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Neuron Nov 2005Synesthesia is a condition in which stimulation of one sensory modality causes unusual experiences in a second, unstimulated modality. Although long treated as a... (Review)
Review
Synesthesia is a condition in which stimulation of one sensory modality causes unusual experiences in a second, unstimulated modality. Although long treated as a curiosity, recent research with a combination of phenomenological, behavioral, and neuroimaging methods has begun to identify the cognitive and neural basis of synesthesia. Here, we review this literature with an emphasis on grapheme-color synesthesia, in which viewing letters and numbers induces the perception of colors. We discuss both the substantial progress that has been made in the past fifteen years and some open questions. In particular, we focus on debates in the field relating to the neural basis of synesthesia, including the relationship between synesthesia and attention and the role of meaning in synesthetic colors. We propose that some, but probably not all, of these differences can be accounted for by differences in the synesthetes studied and discuss some methodological implications of these individual differences.
Topics: Attention; Brain; Cognition; Color Perception; Feedback; Humans; Models, Neurological; Perceptual Disorders; Photic Stimulation
PubMed: 16269367
DOI: 10.1016/j.neuron.2005.10.012 -
Neuropsychopharmacology : Official... Oct 2017All modern clinical studies using the classic hallucinogen lysergic acid diethylamide (LSD) in healthy subjects or patients in the last 25 years are reviewed herein.... (Review)
Review
All modern clinical studies using the classic hallucinogen lysergic acid diethylamide (LSD) in healthy subjects or patients in the last 25 years are reviewed herein. There were five recent studies in healthy participants and one in patients. In a controlled setting, LSD acutely induced bliss, audiovisual synesthesia, altered meaning of perceptions, derealization, depersonalization, and mystical experiences. These subjective effects of LSD were mediated by the 5-HT receptor. LSD increased feelings of closeness to others, openness, trust, and suggestibility. LSD impaired the recognition of sad and fearful faces, reduced left amygdala reactivity to fearful faces, and enhanced emotional empathy. LSD increased the emotional response to music and the meaning of music. LSD acutely produced deficits in sensorimotor gating, similar to observations in schizophrenia. LSD had weak autonomic stimulant effects and elevated plasma cortisol, prolactin, and oxytocin levels. Resting-state functional magnetic resonance studies showed that LSD acutely reduced the integrity of functional brain networks and increased connectivity between networks that normally are more dissociated. LSD increased functional thalamocortical connectivity and functional connectivity of the primary visual cortex with other brain areas. The latter effect was correlated with subjective hallucinations. LSD acutely induced global increases in brain entropy that were associated with greater trait openness 14 days later. In patients with anxiety associated with life-threatening disease, anxiety was reduced for 2 months after two doses of LSD. In medical settings, no complications of LSD administration were observed. These data should contribute to further investigations of the therapeutic potential of LSD in psychiatry.
Topics: Hallucinogens; History, 20th Century; History, 21st Century; Humans; Lysergic Acid Diethylamide; Mental Disorders
PubMed: 28447622
DOI: 10.1038/npp.2017.86 -
Frontiers in Bioscience (Landmark... Jan 2021Synesthesia literally means a "union of the senses" whereby two or more of the five senses that are normally experienced separately are involuntarily and automatically... (Review)
Review
Synesthesia literally means a "union of the senses" whereby two or more of the five senses that are normally experienced separately are involuntarily and automatically joined together in experience (1, 2, 3). For example, some synesthetes experience a color when they hear a sound, although many instances of synesthesia also occur entirely within the visual sense. In this paper, I first mainly engage critically with Sollberger's view that there is reason to think that at least some synesthetic experiences can be viewed as truly veridical perceptions, and not as illusions or hallucinations (4). Among other things, I explore the possibility that many forms of synesthesia can be understood as experiencing what I will call "second-order secondary properties," that is, experiences of properties of objects induced by the secondary qualities of those objects. In doing so, I shed some light on why synesthesia is typically one-directional and its relation to some psychopathologies such as autism.
Topics: Autistic Disorder; Hallucinations; Humans; Synesthesia
PubMed: 33049694
DOI: 10.2741/4918 -
Dementia & Neuropsychologia 2015The present review examined the cross-modal association of sensations and their relationship to musical perception. Initially, the study focuses on synesthesia, its... (Review)
Review
The present review examined the cross-modal association of sensations and their relationship to musical perception. Initially, the study focuses on synesthesia, its definition, incidence, forms, and genetic and developmental factors. The theories of the neural basis of synesthesia were also addressed by comparing theories emphasizing the anatomical aspect against others reinforcing the importance of physiological processes. Secondly, cross-modal sensory associations, their role in perception, and relationship to synesthesia were analyzed. We propose the existence of a lower, unconscious degree of synesthesia in non-synesthetes. This latent synesthesia (without explicit sensory manifestations) would be functional, aiding the construction of abstract associations between different perceptual fields. Musical meaning might be constructed largely by synesthetic processes, where the sensory associations from sound activate memories, images, and emotions.
PubMed: 29213937
DOI: 10.1590/S1980-57642015DN91000004 -
Translational Neuroscience 2017Synesthesia is commonly thought to be a phenomenon of fixed associations between an outside inducer and a vivid concurrent experience. Hence, it has been proposed that...
Synesthesia is commonly thought to be a phenomenon of fixed associations between an outside inducer and a vivid concurrent experience. Hence, it has been proposed that synesthesia occurs due to additional connections in the brain with which synesthetes are born. Here we show that synesthesia can be a much richer and more flexible phenomenon with a capability to creatively construct novel synesthetic experiences as events unfold in people's lives. We describe here cases of synesthetes who occasionally generate novel synesthetic experience, called one-shot synesthesias. These synesthetic experiences seem to share all the properties with the classical synesthetic associations except that they occur extremely rarely, people recalling only a few events over the lifetime. It appears that these one-shots are not created at random but are instead responses to specific life events. We contrast the properties of those rare synesthetic events with other, more commonly known forms of synesthesia that also create novel synesthetic experiences, but at a high rate-sometimes creating novel experiences every few seconds. We argue that one-shot synesthesias indicate that synesthetic associations are by their nature not prewired at birth but are dynamically constructed through mental operations and according to the needs of a synesthetic mind. Our conclusions have implications for understanding the biological underpinnings of synesthesia and the role the phenomenon plays in the lives of people endowed with synesthetic capacities.
PubMed: 29188078
DOI: 10.1515/tnsci-2017-0023 -
Frontiers in Human Neuroscience 2014Learning and synesthesia are profoundly interconnected. On the one hand, the development of synesthesia is clearly influenced by learning. Synesthetic inducers - the... (Review)
Review
Learning and synesthesia are profoundly interconnected. On the one hand, the development of synesthesia is clearly influenced by learning. Synesthetic inducers - the stimuli that evoke these unusual experiences - often involve the perception of complex properties learned in early childhood, e.g., letters, musical notes, numbers, months of the year, and even swimming strokes. Further, recent research has shown that the associations individual synesthetes make with these learned inducers are not arbitrary, but are strongly influenced by the structure of the learned domain. For instance, the synesthetic colors of letters are partially determined by letter frequency and the relative positions of letters in the alphabet. On the other hand, there is also a small, but growing, body of literature which shows that synesthesia can influence or be helpful in learning. For instance, synesthetes appear to be able to use their unusual experiences as mnemonic devices and can even exploit them while learning novel abstract categories. Here we review these two directions of influence and argue that they are interconnected. We propose that synesthesia arises, at least in part, because of the cognitive demands of learning in childhood, and that it is used to aid perception and understanding of a variety of learned categories. Our thesis is that the structural similarities between synesthetic triggering stimuli and synesthetic experiences are the remnants, the fossilized traces, of past learning challenges for which synsethesia was helpful.
PubMed: 24592232
DOI: 10.3389/fnhum.2014.00098 -
Current Psychiatry Reports Jun 2007Synesthesia is an experience in which stimulation in one sensory or cognitive stream leads to associated experiences in a second, unstimulated stream. Although... (Review)
Review
Synesthesia is an experience in which stimulation in one sensory or cognitive stream leads to associated experiences in a second, unstimulated stream. Although synesthesia is often referred to as a "neurological condition," it is not listed in the DSM IV or the ICD classifications, as it generally does not interfere with normal daily functioning. However, its high prevalence rate (one in 23) means that synesthesia may be reported by patients who present with other psychiatric symptoms. In this review, I focus on recent research examining the neural basis of the two most intensively studied forms of synesthesia, grapheme --> color synesthesia and tone --> color synesthesia. These data suggest that these forms of synesthesia are elicited through anomalous activation of color-selective areas, perhaps in concert with hyperbinding mediated by the parietal cortex. I then turn to questions for future research and the implications of these models for other forms of synesthesia.
Topics: Association; Cerebral Cortex; Color Perception; Electroencephalography; Humans; Magnetic Resonance Imaging; Magnetoencephalography; Models, Neurological; Parietal Lobe; Pattern Recognition, Visual; Perceptual Disorders; Pitch Perception; Positron-Emission Tomography; Reading; Semantics; Visual Cortex; Visual Perception
PubMed: 17521514
DOI: 10.1007/s11920-007-0018-6 -
Brain Sciences Oct 2022Number-color synesthesia is a condition in which synesthetes perceive numbers with concurrent experience of specific, corresponding colors. It has been proposed that...
Number-color synesthesia is a condition in which synesthetes perceive numbers with concurrent experience of specific, corresponding colors. It has been proposed that synesthetic association exists primarily between representations of Arabic digit graphemes and colors, and a secondary, semantic connection between numerosity and colors is built via repeated co-activation. However, this distinction between the graphemic and semantic pathways of synesthetic number-color connection has not been empirically tested. The current study aims to dissociate graphemic and semantic aspects of color activations in number-color synesthesia by comparing their time courses. We adopted a synesthetic priming paradigm with varied stimuli onset asynchronies (SOAs). A number (2-6, prime) was presented in one of three notations: digit, dice, or non-canonical dot pattern, and a color patch (target) appeared with an SOA of 0, 100, 300, 400, or 800 ms. Participants reported the color as quickly as possible. Using the congruency effect (i.e., shorter reaction time when target color matched the synesthetic color of number prime) as an index of synesthetic color activation level, we revealed that the effect from the graphemic pathway is quick and relatively persistent, while the effect from the semantic pathway unfolds at a later stage and is more transient. The dissociation between the graphemic and semantic pathways of synesthesia implies further functional distinction within "conceptual synesthesia", which has been originally discussed as a unitary phenomenon. This distinction has been demonstrated by the differential time courses of synesthetic color activations, and suggested that a presumed, single type of synesthesia could involve multiple mechanisms.
PubMed: 36291334
DOI: 10.3390/brainsci12101400 -
Cortex; a Journal Devoted To the Study... Aug 2018When I am looking at my coffee machine that makes funny noises, this is an instance of multisensory perception - I perceive this event by means of both vision and... (Review)
Review
When I am looking at my coffee machine that makes funny noises, this is an instance of multisensory perception - I perceive this event by means of both vision and audition. But very often we only receive sensory stimulation from a multisensory event by means of one sense modality, for example, when I hear the noisy coffee machine in the next room, that is, without seeing it. The aim of this paper is to bring together empirical findings about multimodal perception and empirical findings about (visual, auditory, tactile) mental imagery and argue that on occasions like this, we have multimodal mental imagery: perceptual processing in one sense modality (here: vision) that is triggered by sensory stimulation in another sense modality (here: audition). Multimodal mental imagery is not a rare and obscure phenomenon. The vast majority of what we perceive are multisensory events: events that can be perceived in more than one sense modality - like the noisy coffee machine. And most of the time we are only acquainted with these multisensory events via a subset of the sense modalities involved - all the other aspects of these multisensory events are represented by means of multisensory mental imagery. This means that multisensory mental imagery is a crucial element of almost all instances of everyday perception.
Topics: Auditory Perception; Brain; Humans; Imagination; Multimodal Imaging; Photic Stimulation; Visual Perception
PubMed: 28801065
DOI: 10.1016/j.cortex.2017.07.006 -
Philosophical Transactions of the Royal... Dec 2019In synaesthesia, stimulation of one sensory modality evokes additional experiences in another modality (e.g. sounds evoking colours). Along with these cross-sensory... (Review)
Review
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'.
Topics: Cognition; Color Perception; Humans; Models, Neurological; Models, Psychological; Synesthesia
PubMed: 31630652
DOI: 10.1098/rstb.2019.0029