Acquired auditory-visual synesthesia: A window to early cross-modal sensory interactions

Exploring the subjective experience of rave party participants in Israel who consume psychedelic drugs: a qualitative inquiry

BACKGROUND

Rave music parties (RMP) are a world-wide socio-cultural phenomenon, where people listen to rave music while frequently consuming psychedelic drugs. Epidemiological studies have emphasized the hazardous consequences following the consumption of psychedelic drugs at RMP, and qualitative studies have shown social and psycho-spiritual experiences. Yet, phenomenological inquiry into subjective experiences of attendees is scant. This study aimed to examine physical, emotional, perceptual and social experiences of RMP participants in Israel, and their view on Israel's policy toward rave events. In addition, the study aimed to contribute useful information for policymakers and society on rave music and psychedelic drugs experiences at RMP.

METHOD

Interpretative phenomenological analysis was used to analyze transcriptions of semi-structured interviews with 27 individuals attending RMP regularly and consume psychedelic drugs.

RESULTS

Analysis revealed four significant themes: the first theme, the impact of Israel's drug policy on participants' sense of safety, relates to participants' sense of insecurity and anxiety at Israeli RMP due to government drug ban policy. The second theme, the stigma on rave culture, relates to participants' perception regarding the stigma on rave culture in law enforcement agencies and in society in general. The third theme, negative experiences, describes short-term experiences after consuming psychedelics at RMP, including hallucinations and disorientation. The fourth theme, positive experiences, describes positive sensory, emotional and self/world attitudinal aspects after consuming psychedelics at RMP. Sensory experiences included intensified auditory, visual and tactile experiences; emotional experiences included positive feelings toward others, reduced stress and ability to vent difficult emotions; self/world attitudinal aspects included self-acceptance, higher appreciation of life and connectedness to nature.

CONCLUSIONS

The study highlights RMP participants' sense of insecurity due to Israel's strict drug policy and absence of harm reduction strategies at rave scenes. The study also notes participants' experience of stigmatization as drug addicts by society and law enforcement agencies. Reducing police presence and adopting harm reduction policies at rave scenes in Israel may increase participants' sense of security, reduce stigmatization and decrease overdose risk. Hence, the findings may contribute to new knowledge useful for policymakers and society concerning RMP and the use of psychedelics.

Tinnitus-like "hallucinations" elicited by sensory deprivation in an entropy maximization recurrent neural network

Sensory deprivation has long been known to cause hallucinations or "phantom" sensations, the most common of which is tinnitus induced by hearing loss, affecting 10-20% of the population. An observable hearing loss, causing auditory sensory deprivation over a band of frequencies, is present in over 90% of people with tinnitus. Existing plasticity-based computational models for tinnitus are usually driven by homeostatic mechanisms, modeled to fit phenomenological findings. Here, we use an objective-driven learning algorithm to model an early auditory processing neuronal network, e.g., in the dorsal cochlear nucleus. The learning algorithm maximizes the network's output entropy by learning the feed-forward and recurrent interactions in the model. We show that the connectivity patterns and responses learned by the model display several hallmarks of early auditory neuronal networks. We further demonstrate that attenuation of peripheral inputs drives the recurrent network towards its critical point and transition into a tinnitus-like state. In this state, the network activity resembles responses to genuine inputs even in the absence of external stimulation, namely, it "hallucinates" auditory responses. These findings demonstrate how objective-driven plasticity mechanisms that normally act to optimize the network's input representation can also elicit pathologies such as tinnitus as a result of sensory deprivation.

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'.

Stochastic resonance model of synaesthesia

In synaesthesia, stimulation of one sensory modality evokes additional experiences in another modality (e.g. sounds evoking colours). Along with these cross-sensory experiences, there are several cognitive and perceptual differences between synaesthetes and non-synaesthetes. For example, synaesthetes demonstrate enhanced imagery, increased cortical excitability and greater perceptual sensitivity in the concurrent modality. Previous models suggest that synaesthesia results from increased connectivity between corresponding sensory regions or disinhibited feedback from higher cortical areas. While these models explain how one sense can evoke qualitative experiences in another, they fail to predict the broader phenotype of differences observed in synaesthetes. Here, we propose a novel model of synaesthesia based on the principles of stochastic resonance. Specifically, we hypothesize that synaesthetes have greater neural noise in sensory regions, which allows pre-existing multisensory pathways to elicit supra-threshold activation (i.e. synaesthetic experiences). The strengths of this model are (a) it predicts the broader cognitive and perceptual differences in synaesthetes, (b) it provides a unified framework linking developmental and induced synaesthesias, and (c) it explains why synaesthetic associations are inconsistent at onset but stabilize over time. We review research consistent with this model and propose future studies to test its limits. This article is part of a discussion meeting issue 'Bridging senses: novel insights from synaesthesia'.

Synaesthesia: a distinct entity that is an emergent feature of adaptive neurocognitive differences

In this article, I argue that synaesthesia is not on a continuum with neurotypical cognition. Synaesthesia is special: its phenomenology is different; it has distinct causal mechanisms; and is likely to be associated with a distinct neurocognitive profile. However, not all synaesthetes are the same, and there are quantifiable differences between them. In particular, the number of types of synaesthesia that a person possesses is a hitherto underappreciated variable that predicts cognitive differences along a number of dimensions (mental imagery, sensory sensitivity, attention to detail). Together with enhanced memory, this may constitute a common core of abilities that may go some way to explaining why synaesthesia might have evolved. I argue that the direct benefits of synaesthesia are generally limited (i.e. the synaesthetic associations do not convey novel information about the world) but, nevertheless, synaesthesia may develop due to other adaptive functions (e.g. perceptual ability, memory) that necessitate changes to design features of the brain. The article concludes by suggesting that synaesthesia forces us to reconsider what we mean by a 'normal' mind/brain. There may be multiple 'normal' neurodevelopmental trajectories that can sculpt very different ways of experiencing the world, of which synaesthesia is but one. This article is part of a discussion meeting issue 'Bridging senses: novel insights from synaesthesia'.

Déjà vu and prescience in a case of severe episodic amnesia following bilateral hippocampal lesions

Several studies pertaining to déjà vu have consistently made a connection with the perirhinal region, a region located below the hippocampus. This idea is strengthened by the fact that déjà vu is an erroneous sense of familiarity and that familiarity appears to largely depend on the perirhinal region in healthy subjects. In this context, the role of the hippocampus is particularly unclear as it is unknown whether or not it plays a role in the genesis of déjà vu. We report on the case of OHVR, an epileptic patient who suffers from severe episodic amnesia related to massive isolated bilateral damage to the hippocampus. In contrast, the perirhinal region is intact structurally and functionally. This patient reports frequent déjà vu but also another experiential phenomenon with a prominent feeling of prescience, which shows some of the characteristics of déjà vécu. She clearly distinguishes both. She also developed a form of synaesthesia by attributing affective valence to numbers. This study shows that déjà vu can occur in cases of amnesia with massively damaged hippocampi and confirms that the perirhinal region is a core region for déjà vu, using a different approach from previous reports. It also provides clues about a potential influence of hippocampal alterations in déjà vécu.

A possible key role of vision in the development of schizophrenia

Based on a brief overview of the various aspects of schizophrenia reported by numerous studies, here we hypothesize that schizophrenia may originate (and in part be performed) from visual areas. In other words, it seems that a normal visual system or at least an evanescent visual perception may be an essential prerequisite for the development of schizophrenia as well as of various types of hallucinations. Our study focuses on auditory and visual hallucinations, as they are the most prominent features of schizophrenic hallucinations (and also the most studied types of hallucinations). Here, we evaluate the possible key role of the visual system in the development of schizophrenia.

Early sensory experience influences the development of multisensory thalamocortical and intracortical connections of primary sensory cortices

The nervous system integrates information from multiple senses. This multisensory integration already occurs in primary sensory cortices via direct thalamocortical and corticocortical connections across modalities. In humans, sensory loss from birth results in functional recruitment of the deprived cortical territory by the spared senses but the underlying circuit changes are not well known. Using tracer injections into primary auditory, somatosensory, and visual cortex within the first postnatal month of life in a rodent model (Mongolian gerbil) we show that multisensory thalamocortical connections emerge before corticocortical connections but mostly disappear during development. Early auditory, somatosensory, or visual deprivation increases multisensory connections via axonal reorganization processes mediated by non-lemniscal thalamic nuclei and the primary areas themselves. Functional single-photon emission computed tomography of regional cerebral blood flow reveals altered stimulus-induced activity and higher functional connectivity specifically between primary areas in deprived animals. Together, we show that intracortical multisensory connections are formed as a consequence of sensory-driven multisensory thalamocortical activity and that spared senses functionally recruit deprived cortical areas by an altered development of sensory thalamocortical and corticocortical connections. The functional-anatomical changes after early sensory deprivation have translational implications for the therapy of developmental hearing loss, blindness, and sensory paralysis and might also underlie developmental synesthesia.

Synesthetic hallucinations induced by psychedelic drugs in a congenitally blind man

This case report offers rare insights into crossmodal responses to psychedelic drug use in a congenitally blind (CB) individual as a form of synthetic synesthesia. BP's personal experience provides us with a unique report on the psychological and sensory alterations induced by hallucinogenic drugs, including an account of the absence of visual hallucinations, and a compelling look at the relationship between LSD induced synesthesia and crossmodal correspondences. The hallucinatory experiences reported by BP are of particular interest in light of the observation that rates of psychosis within the CB population are extremely low. The phenomenology of the induced hallucinations suggests that experiences acquired through other means, might not give rise to "visual" experiences in the phenomenological sense, but instead gives rise to novel experiences in the other functioning senses.

Sound Properties Associated With Equiluminant Colours

There is a widespread tendency to associate certain properties of sound with those of colour (e.g., higher pitches with lighter colours). Yet it is an open question how sound influences chroma or hue when properly controlling for lightness. To examine this, we asked participants to adjust physically equiluminant colours until they ‘went best’ with certain sounds. For pure tones, complex sine waves and vocal timbres, increases in frequency were associated with increases in chroma. Increasing the loudness of pure tones also increased chroma. Hue associations varied depending on the type of stimuli. In stimuli that involved only limited bands of frequencies (pure tones, vocal timbres), frequency correlated with hue, such that low frequencies gave blue hues and progressed to yellow hues at 800 Hz. Increasing the loudness of a pure tone was also associated with a shift from blue to yellow. However, for complex sounds that share the same bandwidth of frequencies (100–3200 Hz) but that vary in terms of which frequencies have the most power, all stimuli were associated with yellow hues. This suggests that the presence of high frequencies (above 800 Hz) consistently yields yellow hues. Overall we conclude that while pitch–chroma associations appear to flexibly re-apply themselves across a variety of contexts, frequencies above 800 Hz appear to produce yellow hues irrespective of context. These findings reveal new sound–colour correspondences previously obscured through not controlling for lightness. Findings are discussed in relation to understanding the underlying rules of cross-modal correspondences, synaesthesia, and optimising the sensory substitution of visual information through sound.

The Emergence of Synaesthesia in a Neuronal Network Model via Changes in Perceptual Sensitivity and Plasticity

Synaesthesia is an unusual perceptual experience in which an inducer stimulus triggers a percept in a different domain in addition to its own. To explore the conditions under which synaesthesia evolves, we studied a neuronal network model that represents two recurrently connected neural systems. The interactions in the network evolve according to learning rules that optimize sensory sensitivity. We demonstrate several scenarios, such as sensory deprivation or heightened plasticity, under which synaesthesia can evolve even though the inputs to the two systems are statistically independent and the initial cross-talk interactions are zero. Sensory deprivation is the known causal mechanism for acquired synaesthesia and increased plasticity is implicated in developmental synaesthesia. The model unifies different causes of synaesthesia within a single theoretical framework and repositions synaesthesia not as some quirk of aberrant connectivity, but rather as a functional brain state that can emerge as a consequence of optimising sensory information processing.

Synaesthesia is a remarkable form of altered perception, where one attribute of a stimulus (e.g. sound) leads to the conscious experience of an additional attribute (often colour). Despite being known about for 200 years, there is no commonly agreed upon model for how and why synaesthesia emerges. This study presents a new model of synaesthesia based on computational principles that accounts for the emergence of different types of synaesthesia (acquired and developmental) as well as many of its key characteristics. The model describes how two independent neuronal systems can evolve to interact with one another even though their inputs are statistically uncorrelated. Specifically, synaesthesia arises as a result of instability in the learning process that shapes the network, which can be caused by heightened plasticity or due to sensory deprivation of one of the systems. The model unifies different aspects of synaesthesia and generates novel insights and predictions.

Non-optic vision: Beyond synesthesia?

Patient NS is a 28year-old female who went blind in her early twenties as a result of S-cone syndrome, a degenerative retinal disorder. A few years after losing her vision, she started experiencing visual perceptions of her hands as she moved them and objects that came into contact with her hands. Over the course of a year, these cross-modal sensations evolved to become veridical visual experiences accurately representative of her hands, objects she touched, and to some degree, objects she could infer from her immediate surroundings. We argue that these experiences are distinct from mental imagery as they occurred automatically, remained consistent over time, and were proprioceptively mediated by her head position much like normal optical vision. Moreover, she could neither consciously force these visual experiences to occur without sensory inference nor prevent them from happening when haptically exploring an object. Her previous visual experiences contributed to a strong influence of top-down processing in her perceptions. Though individuals have previously been able to develop limited veridical acquired synesthesia following extensive practice over many years with the use of a special sensory device, none reported experiencing the richness of complexity or degree of top-down processing exhibited by NS. Thus, we posit that NS's case may represent a phenomenon beyond synesthesia altogether.

Subtyping Somatic Tinnitus: A Cross-Sectional UK Cohort Study of Demographic, Clinical and Audiological Characteristics

Somatic tinnitus is the ability to modulate the psychoacoustic features of tinnitus by somatic manoeuvres. The condition is still not fully understood and further identification of this subtype is essential, particularly for the purpose of establishing protocols for both its diagnosis and treatment. This study aimed to investigate the characteristics of somatic tinnitus within a large UK cohort using a largely unselected sample. We believe this to be relatively unique in comparison to current literature on the topic. This was investigated by using a total of 608 participant assessments from a set of recognised tinnitus and audiology measures. Results from a set of chi-square tests of association found that amongst the individuals with somatic tinnitus, a higher proportion had pulsatile tinnitus (different from heartbeat), were under the age of 40, reported variation in the loudness of their tinnitus and reported temporomandibular joint (TMJ) disorder. The same pattern of results was confirmed using a multivariate analysis of the data based on logistic regression. These findings have strong implications towards the profiling of somatic tinnitus as a distinct subtype of general tinnitus.

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

Though synesthesia research has seen a huge growth in recent decades, and tremendous progress has been made in terms of understanding the mechanism and cause of synesthesia, we are still left mostly in the dark when it comes to the mechanistic commonalities (if any) among developmental, acquired and drug-induced synesthesia. We know that many forms of synesthesia involve aberrant structural or functional brain connectivity. Proposed mechanisms include direct projection and disinhibited feedback mechanisms, in which information from two otherwise structurally or functionally separate brain regions mix. We also know that synesthesia sometimes runs in families. However, it is unclear what causes its onset. Studies of psychedelic drugs, such as psilocybin, LSD and mescaline, reveal that exposure to these drugs can induce synesthesia. One neurotransmitter suspected to be central to the perceptual changes is serotonin. Excessive serotonin in the brain may cause many of the characteristics of psychedelic intoxication. Excessive serotonin levels may also play a role in synesthesia acquired after brain injury. In brain injury sudden cell death floods local brain regions with serotonin and glutamate. This neurotransmitter flooding could perhaps result in unusual feature binding. Finally, developmental synesthesia that occurs in individuals with autism may be a result of alterations in the serotonergic system, leading to a blockage of regular gating mechanisms. I conclude on these grounds that one commonality among at least some cases of acquired, developmental and drug-induced synesthesia may be the presence of excessive levels of serotonin, which increases the excitability and connectedness of sensory brain regions.

The induction of synaesthesia with chemical agents: a systematic review

Despite the general consensus that synaesthesia emerges at an early developmental stage and is only rarely acquired during adulthood, the transient induction of synaesthesia with chemical agents has been frequently reported in research on different psychoactive substances. Nevertheless, these effects remain poorly understood and have not been systematically incorporated. Here we review the known published studies in which chemical agents were observed to elicit synaesthesia. Across studies there is consistent evidence that serotonin agonists elicit transient experiences of synaesthesia. Despite convergent results across studies, studies investigating the induction of synaesthesia with chemical agents have numerous methodological limitations and little experimental research has been conducted. Cumulatively, these studies implicate the serotonergic system in synaesthesia and have implications for the neurochemical mechanisms underlying this phenomenon but methodological limitations in this research area preclude making firm conclusions regarding whether chemical agents can induce genuine synaesthesia.

Training, hypnosis, and drugs: artificial synaesthesia, or artificial paradises?

The last few years have seen the publication of a number of studies by researchers claiming to have induced "synaesthesia," "pseudo-synaesthesia," or "synaesthesia-like" phenomena in non-synaesthetic participants. Although the intention of these studies has been to try and shed light on the way in which synaesthesia might have been acquired in developmental synaesthestes, we argue that they may only have documented a phenomenon that has elsewhere been accounted for in terms of the acquisition of sensory associations and is not evidently linked to synaesthesia. As synaesthesia remains largely defined in terms of the involuntary elicitation of conscious concurrents, we suggest that the theoretical rapprochement with synaesthesia (in any of its guises) is unnecessary, and potentially distracting. It might therefore, be less confusing if researchers were to avoid referring to synaesthesia when characterizing cases that lack robust evidence of a conscious manifestation. Even in the case of those other conditions for which conscious experiences are better evidenced, when training has been occurred during hypnotic suggestion, or when it has been combined with drugs, we argue that not every conscious manifestation should necessarily be counted as synaesthetic. Finally, we stress that cases of associative learning are unlikely to shed light on two highly specific characteristic of the majority of cases of developmental synaesthesia in terms of learning patterns: First, their resistance to change through exposure once the synaesthetic repertoire has been fixed; Second, the transfer of conditioned responses between concurrents and inducers after training. We conclude by questioning whether, in adulthood, it is ever possible to acquire the kind of synaesthesia that is typically documented in the developmental form of the condition. The available evidence instead seems to point to there being a critical period for the development of synaesthesia, probably only in those with a genetic predisposition to develop the condition.

Seeing mathematics: perceptual experience and brain activity in acquired synesthesia

We studied the patient JP who has exceptional abilities to draw complex geometrical images by hand and a form of acquired synesthesia for mathematical formulas and objects, which he perceives as geometrical figures. JP sees all smooth curvatures as discrete lines, similarly regardless of scale. We carried out two preliminary investigations to establish the perceptual nature of synesthetic experience and to investigate the neural basis of this phenomenon. In a functional magnetic resonance imaging (fMRI) study, image-inducing formulas produced larger fMRI responses than non-image inducing formulas in the left temporal, parietal and frontal lobes. Thus our main finding is that the activation associated with his experience of complex geometrical images emerging from mathematical formulas is restricted to the left hemisphere.

Synesthesia

Although synesthesia has been known about for 200 years, it is only in the past decade or so that substantial progress has been made in studying it empirically and in understanding the mechanisms that give rise to it. The first part of the review considers the characteristics of synesthesia: its elicited nature, automaticity, prevalence, and consistency, and its perceptual and spatial phenomenology. The second part considers the causes of synesthesia both in terms of candidate neural mechanisms and the distal influences that shape this: genetic differences in developmental synesthesia and plasticity following sensory loss in acquired synesthesia. The final part considers developmental synesthesia as an individual difference in cognition and summarizes evidence of its influence on perception, imagery, memory, art/creativity, and numeracy.

Genuine and drug-induced synesthesia: a comparison

Despite some principal similarities, there is no systematic comparison between the different types of synesthesia (genuine, acquired and drug-induced). This comprehensive review compares the three principal types of synesthesia and focuses on their phenomenological features and their relation to different etiological models. Implications of this comparison for the validity of the different etiological models are discussed. Comparison of the three forms of synesthesia show many more differences than similarities. This is in contrast to their representation in the literature, where they are discussed in many respects as being virtually similar. Noteworthy is the much broader spectrum and intensity with the typical drug-induced synesthesias compared to genuine and acquired synesthesias. A major implication of the phenomenological comparison in regard to the etiological models is that genuine and acquired synesthesias point to morphological substrates, while drug-induced synesthesia appears to be based on functional changes of brain activity.

Neurophysiological investigation of idiopathic acquired auditory-visual synesthesia

We present a case of acquired auditory-visual synesthesia and its neurophysiological investigation in a healthy 42-year-old woman. She started experiencing persistent positive and intermittent negative visual phenomena at age 37 followed by auditory-visual synesthesia. Her neurophysiological investigation included video-EEG, fMRI, and MEG. Auditory stimuli (700 Hz, 50 ms duration, 0.5 s ISI) were presented binaurally at 60 db above the hearing threshold in a dark room. The patient had bilateral symmetrical auditory-evoked neuromagnetic responses followed by an occipital-evoked field 16.3 ms later. The activation of occipital cortex following auditory stimuli may represent recruitment of existing cross-modal sensory pathways.

"Blue is music to my ears": multimodal synesthesias after a thalamic stroke

Acquired synesthesias have been reported in association with deafferentation of the visual system, temporal lobe seizures, and the use of psychedelics. Based on our review of the literature, the appearance of synesthesias after a thalamic stroke has been reported only once. We present the case of a 45-year-old hypertensive male who, 9 months after a hemorrhagic stroke involving the left lateral posterior nucleus of the thalamus developed persistent sound-tactile, sound-color, and grapheme-gustatory synesthesias. Moreover, the patient noted that even thinking about a sensory stimulus could trigger the experience of another sensory modality, a conceptual type of synesthesia.