N = 70; n = 31, 0.16 mg/kg; n = 10, 0.2 mg/kg; n = 29, 0.215 mg/kg). First, we quantified psilocybin-induced changes in relative and absolute CBF. Second, in an exploratory analysis, we assessed whether individual baseline characteristics and subjective psychedelic experience are associated with changes in CBF. Psychological and neurobiological baseline characteristics correlated with the psilocybin-induced reduction in relative CBF and the psilocybin-induced subjective experience. Furthermore, the psilocybin-induced subjective experience was associated with acute changes in relative and absolute CBF. The results demonstrated that inter-individual heterogeneity in the neural response to psilocybin is associated with baseline characteristics and shed light on the mechanisms underlying the psychedelic-induced altered state. Overall, these findings help guide the search for biomarkers, paving the way for a personalized medicine approach within the framework of psychedelic-assisted therapy.

The chemical induction of synaesthesia

OBJECTIVE

Preliminary research suggests that experiences resembling synaesthesia are frequently reported under the influence of a diverse range of chemical substances although the incidence, chemical specificity, and characteristics of these effects are poorly understood.

METHODS

Here we surveyed recreational drug users and self-reported developmental synaesthetes regarding their use of 28 psychoactive drugs from 12 different drug classes and whether they had experienced synaesthesia under the influence of these substances.

RESULTS

The drug class of tryptamines exhibited the highest incidence rates of drug-induced synaesthesia in controls and induction rates of novel forms of synaesthesia in developmental synaesthetes. Induction incidence rates in controls were strongly correlated with the corresponding induction and enhancement rates in developmental synaesthetes. In addition, the use of lysergic acid diethylamide (LSD) was the strongest predictor of drug-induced synaesthesia in both controls and developmental synaesthetes. Clear evidence was observed for a clustering of synaesthesia-induction rates as a function of drug class in both groups, denoting non-random incidence rates within drug classes. Sound-colour synaesthesia was the most commonly observed type of induced synaesthesia. Further analyses suggest the presence of synaesthesia-prone individuals, who were more likely to experience drug-induced synaesthesia with multiple drugs.

CONCLUSIONS

These data corroborate the hypothesized link between drug-induced synaesthesia and serotoninergic activity, but also suggest the possibility of alternative neurochemical pathways involved in the induction of synaesthesia. They further imply that the induction and modulation of synaesthesia in controls and developmental synaesthetes share overlapping mechanisms and that certain individuals may be more susceptible to experiencing induced synaesthesia with different drugs.

Philosophy and classic psychedelics: A review of some emerging themes

Serotonergic (or “classic”) psychedelics have struck many researchers as raising significant philosophical questions that, until recently, were largely unexplored by academic philosophers. This paper provides an overview of four emerging lines of research at the intersection of academic philosophy and psychedelic science that have gained considerable traction in the last decade: selfless consciousness, psychedelic epistemology, psychedelic ethics, and spiritual/religious naturalism. In this paper, we highlight philosophical questions concerning (i) psychedelics, self-consciousness, and phenomenal consciousness, (ii) the epistemic profile of the psychedelic experience; (iii) ethical concerns about the appropriate use of psychedelics; and (iv) whether spiritual or religious dimensions of psychedelic use are compatible with a naturalistic worldview.

Beating Pain with Psychedelics: Matter over Mind?

Basic pain research has shed light on key cellular and molecular mechanisms underlying nociceptive and phenomenological aspects of pain. Despite these advances, [[we still yearn for] the discovery of novel therapeutic strategies to address the unmet needs of about 70% of chronic neuropathic pain patients whose pain fails to respond to opioids as well as to other conventional analgesic agents. Importantly, a substantial body of clinical observations over the past decade cumulatively suggests that the psychedelic class of drugs may possess heuristic value for understanding and treating chronic pain conditions. The present review presents a theoretical framework for hitherto insufficiently understood neuroscience-based mechanisms of psychedelics' potential analgesic effects. To that end, searches of PubMed-indexed journals were performed using the following Medical Subject Headings' terms: pain, analgesia, inflammatory, brain connectivity, ketamine, psilocybin, functional imaging, and dendrites. Recursive sets of scientific and clinical evidence extracted from this literature review were summarized within the following key areas: (1) studies employing psychedelics for alleviation of physical and emotional pain; (2) potential neuro-restorative effects of psychedelics to remediate the impaired connectivity underlying the dissociation between pain-related conscious states/cognitions and the subcortical activity/function leading to the eventual chronicity through immediate and long-term effects on dentritic plasticity; (3) anti-neuroinflammatory and pro-immunomodulatory actions of psychedelics as the may pertain to the role of these factors in the pathogenesis of neuropathic pain; (4) safety, legal, and ethical consideration inherent in psychedelics' pharmacotherapy. In addition to direct beneficial effects in terms of reduction of pain and suffering, psychedelics' inclusion in the analgesic armamentarium will contribute to deeper and more sophisticated insights not only into pain syndromes but also into frequently comorbid psychiatric condition associated with emotional pain, e.g., depressive and anxiety disorders. Further inquiry is clearly warranted into the above areas that have potential to evolve into further elucidate the mechanisms of chronic pain and affective disorders, and lead to the development of innovative, safe, and more efficacious neurobiologically-based therapeutic approaches.

An internet-based survey of synesthesia in multiple sclerosis: Incidence, characteristics and implications

Objective Prior work raises the interesting possibility that both multiple sclerosis and synesthesia share a common etiology, that being immune system dysfunction, as well as neuroanatomical and neurochemical abnormalities, including those involving white matter and serotonergic pathways, respectively. Given these links between these two syndromes, we examined the possibility that prevalence of synesthesia would be elevated in a population of individuals with MS, relative to what is thought to be the prevalence in the neurotypical population. It was not known whether synesthesia might be a marker for subsequent development of MS, or if synesthesia might reflect neurological damage resulting from MS disease progression. Method Individuals with self- reported clinically definite MS were recruited online via the internet and social media using sites specifically relevant to the MS community. Data from 147 individuals who completed several questionnaires related to synesthesia were analyzed. Results Depending on criteria, between approximately 7 and 16% of individuals with MS reported synesthesia here. This is an estimated 1.57 to 3.55 times increased incidence of synesthesia here relative to previous findings in neurotypical samples. Limitations of the study include that this was an internet survey, and that synesthesia was not directly assessed in this sample. Conclusions Results suggest a link between the syndromes, primarily indicating that synesthesia may be a marker for subsequent MS development, and the implications and directions for future study are discussed.

Psychedelic drugs and perception: a narrative review of the first era of research

Psychedelic drugs are well-known for transiently altering perception, and in particular, for their visual effects. Although scientific interest into the substances' effects on perception increased during the first era of psychedelic research during the early to mid-20th century, there is currently no source where these findings have been synthesized. In addressing this gap, the current narrative review found that psychedelics were examined for their influences across all levels of the visual system (e.g., retinal, cortical, subcortical, simple visual processing, complex imagery, hallucinations). Psychedelics were also shown to affect auditory discrimination/generalization, neural correlates of auditory processing, and led to auditory hallucinations in subsets of participants. Several studies demonstrated that psychedelics can distort representations of body schema and time perception. Concerns regarding methodological standards of this era are a limitation to the findings and are discussed. Collectively, this review preserves and increases the accessibility of the work done by pioneering psychedelic/perception researchers, synthesizes findings, and critically analyzes areas of discrepancy to inform future studies.

Synesthesia and the Perception of Unseen Realities

Exceptional experiences (ExE) incorporate a range of phenomena including subjective paranormal and transpersonal experiences. Synesthesia and synesthetic experiences are discussed as important variables in understanding the etiologies of ExE. The neural and psychological correlates of synesthetic experiences (associated with hyperconnectivity) are discussed in relation to ExE. It is argued that synesthetic processes enable both the detection and conscious perception of information from a range of sources that are usually unseen or inaccessible, including abstract, unlanguaged, preconscious, and potentially other nonlocal sources.

Can Psychedelics Alleviate Symptoms of Cluster Headache and Accompanying Mental Health Problems? A Case Report Involving Hawaiian Baby Woodrose

Preliminary evidence supports the efficacy of psychedelics in the alleviation of cluster headache and mental health problems. We describe a case of an individual with cluster headache and mood disorder who claims to have benefited from her use of psychedelics. A forty-eight-year-old woman was referred to a private Australian mental health clinic for the management of chronic pain and depression. She reported using Hawaiian baby woodrose to successfully alleviate symptoms of cluster headache and the accompanying mental health problems. Incidental observation also highlighted the potential therapeutic benefits of antiviral therapy. This is the first case report to concurrently examine the analgesic and psycho-spiritual effects of Hawaiian baby woodrose, with results in support of nascent research in this field. The results of this case report highlight the need for further research into the use of psychedelics in the management of cluster headache and mental illness.

Neurophenomenology of induced and natural synaesthesia

People with synaesthesia have additional perceptual experiences, which are automatically and consistently triggered by specific inducing stimuli. Synaesthesia therefore offers a unique window into the neurocognitive mechanisms underlying conscious perception. A long-standing question in synaesthesia research is whether it is possible to artificially induce non-synaesthetic individuals to have synaesthesia-like experiences. Although synaesthesia is widely considered a congenital condition, increasing evidence points to the potential of a variety of approaches to induce synaesthesia-like experiences, even in adulthood. Here, we summarize a range of methods for artificially inducing synaesthesia-like experiences, comparing the resulting experiences to the key hallmarks of natural synaesthesia which include consistency, automaticity and a lack of 'perceptual presence'. We conclude that a number of aspects of synaesthesia can be artificially induced in non-synaesthetes. These data suggest the involvement of developmental and/or learning components in the acquisition of synaesthesia, and they extend previous reports of perceptual plasticity leading to dramatic changes in perceptual phenomenology in adults. This article is part of a discussion meeting issue 'Bridging senses: novel insights from synaesthesia'.

Double-blind study of visual imagery in grapheme-color synesthesia

Synesthesia is an atypical perceptual phenomenon that has been associated with generalized differences in other cognitive and perceptual domains. Given similarities in the qualitative nature of synesthetic experiences to visual imagery perceptions, several studies have sought to examine whether synesthetes demonstrate increased visual imagery abilities. Using subjective imagery questionnaires, some studies have identified superior imaging abilities in synesthetes, while others have not. However, because most research on synesthesia uses un-blinded group membership prior to data collection, such methods for studying group differences may be prone to participant and experimenter biases (e.g., a motivated synesthete may rate themselves as having stronger visual imagery abilities due to their own bias and perceived experimenter expectations). To address this issue, we demonstrate the feasibility of double-blind designs in synesthesia research, applied here to examine differences in subjectively reported levels of imagery usage and intensity. Prior to identifying synesthetes' and non-synesthetes' group membership (in order to eliminate the potential for bias), subjects completed two common measures of visual imagery experiences. Using this approach, we replicated findings of greater visual imagery usage in synesthetes on the Spontaneous Use of Imagery Scale (SUIS) measure, but not of enhanced imagery abilities on the standardized Vividness of Visual Imagery Questionnaire (VVIQ) measure. The present study strengthens prior evidence that synesthesia is associated with heightened visual imagery and demonstrates the utility of double-blind designs in order to limit biases and promote further replicability of other findings in research on synesthesia.

Inducing synesthesia in non-synesthetes: Short-term visual deprivation facilitates auditory-evoked visual percepts

Sounds can modulate activity in visual cortex, facilitating the detection of visual targets. However, these sound-driven modulations are not thought to evoke conscious visual percepts in the general population. In individuals with synesthesia, however, multisensory interactions do lead to qualitatively different experiences such as sounds evoking flashes of light. Why, if multisensory interactions are present in all individuals, do only synesthetes experience abnormal qualia? Competing models differ in the time required for synesthetic experiences to emerge. The cross-activation model suggests synesthesia arises over months or years from the development of abnormal neural connections. Here we demonstrate that after ∼5 min of visual deprivation, sounds can evoke synesthesia-like percepts (vivid colors and Klüver form-constants) in ∼50% of non-synesthetes. These results challenge aspects of the cross-activation model and suggest that synesthesia exists as a latent feature in all individuals, manifesting when the balance of activity across the senses has been altered.

Is there a burden attached to synaesthesia? Health screening of synaesthetes in the general population

Synaesthesia has long been considered a benign alternative form of perception most often associated with positive rather than negative outcomes. The condition has been associated with a variety of cognitive and perceptual advantages, including benefits in memory, processing speed, and creativity. It is not currently recognized in the DSM-IV. Recently, however, several studies have raised the question of a possible link between synaesthesia and clinical conditions. Here, we present the first large-scale screening of the general population in which we (1) objectively identified grapheme-colour synaesthetes and (2) elicited information from our participants about a range of clinical conditions. We compared the prevalence rates of these conditions in synaesthetes versus non-synaesthetes to establish whether any conditions were found at a higher rate among synaesthetes. In our initial study, screening 3,742 people (95 synaesthetes and 3,647 controls), we found initially that grapheme-colour was significantly comorbid with two conditions (anxiety disorder and obsessive compulsive disorder). In our second study, screening a new population of 120 synaesthetes and 166 non-synaesthetes, we replicated our finding that grapheme-colour synaesthesia is comorbid with anxiety disorder. At the same time, we also addressed a methodological concern that likely elevated rates of OCD in Study 1. We consider the aetiology of synaesthesia to determine whether there may be a shared genetic or neurological basis with anxiety disorder, and we question the status of synaesthesia within a mental health framework.

Relief from intractable phantom pain by combining psilocybin and mirror visual-feedback (MVF)

AL's leg was amputated resulting in phantom-limb pain (PLP). (1) When a volunteer placed her foot on or near the phantom - touching it evoked organized sensations in corresponding locations on AL's phantom. (2) Mirror-visual-feedback (MVF) relieved PLP, as did, "phantom massage". (3) Psilocybin-MVF pairing produced synergistic effects, complete elimination of PLP, and reduction in paroxysmal episodes. (4) Touching the volunteer's leg where AL previously had external fixators, evoked sensation of nails boring through the leg. Using a "telescoping" nail, we created the illusion of a nail being removed with corresponding pain relief. (5) Artificial flames produced warmth in the phantom.

Unifying Theories of Psychedelic Drug Effects

How do psychedelic drugs produce their characteristic range of acute effects in perception, emotion, cognition, and sense of self? How do these effects relate to the clinical efficacy of psychedelic-assisted therapies? Efforts to understand psychedelic phenomena date back more than a century in Western science. In this article I review theories of psychedelic drug effects and highlight key concepts which have endured over the last 125 years of psychedelic science. First, I describe the subjective phenomenology of acute psychedelic effects using the best available data. Next, I review late 19th-century and early 20th-century theories-model psychoses theory, filtration theory, and psychoanalytic theory-and highlight their shared features. I then briefly review recent findings on the neuropharmacology and neurophysiology of psychedelic drugs in humans. Finally, I describe recent theories of psychedelic drug effects which leverage 21st-century cognitive neuroscience frameworks-entropic brain theory, integrated information theory, and predictive processing-and point out key shared features that link back to earlier theories. I identify an abstract principle which cuts across many theories past and present: psychedelic drugs perturb universal brain processes that normally serve to constrain neural systems central to perception, emotion, cognition, and sense of self. I conclude that making an explicit effort to investigate the principles and mechanisms of psychedelic drug effects is a uniquely powerful way to iteratively develop and test unifying theories of brain function.

Serotonergic Hallucinogen-Induced Visual Perceptual Alterations

Serotonergic hallucinogens, such as lysergic acid diethylamide (LSD), psilocybin, and N,N-dimethyltryptamine (DMT), are famous for their capacity to temporally and profoundly alter an individual's visual experiences. These visual alterations show consistent attributes despite large inter- and intra-individual variances. Many reports document a common perception of colors as more saturated, with increased brightness and contrast in the environment ("Visual Intensifications"). Environmental objects might be altered in size ("Visual illusions") or take on a modified and special meaning for the subject ("Altered self-reference"). Subjects may perceive light flashes or geometrical figures containing recurrent patterns ("Elementary imagery and hallucinations") influenced by auditory stimuli ("Audiovisual synesthesia"), or they may envision images of people, animals, or landscapes ("Complex imagery and hallucinations") without any physical stimuli supporting their percepts. This wide assortment of visual phenomena suggests that one single neuropsychopharmacological mechanism is unlikely to explain such vast phenomenological diversity. Starting with mechanisms that act at the cellular level, the key role of 5-HT2A receptor activation and the subsequent increased cortical excitation will be considered. Next, it will be shown that area specific anatomical and dynamical features link increased excitation to the specific visual contents of hallucinations. The decrease of alpha oscillations by hallucinogens will then be introduced as a systemic mechanism for amplifying internal-driven excitation that overwhelms stimulus-induced excitations. Finally, the hallucinogen-induced parallel decrease of the N170 visual evoked potential and increased medial P1 potential will be discussed as key mechanisms for inducing a dysbalance between global integration and early visual gain that may explain several hallucinogen-induced visual experiences, including visual hallucinations, illusions, and intensifications.

Phenomenology, Structure, and Dynamic of Psychedelic States

Classic serotonergic hallucinogens or psychedelics produce an altered states of consciousness (ASC) that is characterized by profound alterations in sensory perception, mood, thought including the perception of reality, and the sense of self. Over the past years, there has been considerable progress in the search for invariant and common features of psychedelic states. In the first part of this review, we outline contemporary approaches to characterize the structure of ASCs by means of three primary etiology-independent dimensions including oceanic boundlessness, anxious ego-dissolution, and visionary restructuralization as well as by 11 lower-order factors, all of which can be reliably measured by the altered state of consciousness questionnaire (APZ-OAV). The second part sheds light on the dynamic nature of psychedelic experiences. Frequently, psychedelic subjects progress through different stages over time and levels of changes along a perception-hallucination continuum of increasing arousal and ego-dissolution. We then review in detail the acute effects of psychedelics on sensory perception, emotion, cognition, creativity, and time perception along with possible neural mechanisms underlying them. The next part of this review outlines the influence of non-pharmacological factors (predictors) on the acute psychedelic experience, such as demographics, genetics, personality, mood, and setting, and also discusses some long-term effects succeeding the acute experience. The last part presents some recent concepts and models attempting to understand different facets of psychedelic states of consciousness from a neuroscientific perspective.

Unconscious Imagination and the Mental Imagery Debate

Traditionally, philosophers have appealed to the phenomenological similarity between visual experience and visual imagery to support the hypothesis that there is significant overlap between the perceptual and imaginative domains. The current evidence, however, is inconclusive: while evidence from transcranial brain stimulation seems to support this conclusion, neurophysiological evidence from brain lesion studies (e.g., from patients with brain lesions resulting in a loss of mental imagery but not a corresponding loss of perception and vice versa) indicates that there are functional and anatomical dissociations between mental imagery and perception. Assuming that the mental imagery and perception do not overlap, at least, to the extent traditionally assumed, then the question arises as to what exactly mental imagery is and whether it parallels perception by proceeding via several functionally distinct mechanisms. In this review, we argue that even though there may not be a shared mechanism underlying vision for perception and conscious imagery, there is an overlap between the mechanisms underlying vision for action and unconscious visual imagery. On the basis of these findings, we propose a modification of Kosslyn's model of imagery that accommodates unconscious imagination and explore possible explanations of the quasi-pictorial phenomenology of conscious visual imagery in light of the fact that its underlying neural substrates and mechanisms typically are distinct from those of visual experience.

The subjective experience of acute, experimentally-induced Salvia divinorum inebriation

This study examined the overall psychological effects of inebriation facilitated by the naturally-occurring plant hallucinogen Salvia divinorum using a double-blind, randomized, placebo-controlled trial. Thirty healthy individuals self-administered Salvia divinorum via combustion and inhalation in a quiet, comfortable research setting. Experimental sessions, post-session interviews, and 8-week follow-up meetings were audio recorded and transcribed to provide the primary qualitative material analyzed here. Additionally, post-session responses to the Hallucinogen Rating Scale provided a quantitative groundwork for mixed-methods discussion. Qualitative data underwent thematic content analysis, being coded independently by three researchers before being collaboratively integrated to provide the final results. Three main themes and 10 subthemes of acute intoxication emerged, encompassing the qualities of the experience, perceptual alterations, and cognitive-affective shifts. The experience was described as having rapid onset and being intense and unique. Participants reported marked changes in auditory, visual, and interoceptive sensory input; losing normal awareness of themselves and their surroundings; and an assortment of delusional phenomena. Additionally, the abuse potential of Salvia divinorum was examined post hoc. These findings are discussed in light of previous research, and provide an initial framework for greater understanding of the subjective effects of Salvia divinorum, an emerging drug of abuse.

Color synesthesia. Insight into perception, emotion, and consciousness

Purpose of review

Synesthesia is an extraordinary perceptual phenomenon, in which individuals experience unusual percepts elicited by the activation of an unrelated sensory modality or by a cognitive process. Emotional reactions are commonly associated. The condition prompted philosophical debates on the nature of perception and impacted the course of art history. It recently generated a considerable interest among neuroscientists, but its clinical significance apparently remains underevaluated. This review focuses on the recent studies regarding variants of color synesthesia, the commonest form of the condition.

Recent findings

Synesthesia is commonly classified as developmental and acquired. Developmental forms predispose to changes in primary sensory processing and cognitive functions, usually with better performances in certain aspects and worse in others, and to heightened creativity. Acquired forms of synesthesia commonly arise from drug ingestion or neurological disorders, including thalamic lesions and sensory deprivation (e.g., blindness). Cerebral exploration using structural and functional imaging has demonstrated distinct patterns in cortical activation and brain connectivity for controls and synesthetes. Artworks of affected painters are most illustrative of the nature of synesthetic experiences.

Summary

Results of the recent investigations on synesthesia offered a remarkable insight into the mechanisms of perception, emotion and consciousness, and deserve attention both from neuroscientists and from clinicians.

Probing the neurochemical basis of synaesthesia using psychophysics

The neurochemical mechanisms that contribute to synaesthesia are poorly understood, but multiple models implicate serotonin and GABA in the development of this condition. Here we used psychophysical tasks to test the predictions that synaesthetes would display behavioral performance consistent with reduced GABA and elevated serotonin in primary visual cortex. Controls and synaesthetes completed the orientation-specific surround suppression (OSSS) and tilt-after effect (TAE) tasks, previously shown to relate to GABA and serotonin levels, respectively. Controls and synaesthetes did not differ in the performance parameter previously associated with GABA or in the magnitude of the TAE. However, synaesthetes did display lower contrast difference thresholds in the OSSS task than controls when no surround (NS) was present. These results are inconsistent with the hypothesized roles of GABA and serotonin in this condition, but provide preliminary evidence that synaesthetes exhibit enhanced contrast discrimination.