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Schwartzman DJ, Bor D, Rothen N, Seth AK. Neurophenomenology of induced and natural synaesthesia. Philos Trans R Soc Lond B Biol Sci 2019; 374:20190030. [PMID: 31630656 PMCID: PMC6834010 DOI: 10.1098/rstb.2019.0030] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 09/06/2019] [Indexed: 12/15/2022] Open
Abstract
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'.
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Affiliation(s)
- David J. Schwartzman
- Sackler Centre for Consciousness Science, University of Sussex, Brighton BN1 9QJ, UK
- Department of Informatics, University of Sussex, Brighton BN1 9QJ, UK
| | - Daniel Bor
- Department of Psychology, University of Cambridge, Downing Street, Cambridge CB2 3EB, UK
| | - Nicolas Rothen
- Sackler Centre for Consciousness Science, University of Sussex, Brighton BN1 9QJ, UK
- Faculty of Psychology, Swiss Distance University Institute, 3900 Brig, Switzerland
| | - Anil K. Seth
- Sackler Centre for Consciousness Science, University of Sussex, Brighton BN1 9QJ, UK
- Department of Informatics, University of Sussex, Brighton BN1 9QJ, UK
- Azrieli Programme on Brain, Mind, and Consciousness, Canadian Institute for Advanced Research, Toronto, Ontario, Canada
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Newell FN, Mitchell KJ. Multisensory integration and cross-modal learning in synaesthesia: A unifying model. Neuropsychologia 2015; 88:140-150. [PMID: 26231979 DOI: 10.1016/j.neuropsychologia.2015.07.026] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/13/2015] [Revised: 05/28/2015] [Accepted: 07/27/2015] [Indexed: 10/23/2022]
Abstract
Recent research into synaesthesia has highlighted the role of learning, yet synaesthesia is clearly a genetic condition. Here we ask how can the idea that synaesthesia reflects innate, genetic differences be reconciled with models that suggest it is driven by learning. A number of lines of evidence suggest that synaesthesia relies on, or at least interacts with, processes of multisensory integration that are common across all people. These include multisensory activations that arise in early regions of the brain as well as feedback from longer-term cross-modal associations generated in memory. These cognitive processes may interact independently to influence the phenomenology of the synaesthetic experience, as well as the individual differences within particular types of synaesthesia. The theoretical framework presented here is consistent with both an innate difference as the fundamental driver of the condition of synaesthesia, and with experiential and semantic influences on the eventual phenotype that emerges. In particular, it proposes that the internally generated synaesthetic percepts are treated similarly to other sensory information as the brain is learning the multisensory attributes of objects and developing cross-modal associations that merge in the concept of the object.
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Affiliation(s)
- Fiona N Newell
- School of Psychology, Trinity College Dublin, Ireland; Institute of Neuroscience, Trinity College Dublin, Ireland.
| | - Kevin J Mitchell
- Institute of Neuroscience, Trinity College Dublin, Ireland; Smurfit Institute of Genetics, Trinity College Dublin, Ireland.
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Cohen MX, Weidacker K, Tankink J, Scholte HS, Rouw R. Grapheme-color synesthesia subtypes: Stable individual differences reflected in posterior alpha-band oscillations. Cogn Neurosci 2015; 6:56-67. [DOI: 10.1080/17588928.2015.1017450] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/23/2022]
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Hupé JM, Dojat M. A critical review of the neuroimaging literature on synesthesia. Front Hum Neurosci 2015; 9:103. [PMID: 25873873 PMCID: PMC4379872 DOI: 10.3389/fnhum.2015.00103] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/12/2014] [Accepted: 02/11/2015] [Indexed: 11/13/2022] Open
Abstract
Synesthesia refers to additional sensations experienced by some people for specific stimulations, such as the systematic arbitrary association of colors to letters for the most studied type. Here, we review all the studies (based mostly on functional and structural magnetic resonance imaging) that have searched for the neural correlates of this subjective experience, as well as structural differences related to synesthesia. Most differences claimed for synesthetes are unsupported, due mainly to low statistical power, statistical errors, and methodological limitations. Our critical review therefore casts some doubts on whether any neural correlate of the synesthetic experience has been established yet. Rather than being a neurological condition (i.e., a structural or functional brain anomaly), synesthesia could be reconsidered as a special kind of childhood memory, whose signature in the brain may be out of reach with present brain imaging techniques.
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Affiliation(s)
- Jean-Michel Hupé
- Centre de Recherche Cerveau et Cognition, Université de Toulouse and Centre National de la Recherche Scientifique, Toulouse France
| | - Michel Dojat
- Grenoble Institut des Neurosciences, Institut National de la Santé et de la Recherche Médicale U836 and Université Grenoble Alpes, Grenoble France
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Adults can be trained to acquire synesthetic experiences. Sci Rep 2014; 4:7089. [PMID: 25404369 PMCID: PMC4235407 DOI: 10.1038/srep07089] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/10/2014] [Accepted: 10/29/2014] [Indexed: 01/22/2023] Open
Abstract
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.
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Melero H, Ríos-Lago M, Peña-Melián A, Álvarez-Linera J. Achromatic synesthesias - a functional magnetic resonance imaging study. Neuroimage 2014; 98:416-24. [PMID: 24845620 DOI: 10.1016/j.neuroimage.2014.05.019] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/16/2014] [Revised: 04/22/2014] [Accepted: 05/09/2014] [Indexed: 10/25/2022] Open
Abstract
Grapheme-color synesthetes experience consistent, automatic and idiosyncratic colors associated with specific letters and numbers. Frequently, these specific associations exhibit achromatic synesthetic qualities (e.g. white, black or gray). In this study, we have investigated for the first time the neural basis of achromatic synesthesias, their relationship to chromatic synesthesias and the achromatic congruency effect in order to understand not only synesthetic color but also other components of the synesthetic experience. To achieve this aim, functional magnetic resonance imaging experiments were performed in a group of associator grapheme-color synesthetes and matched controls who were stimulated with real chromatic and achromatic stimuli (Mondrians), and with letters and numbers that elicited different types of grapheme-color synesthesias (i.e. chromatic and achromatic inducers which elicited chromatic but also achromatic synesthesias, as well as congruent and incongruent ones). The information derived from the analysis of Mondrians and chromatic/achromatic synesthesias suggests that real and synesthetic colors/achromaticity do not fully share neural mechanisms. The whole-brain analysis of BOLD signals in response to the complete set of synesthetic inducers revealed that the functional peculiarities of the synesthetic brain are distributed, and reflect different components of the synesthetic experience: a perceptual component, an (attentional) feature binding component, and an emotional component. Additionally, the inclusion of achromatic experiences has provided new evidence in favor of the emotional binding theory, a line of interpretation which constitutes a bridge between grapheme-color synesthesia and other developmental modalities of the phenomenon.
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Affiliation(s)
- H Melero
- Departamento de Psicobiología, Facultad de Psicología, Universidad Complutense de Madrid, 28223, Spain; Departamento de Investigación, Desarrollo y Promoción, Fundación Internacional Artecittá, Spain.
| | - M Ríos-Lago
- Departamento de Psicología Básica II, UNED, Madrid 28040, Spain; Unidad de daño Cerebral, Hospital Beata María Ana, Madríd 28007, Spain; Laboratorio de Análisis de Imagen Médica, Fundación CIEN-Fundación Reina Sofía, Madrid 28031, Spain
| | - A Peña-Melián
- Departamento de Anatomía y Embriología Humana I, Facultad de Medicina, Universidad Complutense de Madrid, 28040, Spain
| | - J Álvarez-Linera
- Laboratorio de Análisis de Imagen Médica, Fundación CIEN-Fundación Reina Sofía, Madrid 28031, Spain; Servicio de Neurorradiología, Hospital Ruber Internacional, Madrid 28034, Spain
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Rothen N, Meier B. Acquiring synaesthesia: insights from training studies. Front Hum Neurosci 2014; 8:109. [PMID: 24624072 PMCID: PMC3939620 DOI: 10.3389/fnhum.2014.00109] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/29/2013] [Accepted: 02/12/2014] [Indexed: 11/29/2022] Open
Abstract
Synaesthesia denotes a condition of remarkable individual differences in experience characterized by specific additional experiences in response to normal sensory input. Synaesthesia seems to (i) run in families which suggests a genetic component, (ii) is associated with marked structural and functional neural differences, and (iii) is usually reported to exist from early childhood. Hence, synaesthesia is generally regarded as a congenital phenomenon. However, most synaesthetic experiences are triggered by cultural artifacts (e.g., letters, musical sounds). Evidence exists to suggest that synaesthetic experiences are triggered by the conceptual representation of their inducer stimuli. Cases were identified for which the specific synaesthetic associations are related to prior experiences and large scale studies show that grapheme-color associations in synaesthesia are not completely random. Hence, a learning component is inherently involved in the development of specific synaesthetic associations. Researchers have hypothesized that associative learning is the critical mechanism. Recently, it has become of scientific and public interest if synaesthetic experiences may be acquired by means of associative training procedures and whether the gains of these trainings are associated with similar cognitive benefits as genuine synaesthetic experiences. In order to shed light on these issues and inform synaesthesia researchers and the general interested public alike, we provide a comprehensive literature review on developmental aspects of synaesthesia and specific training procedures in non-synaesthetes. Under the light of a clear working definition of synaesthesia, we come to the conclusion that synaesthesia can potentially be learned by the appropriate training.
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Affiliation(s)
- Nicolas Rothen
- Department of Psychology and Sackler Centre for Consciousness Science, University of Sussex Brighton, UK
| | - Beat Meier
- Department of Psychology and Center for Cognition, Learning and Memory, University of Bern Bern, Switzerland
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