Cross-modal synesthesia

“Synesthesia is an inherited condition that gives rise to involuntary cross-sensations. for synesthetes, the presentation of a particular triggering stimulus (e.g., a sound) elicits a secondary experience (e.g., a color) in addition to the usual sensory phenomenology. synesthetic experiences are automatic, involuntary, and consistent over time. synesthesia is a multivariant condition, with a range of triggers and resultant experiences (e.g., sounds may trigger tastes, tastes may trigger colors, etc.). the condition has a long history or study, a known genetic basis, and a neurological profile that can be seen in both functional and structure imaging.”

“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.”

“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. copyright ©2005 by elsevier inc.”

“Aims: although classic hallucinogens have relatively low physiological toxicity and do not produce compulsive drug seeking, they can produce adverse psychological reactions (i.e., ‘bad trips’). an online survey study sought to better characterize such experiences. methods: participants completed an anonymous online survey in reference to their most challenging experience after ingesting psilocybin mushrooms. the experience was required not to have occurred in the context of a research study. the questionnaire assessed the phenomenology of the experience. results: there were 1853 participants (mean age 29 yrs; 81% male; 50% college or graduate degree). 39% rated the experience as among the 5 most challenging experiences of their lives, and 34% rated the experience as among the 5 most meaningful of their lives. scores on the challenging experience questionnaire showed a reasonably high correlation with ratings of difficulty of experience (r = 0.516), but not with ratings of the degree of spiritual significance (r = 0.125). in contrast, scores on the mystical experience questionnaire correlated moderately highly with spiritual significance ratings (r = 0.658) but not with degree of difficulty (r = 0.198). consistent with known effects of psilocybin, participants endorsed visual effects (61%) and auditory (23%) and visual (21%) synesthesia. anomalous experiences were endorsed, including: interactions with entities/persons not physically present (29%), convincing experiences of prior lives (10%), and experiences of jamais vu (17%). 11% reported that they put themselves or others at risk for physical harm during the experience. conclusions: participants completed a survey about their most challenging experience after psilocybin. scores of challenging experience and mystical experience showed different patterns of correlations. visual and auditory effects and unusual phenomenology were reported.”

“Synesthesia is a conscious experience of systematically induced sensory attributes that are not experienced by most people under comparable conditions. recent findings from cognitive psychology, functional brain imaging and electrophysiology have shed considerable light on the nature of synesthesia and its neurocognitive underpinnings. these cognitive and physiological findings are discussed with respect to a neuroanatomical framework comprising hierarchically organized cortical sensory pathways. we advance a neurobiological theory of synesthesia that fits within this neuroanatomical framework. copyright © 2001 elsevier science ltd.”

“Studies have shown that both serotonin and glutamate receptor systems play a crucial role in the mechanisms underlying drug-induced synesthesia. the specific nature of these mechanisms, however, continues to remain elusive. here we propose two distinct hypotheses for how synesthesia triggered by hallucinogens in the serotonin agonist family may occur. one hypothesis is that the drug-induced destabilization of thalamic projections via ?-aminobutyric acidergic neuronal circuits from sensory areas leads to a disruption of low-level, spontaneous integration of multisensory stimuli. this sort of integration regularly occurs when spatial and temporal attributes match. destabilization of feedback loops, however, can result in incongruent experiences or binding of random thalamus activation with sensory input in a particular sensory modality. the second hypothesis builds on embodied cognition, cases in which visual images of external stimuli activate task-related neural regions. in this proposal, binding processes that do not normally generate awareness become accessible to consciousness as a result of decreased attentional discrimination among incoming stimuli.”

“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.”

“Enhanced color perception and vividly colored visual hallucinations are frequently reported by persons treated with psyehotomimetie drugs (klt~ve~ 1942; landis and clavson, 1954; a]3~a~so~ et al., 1955a; a]3ramson et al., 1955b). these findings are not explained readily by neurophysiologie or psychophysiologie studies of the action of these drugs. in this respect, the vivid and unusual experiences of color asso-ciated with lysergic acid diethylanfide (lsd-25) or mescaline intoxi-cation seem paradoxical in view of evidence of increased thresholds for axonal response beyond the thalamic synapses (evarts, 1955; bishop, 1958), and demonstrations of increased absolute and differential limens for vision (carlson, 1958). if anything, these data suggest that the marked visual effects associated with the action of the psychotomimetie drugs are not due to increased sensitivity to the normally adequate stimuli for color visual experience. some clues to this apparent paradox are provided by the facilitating effects of sensory input from other modalities on the perception of visual hallucinations, commonly referred to as synesthesia. in such cases, perceptual responses may be evoked by stimuli which ordinarily evoke a separate sensory response, for example, sounds evoking colored patterns. one might hypothesize that some psychotomimetics interfere with the normal integration of experience within specific sensory modalities. the way we chose to test this hypothesis was to study the effects of stim-uli varying in degree of adequacy for evoking color experience, both before and during the administration of three psyehotomimetie drugs. the stimuli employed included those directly involving color perception (discrimination of hues and reports of colors of after-images), those usually marginal in terms of color experience (gazing at an episcotister),”

“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.”

“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 vari- ances. 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 hal- lucinations. the decrease of alpha oscillations by hallucinogens will then be intro- duced 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”

“Synesthesia is a perceptual experience in which stimuli presented through one modality will spontaneously evoke sensations in an unrelated modality. the condition occurs from increased communication between sensory regions and is involuntary, automatic, and stable over time. while synesthesia can occur in response to drugs, sensory deprivation, or brain damage, research has largely focused on heritable variants comprising roughly 4% of the general population. genetic research on synesthesia suggests the phenomenon is heterogeneous and polygenetic, yet it remains unclear whether synesthesia ever provided a selective advantage or is merely a byproduct of some other useful selected trait. progress in uncovering the genetic basis of synesthesia will help us understand why synesthesia has been conserved in the population.”

“Altered states of consciousness induced by hallucinogens (h-asc) is still a vaguely understood phenomenon. taken the diverse psychological effects they exert, the main mechanism of action of hallucinogens; lsd, ibogaine, thc, pcp, mdma, methamphetamine, mescaline, psilocybin and dmt, of which psychological effects are discussed in the article, are not properly understood and explained by the modern methods of neuroscience due to the lack of vigorous research. the involvement of some receptors, such as, 5-ht2 (and probably other 5-ht receptors), glutamate and dopamine receptors, adrenergic and cannabinoid receptors, is one of the mechanisms, however it is not easy to explain such incongruent psychological effects by only receptor and neurotransmitter systems alone, since h-ascs have, sometimes, their own unity and gestalt, unfolding the subconscious, in the ‘voyages’ they induce, although the perception may, or may not, be distorted depending on the person, and ‘set and setting’. they induce visual, tactile and auditory hallucinations; synesthesia; perception of fractals, geometrical and kaleidoscopic images with vivid colors; perception of two dimensional pictures as three dimensional, animated and moving; distortions and alterations in the body perception; alterations in the perceptions of temporal-spatial continuum and time; changes in the perception of the ego and the self; feelings of unification with nature and universe, peak experiences – mimicking satori or nirvana-, ecstasy, rapture, extreme euphoria, excitement and happiness, oceanic bliss, self-fulfillment, referred as ‘good trip’, as well as, dysphoria, anxiety, mania, delirium, psychosis, acute schizophrenia, collapse of the self, known as ‘bad trip’; depending upon the mood, affection and psychology of the person, and ‘set and setting’. mysticomimetic effects of h-ascs, imitate the consciousness states of ancient mystics, probably, by means of activating prefrontal cortex, limbic system and the right temporal lobe. a hypothetical ‘holographic brain theory’ may give some extra insights about the explanation of some of the effects of h-ascs. it should be taken into account that h-ascs, can be accepted as a good tool to investigate the nature of consciousness, brain and the human psyche, as well as some of the h-ascs are good models of psychosis, too. more detailed scientific research should be performed to understand the basic and real mechanisms of h-ascs, to comprehend and unravel the myste…”

“Diffusion tensor imaging allowed us to validate for the first time the hypothesis that hyperconnectivity causes the added sensations in synesthesia. grapheme-color synesthetes (n = 18), who experience specific colors with particular letters or numbers (for example, ’r is sky blue’), showed greater anisotropic diffusion compared with matched controls. greater anisotropic diffusion indicates more coherent white matter. anisotropy furthermore differentiated subtypes of grapheme-color synesthesia. greater connectivity in the inferior temporal cortex was particularly strong for synesthetes who see synesthetic color in the outside world (‘projectors’) as compared with synesthetes who see the color in their ‘mind’s eye’ only (‘associators’). in contrast, greater connectivity (as compared with non-synesthetes) in the superior parietal or frontal cortex did not differentiate between subtypes of synesthesia. in conclusion, we found evidence that increased structural connectivity is associated with the presence of grapheme-color synesthesia, and has a role in the subjective nature of synesthetic color experience.”

“Objective: serotonergic hallucinogens such as psilocybin produce positive-like symptoms and perceptual disturbances including visual hallucinations and deficits in visual contour processing that are reminiscent to that observed in some patients in early or acute stages of schizophrenia. however, the neurophysiological mechanisms and 5-ht receptor sites that mediate these visual perceptual alterations remain largely unknown. methods: to further elucidate the role of 5-ht2a receptors in visual processing, we performed a double-blind placebo-controlled randomized trial in healthy human subjects (n=20) to assess the effects of psilocybin (215 mg/kg p.o.), on prestimulus and subsequent visual stimulus-induced parieto-occipital alpha oscillations and early visual-evoked potentials and to determine whether these psilocybin-induced effects are (ii) reversed by the preferential 5-ht2a antagonist ketanserin (50 mg p.o.) or the partial 5-ht1a agonist buspirone. results: analysis of the visual-evoked potentials revealed that psilocybin selectively increased the medial p1 potential, whereas ketanserin selectively decreased the medial p1 potential. the subsequent n170 potential was strongly decreased over the lateral occipital cortex by psilocybin and associated with the appearance of visual hallucinations and audiovisual synesthesia, both of which were completely blocked by the 5-ht2a antagonist ketanserin. a correlational analysis showed that the decease the n170 potential correlated with the intensity of psilocybin-induced subjective alterations in visual perceptions. in contrast to this, the partial 5-ht1a agonist buspirone did only moderately reduced psilocybin-induced visual disturbances and partially reversed the reduction in the n170 potential. conclusion: the present results show that 5-ht2a rather than 5-ht1a receptor stimulation is the key mechanism for the generation of visual hallucinations and audiovisual synesthesia in psilocybin states and suggests that such a mechanism may also be responsible for the visual disturbances observed in early schizophrenia and/or parkinson’s disease.”

“Synesthesia is an unusual condition in which stimulation of one modality evokes sensation or experience in another modality. although discussed in the literature well over a century ago, synesthesia slipped out of the scientific spotlight for decades because of the difficulty in verifying and quantifying private perceptual experiences. in recent years, the study of synesthesia has enjoyed a renaissance due to the introduction of tests that demonstrate the reality of the condition, its automatic and involuntary nature, and its measurable perceptual consequences. however, while several research groups now study synesthesia, there is no single protocol for comparing, contrasting and pooling synesthetic subjects across these groups. there is no standard battery of tests, no quantifiable scoring system, and no standard phrasing of questions. additionally, the tests that exist offer no means for data comparison. to remedy this deficit we have devised the synesthesia battery. this unified collection of tests is freely accessible online (http://www.synesthete.org). it consists of a questionnaire and several online software programs, and test results are immediately available for use by synesthetes and invited researchers. performance on the tests is quantified with a standard scoring system. we introduce several novel tests here, and offer the software for running the tests. by presenting standardized procedures for testing and comparing subjects, this endeavor hopes to speed scientific progress in synesthesia research. © 2006 elsevier b.v. all rights reserved.”

“The alkaloids of the third magic mexican drug ‘ololiuqui’ – chemically closely related to lsd 25 – were studied in normal subjects in a laboratorysituation. a standardized method (developed and applied before in testing other psychotomimetic drugs) included the following procedure: a short interview taken every 45 min was registered on sound-movie and then analyzed for changes in expression and motor behaviour. a battery of psychometric tests also given every 45th minute was studied for trend-changes over time. finally, symptoms reported by the subjects were divided into 8 categories, reflecting a spectrum going from autonomic side-effects to psychotic symptoms and alteration of consciousness. the results indicate that the effect of the two major compounds, d-lysergicacid-amide and d-iso-lysergic-acid-amide, determine the effect of the total alkaloids of ololiuqui. with d-lysergic-acid-amide the clinical picture was one of intoxication with strong autonomic side-effects; with d-iso-lysergic-acid-amide euphoria synesthesia and altered time experience were observed. in small doses the response to total-alkaloids resembled that of d-iso-lysergic-acid-amide; in higher doses it resembled that of d-lysergic-acid-amide, i.e. a heavy intoxication with reduced consciousness and autonomic side-effects. overall, there was little difference between the total alkaloids and the compounds of ololiuqui. psychometric tests taken at each 45-min-phase also reflected the progressive intoxication: the performance in each of the five tests measuring the ability to concentrate was increasingly reduced, as compared to placebo-subjects, during the time of observation. this is in contrast to observations on psilocybin: here the psychometric results were worst after the first 45-min-phase, whereas changes in expression and motor behaviour were greatest in the last of the phases, i.e. after 2-3 hours. not only are there no phase-dependent alterations with qualitative modification of the effects in ololiuqui as compared with psilocybin; in addition there was little evidence of typical alteration in consciousness and hallucinations as typically seen with psychotomimetic drugs like lsd 25 and psilocybin. in conclusion, ololiuqui tested in normal subjects in a standard laboratorysituation lacks the characterictics of the typical psychotomimetic drugs. it resembles more drugs like scopolamine and ibogalin, inducing a toxic-psychosis. ? 1968 springer-verlag.”

“In this review, the authors distinguish strong and weak forms of synesthesia. strong synesthesia is characterized by a vivid image in one sensory modality in response to stimulation in another one. weak synesthesia is characterized by cross-sensory correspondences expressed through language, perceptual similarity, and perceptual interactions during information processing. despite important phenomenological dissimilarities between strong and weak synesthesia, we maintain that the two forms draw on similar underlying mechanisms. the study of strong and weak synesthetic phenomena provides an opportunity to enrich scientists’ understanding of basic mechanisms involved in perceptual coding and cross-modal information processing. (psycinfo database record (c) 2010 apa, all rights reserved)”

“Watching another person being touched activates a similar neural circuit to actual touch and, for some people with ‘mirror-touch’ synesthesia, can produce a felt tactile sensation on their own body. in this study, we provide evidence for the existence of this type of synesthesia and show that it correlates with heightened empathic ability. this is consistent with the notion that we empathize with others through a process of simulation.”

“Synesthesia provides an elegant model to investigate neural mechanisms underlying individual differences in subjective experience in humans. in grapheme-color synesthesia, written letters induce color sensations, accompanied by activation of color area v4. competing hypotheses suggest that enhanced v4 activity during synesthesia is either induced by direct bottom-up cross-activation from grapheme processing areas within the fusiform gyrus, or indirectly via higher-order parietal areas. synesthetes differ in the way synesthetic color is perceived: ‘projector’ synesthetes experience color externally colocalized with a presented grapheme, whereas ‘associators’ report an internally evoked association. using dynamic causal modeling for fmri, we show that v4 cross-activation during synesthesia was induced via a bottom-up pathway (within fusiform gyrus) in projector synesthetes, but via a top-down pathway (via parietal lobe) in associators. these findings show how altered coupling within the same network of active regions leads to differences in subjective experience. our findings reconcile the two most influential cross-activation accounts of synesthesia.”

“In synesthesia, stimulation of one sensory modality (e.g., hearing) triggers a percept in another, non-stimulated sensory modality (e.g., vision). likewise, perception of a form (e.g., a letter) may induce a color percept (i.e., grapheme-color synesthesia). to date, the neural mechanisms underlying synesthesia remain to be elucidated. we disclosed by fmri, while controlling for surface color processing, enhanced activity in the left intraparietal cortex during the experience of grapheme-color synesthesia (n = 9). in contrast, the perception of surface color per se activated the color centers in the fusiform gyrus bilaterally. the data support theoretical accounts that grapheme-color synesthesia may originate from enhanced cross-modal binding of form and color. a mismatch of surface color and grapheme induced synesthetically felt color additionally activated the left dorsolateral prefrontal cortex (dlpfc). this suggests that cognitive control processes become active to resolve the perceptual conflict resulting from synesthesia. © 2005 elsevier inc. all rights reserved.”

“Synesthesia is a perceptual phenomenon in which stimuli can trigger experiences in non-stimulated sensory dimensions. the literature has focused on forms of synesthesia in which stimuli (e.g. music, touch or numbers) trigger experiences of color. generally missing, however, is the observation that synesthetic colors are often accompanied by the experience of other surface properties such as texture (e.g. a visual experience of linen, metal, marble, velvet, etc). current frameworks for synesthesia focus only upon the involvement of brain regions such as the v4 color complex. here, we propose an expanded framework that includes brain regions involved in the encoding of material properties – specifically, larger regions of the medial ventral stream. the overlap of visual texture and color processing within ventral regions might explain why many experiences of synesthesia extend beyond color to other material properties. © 2009 elsevier ltd. all rights reserved.”

“People with color-grapheme synesthesia experience color when viewing written letters or numerals, usually with a particular color evoked by each grapheme. here, we report on data from 11 color-grapheme synesthetes who had startlingly similar color-grapheme pairings traceable to childhood toys containing colored letters. these are the first and only data to show learned synesthesia of this kind in more than a single individual. whereas some researchers have focused on genetic and perceptual aspects of synesthesia, our results indicate that a complete explanation of synesthesia must also incorporate a central role for learning and memory. we argue that these two positions can be reconciled by thinking of synesthesia as the automatic retrieval of highly specific mnemonic associations, in which perceptual contents are brought to mind in a manner akin to mental imagery or the perceptual-reinstatement effects found in memory studies.”

“In this article, the authors introduce a new theoretical framework for understanding intersensory development. their approach is based upon insights gained from adults who experience synesthesia, in whom sensory stimuli induce extra cross-modal or intramodal percepts. synesthesia appears to represent one way that typical developmental mechanisms can play out by magnifying connections present in early life that are pruned and/or inhibited during development but persist in muted form in all adults. as such, the study of synesthesia provides valuable insights into the nature of intersensory development. the authors review evidence on the perceptual reality and neural basis of synesthesia, then summarize developmental models and evidence that its underlying mechanisms are universal in adults. they illustrate how evidence for consistent sensory associations in adults leads to predictions about toddlers’ perception and present 3 bodies of work that have confirmed those hypotheses. they end by describing novel hypotheses about intersensory development that arise from this framework. such intersensory associations appear to reflect intrinsic sensory cortical organization that influences the development of perception and of language and that may constrain the learning of environmentally based associations.”

“Synesthesia is an unusual condition characterized by the over-binding of two or more features and the concomitant automatic and conscious experience of atypical, ancillary images or perceptions [1-3]. previous research suggests that synesthetes display enhanced modality-specific perceptual processing [4-7], but it remains unclear whether enhanced processing contributes to conscious awareness of color photisms. in three experiments, we investigated whether grapheme-color synesthesia is characterized by enhanced cortical excitability in primary visual cortex and the role played by this hyperexcitability in the expression of synesthesia. using transcranial magnetic stimulation, we show that synesthetes display 3-fold lower phosphene thresholds than controls during stimulation of the primary visual cortex. we next used transcranial direct current stimulation to discriminate between two competing hypotheses of the role of hyperexcitability in the expression of synesthesia. we demonstrate that synesthesia can be selectively augmented with cathodal stimulation and attenuated with anodal stimulation of primary visual cortex. a control task revealed that the effect of the brain stimulation was specific to the experience of synesthesia. these results indicate that hyperexcitability acts as a source of noise in visual cortex that influences the availability of the neuronal signals underlying conscious awareness of synesthetic photisms. © 2011 elsevier ltd. all rights reserved.”

“Reviews the book, ‘synesthesia: perspectives from cognitive neuroscience’ by l. c. robertson and n. sagiv, editors (see record 2005-09449-000). synaesthesia is a familial condition in which ordinary activities (eg, reading, listening to music) trigger extra-ordinary experiences (eg, colours, tastes). twelve articles address this unusual phenomenon with a particular neuroscience slant in the book. topics covered by the book include the development of synesthesia, the role of attention in synaesthetic binding, and first-person accounts of synaesthetic experiences. the reviewer states that the work provides a broad cross-section of interest for synaesthesia researchers, and does so in a readable and comprehensive way. the book has some faults, most notably the use of a number of different terms to describe the same phenomena, which may be confusing to the reader. however, the range of viable terminology used in the volume might simply reflect the lack of terminological agreement peppered throughout the literature. because of its breadth of coverage and readability, the book is recommended to researchers and students, in philosophy, psychology, or neuroscience, and it is a must-read for those wishing to get acquainted with this unusual and fascinating phenomenon. (psycinfo database record (c) 2010 apa, all rights reserved)”

“Synesthesia, a condition in which a stimulus in one sensory modality consistently and automatically triggers concurrent percepts in another modality, provides a window into the neural correlates of cross-modal associations. while research on grapheme-color synesthesia has provided evidence for both hyperconnectivity-hyperbinding and disinhibited feedback as potential underlying mechanisms, less research has explored the neuroanatomical basis of other forms of synesthesia. in the current study we investigated the white matter correlates of colored-music synesthesia. as these synesthetes report seeing colors upon hearing musical sounds, we hypothesized that they might show unique patterns of connectivity between visual and auditory association areas. we used diffusion tensor imaging to trace the white matter tracts in temporal and occipital lobe regions in 10 synesthetes and 10 matched non-synesthete controls. results showed that synesthetes possessed hemispheric patterns of fractional anisotropy, an index of white matter integrity, in the inferior fronto-occipital fasciculus (ifof), a major white matter pathway that connects visual and auditory association areas to frontal regions. specifically, white matter integrity within the right ifof was significantly greater in synesthetes than controls. furthermore, white matter integrity in synesthetes was correlated with scores on audiovisual tests of the synesthesia battery, especially in white matter underlying the right fusiform gyrus. our findings provide the first evidence of a white matter substrate of colored-music synesthesia, and suggest that enhanced white matter connectivity is involved in enhanced cross-modal associations. © 2013 elsevier inc.”

“Synesthesia is a condition where presentation of one perceptual class consistently evokes additional experiences in different perceptual categories. synesthesia is widely considered a congenital condition, although an alternative view is that it is underpinned by repeated exposure to combined perceptual features at key developmental stages. here we explore the potential for repeated associative learning to shape and engender synesthetic experiences. non-synesthetic adult participants engaged in an extensive training regime that involved adaptive memory and reading tasks, designed to reinforce 13 specific letter-color associations. following training, subjects exhibited a range of standard behavioral and physiological markers for grapheme-color synesthesia; crucially, most also described perceiving color experiences for achromatic letters, inside and outside the lab, where such experiences are usually considered the hallmark of genuine synesthetes. collectively our results are consistent with developmental accounts of synesthesia and illuminate a previously unsuspected potential for new learning to shape perceptual experience, even in adulthood”

“In synesthesia, certain stimuli (‘inducers’) may give rise to perceptual experience in additional modalities not normally associated with them (‘concurrent’). for example, color-grapheme synesthetes automatically perceive achromatic numbers as colored (e.g., 7 is turquoise). although synesthetes know when a given color matches the one evoked by a certain number, colors do not automatically give rise to any sort of number experience. the behavioral consequences of synesthesia have been documented using stroop-like paradigms, usually using color judgments. owing to the unidirectional nature of the synesthetic experience, little has been done to obtain performance measures that could indicate whether bidirectional cross-activation occurs in synesthesia. here it is shown that colors do implicitly evoke numerical magnitudes in color-grapheme synesthetes, but not in nonsynesthetic participants. it is proposed that bidirectional coactivation of brain areas is responsible for the links between color and magnitude processing in color-grapheme synesthesia and that unidirectional models of synesthesia might have to be revised.”