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Synesthesia is the phenomenon where stimulation in one sensory modality elicits an associative experience with another sensory modality which is normally unstimulated[1]. This is not limited to basic sensory modalities like gustatory, auditory, visual, olfactory, and touch, but incorporates other aspects of the human experience like language (grapheme-colour and personification based), space-time orientation, emotions, and many others. As such, different regions of the brain are stimulated depending on the specific type of synesthesia experienced. Synesthesia is more common than one might think, as synesthetes comprise of approximately 4% of the general population, and has been found to be highly attributed to genetics[2]. Approximately 40% of synesthetes have a close relative that is also a synesthete. As the phenomenon has been formally identified relatively recently, much of the research is still ongoing, and much remains unknown about this varied phenomenon. Through the study of synesthesia, scientists hope to learn more about the incorporation of senses in the everyday lives of neurotypicals, as well as how synesthesia links to a variety of other neurological phenomena and disorders. Specifically, recent research has been focusing on grapheme-colour, auditory-visual, personification-based, emotion-evoked, and acquired synesthesia, as well as synesthesia in relation to mental disorders and memory.

1. Hubbard, E. M. & Ramachandran, V. S. Neurocognitive Mechanisms of Synesthesia. Neuron. 48, 509-520 (2005)
2. Brang, D. & Ramachandran, V. S. Survival of the synesthesia gene: why do people hear colors and taste words? PLoS Biol. 9, 1–6 (2011)

Acquired Synesthesia

main article: Acquired Synesthesia
author: Michelle Li

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Artistic rendition of synesthesia
Retrieved March 26, 2014, from

Acquired synesthesia is a phenomenon which is most commonly induced by physical trauma, neuropathology, or stroke leading to sensory deafferentation[1]. In most cases, it is believed this deafferentation leads to the rewiring of neural pathways such that stimulation of other sensory systems are able to elicit a response in the deafferentated sensory system[2]. The creation of these new associations are generally limited to regions of the brain close to the deafferentated sensory region[2]. Due to the many forms of acquired synesthesia, there are some exceptions that do not appear to fit this model, thus the exact cause of synesthesia still remains unknown. Other factors have been suggested as possible candidates for eliciting synesthetic experiences, such as the serotonin S2a receptor which appears to play a role in all three forms of synesthesia (developmental, acquired, and drug-induced)[1]. A form of synesthesia which appears to be exclusive in acquired synesthetes is the experience of pain in response to seeing or thinking about pain[3]. Specific research into the phenomenon of synesthesia for pain and phantom limbs may give insight into human perception of pain and the role of empathy in the processing of pain. More recently, there has been research into whether synesthesia can be artificially developed in individuals either through reading in coloured letters or through sensory substitution[4][5]. However, there has been some debate over whether these experiences constitute as genuine synesthetic experiences[4][5].

1. Brogaard, B. Serotonergic Hyperactivity as a Potential Factor in Developmental, Acquired and Drug-Induced Synesthesia. Frontiers in Human Neuroscience. 7(657), doi:10.3389/fnhum.2013.00657 (2013).
2. Ward, J. Acquired auditory-tactile synesthesia. Annals of Neurology. 62(5), 429-430 (2007).
3. Fitzgibbon, B.M., Giummarra, M.J., Georgiou-Karistianis, N., Enticott, P.G., Bradshaw, J.L. Shared pain: from empathy to synesthesia. Neuroscience & Biobehavioral Reviews. 34(4), 500-512 (2010).
4. Colizoli, O., Murre, J.M., Rouw, R. Pseudo-synesthesia through reading books with coloured letters. PLoS One. 7, doi:10.1371/journal.pone.0039799 (2012).
5. Ward, J., Wright, T. Sensory substitution as an artificially acquired synesthesia. Neuroscience & Biobehavioral Reviews. doi:10.1016/j.neubiorev.2012.07.007 (2012).

Auditory-Visual Synesthesia

main article: Auditory-Visual Synesthesia
author: Charmaine Chan

Petal Photism
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Figure 1. Example of a visual percept seen by an auditory-visual synesthete

Auditory-Visual synesthesia, a particular type of synesthesia, is a perceptual phenomenon in which the perception of an auditory stimuli (aka. the inducer) elicits a simultaneous visual percept (aka. the concurrent) or vice versa.[1] This usually occurs unilaterally (ie. Auditory → Visual or Visual → Auditory), although there have been few documented cases of synesthesthetes with bilateral sensory stimulation.[2] The auditory and visual percepts experienced by the synesthete may vary from individual to individual but the inducer-concurrent percept combinations are marked by consistency and automaticity. This means that if an auditory-visual synesthete sees the colour blue whenever he/she hears a middle C on the piano, this same sensation will be automatically perceived consistently over time.[3] Within auditory-visual synesthesia there are various types of auditory and visual percepts experienced, out of which the most common ones are musical tones and color percepts.[4] An example of a coloured visual percept (Fig.1) can be seen on the right.[5]
Particular chromosomes, genetic linkages and structural differences in neural pathways have been implicated in auditory-visual synesthetes. Through investigating the suggested genetic and neural mechanisms that underlie this phenomenon, one may hope to further understand the workings of cross-modal interactions of both synesthetes and non-synesthetes.

1. 1. Sinke, C., Neufeld, J., Zedler, M., Emrich, H.M., Bleich, S., Münte, T.F., Szycik, G.R: Reduced audiovisual integration in synesthesia - evidence from bimodal speech perception. Journal of Neuropsychology 2014, 8:94-106.
2. 2. Jäncke, L., Rogenmoser, L., Meyer, M., Elmer, S: Pre-attentive modulation of brain responses to coloured-hearing synesthetes. BMC Neuroscience 2012, 13:151-165.
3. 3. Gaschler-Markefski, B., Szycik, G.R., Sinke, C., Neufeld, J., Schneider, U., Baumgart, F., Dierks, O., Steigemann, U., Scheich, H., Emrich, H.M., Zedler, M: Anomalous Auditory Cortex Activations in Colored Hearing Synaesthetes: An fMRI-Study. Seeing and Perceiving 2011, 24:391-405
4. 4. Chiou, R., Stelter, M., Rich, A.N: Beyond colour perception: Auditory-visual synaesthesia induces experiences of geometric objects in specific locations. Cortex 2013, 49:1750-1763.
5. 5. Jacobs, L., Karpik, A., Bozian, D., Gøthgen, S: Auditory-Visual Synesthesia Sound-Induced Photisms. Arch Neurol 1981, 38:211-216.

Emotion-evoked synesthesia

main article: Emotion-evoked synesthesia
author: Jennifer Cheng

Emotion-colour synesthesia
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Coloured auras or haloes experienced by the synesthete based on how they feel

A relatively new form of synesthesia is person or emotion-colour synesthesia which consists of coloured halos or "auras" around a person's face or body [1]. One of the earliest cases was reported in 1934, where a 7-year old synesthete associated colours with people [2]. The colours were only elicited on people, inanimate objects, such as plaster heads did not elicit any colours showing that it is based on familiarity. He described strangers as "bright orange with a black outline" and as he got to know them better the colour would become blue or pinkish purple [2]. Once the colours stopped changing, he said it was because he got to know them better [2]. This is seen when he watched the Oliver Twist movie, he associated unpleasant characters with dark, gloomier colours such as black and grey, whereas characters he liked had more saturated colours [2].

Another case study was reported in 1989 by Cytowic, with subject BB who saw coloured "auras" around people that were linked to his emotions: "The feeling lasts a few seconds…sometimes I think I see the colour and react emotionally; others it may be reversed - I get an emotion and then see this colour." [3]. Although similar cases were reported in the past on this phenomenon, some were ruled out because they may have been a result of co-occurrence with other types of synesthesia or some case studies were not specific enough [3-4]. Cross-culturally there are a lot of similar associations between certain colours with emotions (brighter and more saturated colours associated with positive emotions, etc.), which can explain why some synesthetes associate certain colours emotions too [4]. Some synesthetes can also see synesthetic colours that are different and unique than those presented in the real world [5].

1. Ramachandran, V.S., Miller, L., Livingstone, M.S., Brang, D. Colored halos around faces and emotion-evoked colors: A new form of synesthesia. Neurocase. 18(4), 352-358 (2013).
2. Riggs, L.A. & Karwoski, T. Synaesthetsia. British Journal of Psychology. 25,29-41 (1934).
3. Cytowic, R.E. Synesthesia: A Union of the senses. Hong Kong. Massachusetts Institute of Technology, (2002).
4. Ward, J. Emotionally Mediated Synaesthesia. Cognitive Neuropsychology. 21(7), 761-772 (2004).
5. Perry, A. & Henik, A. The emotional valence of a conflict: implications from synesthesia. Frontiers in Psychology: Cognitive Science. 4, 1-8 (2013).
6. Callejas, A., Acosta, A., Lupiáñez, J. Green love is ugly: Emotions elicited by synesthetic grapheme-color perceptions. Brain Research. 7, 99-107 (2007).
7. Milán, E.G. et al. Experimental study of phantom colors in a color blind synaesthete. Journal of Consciousness Studies. 14(4), 75-95 (2007).
8. Dael, N., Sierro G., Mohr C. Affect-related synesthesias: a prospective view on their existence, expression and underlying mechanisms. Frontiers in Psychology. 4, 1-9 (2013).
9. Fitzgibbon, B.M. et al. Shared pain: From empathy to synaesthesia. Neuroscience and Biobehavioural Reviews. 34, 500-512 (2010).
10. Palmer, S.E. Music-color associations are mediated by emotion. PNAS. 110(22), 8836-8841 (2013).
11. Melero, H. et al. Grapheme-color synesthetes show peculiarities in their emotional brain: cortical and subcortical evidence from VBM analysis of 3D-T1 and DTI data. Exp Brain Res. 227, 343-353 (2013).
12. Milán E.G. et al. Auras in mysticism and synaesthesia: A comparison. Consciousness and Cognition. 21, 258-268 (2012).
13. Eagleman, D.M. et al. A standardized test battery for the study of synesthesia. Journal of Neuroscience Methods. 159(1), 139-145 (2007).
14. Hubbard, E.M., Manohar, S., Ramachandran, V.S. Contrast Affects the Strength of Synesthetic Colors. Cortex. 42(2), 184-194 (2006).

Grapheme-Colour Synesthesia

main article: Grapheme-Colour Synesthesia
author: Jenny Chong

Grapheme-Colour Synesthesia
One individual's colour perceptions of graphemes with interesting patterns.
Retrieved from:

Grapheme-colour synesthesia is when graphemes, which include letters and numbers, consistently induce a particular colour sensation. The specific colour associations differ between individuals and are constant throughout life, usually starting from a very young age. The graphemes are referred to as the inducers, and the colour sensation as the concurrent [1]. Individuals with any type of synesthesia are referred to as synesthetes. Grapheme-colour synesthesia is one of the most common types found across populations. Key regions of the brain consistently activated during grapheme-colour synesthetic experiences are the colour area V4, grapheme areas, and specific regions of the superior parietal lobe (SPL) [2]. Although these areas of the brain are strongly implicated in all research papers, the pathways connecting these regions are currently under debate. Understanding grapheme-colour synesthesia may help us to understand what makes people different, and how our brains shape our individual perceptions of the world.

1. Brang, D., Kanai, S., Ramachandran, V. S. & Coulson, S. Contextual priming in grapheme-color synesthetes and yoked controls: 400 msec in the life of a synesthete. J. Cogn. Neurosci. 23, 1681–96 (2011).
2. van Leeuwen, T. M., den Ouden, H. E. M. & Hagoort, P. Effective connectivity determines the nature of subjective experience in grapheme-color synesthesia. J. Neurosci. 31, 9879–84 (2011).

Personification-Based Synesthesia

main article: Personification-Based Synesthesia
author: Sally Moy

Personification-Based Synesthesia
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Letters are given human-like qualities like personalities and gender

Personification-based synesthesia is a rare form of synesthesia that occurs when letters, numbers, objects and days are automatically associated with a specific personality or gender. In literature, three main types of personification were identified and these phenomena often co-occurred with other forms of synesthesia.[1] During this resurgence in synesthesia research, neural imaging has helped to identify the neural mechanisms that underlie this form of synesthesia.[2] Practical applications for personification have also been used to improve learning and memory.[7]

1. Simner, J. & Holenstein, E. (2007). Ordinal Linguistic Personification as a Variant of Synesthesia. Journal of Cognitive Neuroscience, 19(4), 694-703.
2. Smilek, D., Malcolmson, K. A., Carriere, J. S. A., Eller, M., Kwan, D., & Reynolds, M. (2007). When “3” is a Jerk and “E” is a King: Personifying Inanimate Objects in Synesthesia. Journal of Cognitive Neuroscience, 19, 981-992.
3. Simner, J., & Hubbard, E. M. (2006). Variants of synesthesia interact in cognitive tasks: Evidence for implicit associations and late connectivity in cross-talk theories. Neuroscience, 143(3), 805-814.
4. Calkins, M. W. (1893). A Statistical Study of Pseudo-Chromesthesia and of Mental-Forms. The American Journal of Psychology, 5(4), 439-464.
5. Cytowic, R. E. (2002). Synesthesia: A union of the senses. New York: Springer-Verlag.
6. Flournoy, T. (1893). Des phenomenes de synopsie. Paris: Felix Alcan.
7. Gruca, J. M., & Douglas, M. (1994). Bug of the week: A personification teaching strategy.Journal of Nursing Education, 33(4), 153-154.

Synesthesia and Memory

main article: Synesthesia and Memory
author: Yu Feng

Daniel Tammet
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Daniel Tammet's pi landscape, taken from

Synaesthesia has been linked to superior memory abilities in many anecdotal accounts. One very popularized case being Daniel Tammet’s description in his autobiography Born on a Blue Day, where he recounts his memorization of 22,000 digits of pi as walking through a landscape of numbers[1]. The link between synaesthesia and memory was not extensively studied in scientifically controlled settings until recently, and many new discoveries are still being made. Current evidence suggest memory abilities in synaesthetes is linked to the superior processing in the involved synaesthetic modalities (visual memory for grapheme-colour synaesthetes, positional memory and rotation for space-time synaesthetes etc[2]). The memory-modality linkages By studying the different sensory modalities and correlation with memory, scientists can investigate the border between perception and memory, the neurological basis of memory, and how different memory techniques aid the memory coding process.

1. Tammet, D. Born on a Blue Day: Inside the Extraordinary Mind of an Autistic Savant. Hodder and Stoughton: Great Britain (2006)
2. Terhune, D. B., Wudarczyk, O. A., Kochuparampil, P., Kadosh, C. Enhanced Dimension-specific Visual Working Memory in Grapheme–color Synesthesia. Cognition 129, 123-137. (2013)

Synesthesia’s links with other mental disorders

main article: Synesthesia’s links with other mental disorders
author: Amanda Tan

Figure 1. Daniel Tammet, an autistic with synesthesia
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There is evidence showing links between synesthesia and other mental disorders, such as autism spectrum conditions (ASC). Synesthesia occurs when a stimulation of a specific sensory modality automatically affects the perception of another unstimulated modality. These altered sensory processing are also common with ASC individuals. Compared to other mental disabilities, ASC individuals demonstrate a higher sensory responsiveness [3]. Neuroimaging studies show that an autistic brain develops and functions differently than a typically developing brain, in a similar fashion to a synaesthete. For example, auditory stimuli can trigger a response in visual brain regions in autistic individuals [4]. While synesthesia typically occurs at a rate of 7.2% in normal individuals, with autistic individuals it occurs at a much higher rate of 18.9%[1]. The co-occurrence of ASC and synesthesia can be explained further through genetics as it is likely that synesthesia has a genetic component. 40-50% of synaesthetes have a first-degree relative who is also a synesthete [2]. Specific gene loci have been identified with synesthesia, such as chromosome 2, and this is an area previously linked to autism [5].

1. Baren-Cohen, S., Johnson, D., Asher, J., Wheelwright, S., Fisher, S., Gregerson, P., & Allison, C. (2013). Is synaesthesia more common in autism?. Molecular Autism, 4(40), doi:10.1186/2040-2392-4-40
2. Barnett K. J., Finucane C., Asher J. E., Bargary G., Corvin A. P., Newell F. N., et al. (2008).Familial patterns and the origins of individual differences in synaesthesia. Cognition 106, 871-893, doi: 10.1016/j.cognition.2007.05.003
3. Marco E. J., Hinkley L. B., Hill S. S., Nagarajan S. S. (2011). Sensory processing in autism: a review of neurophysiologic findings. Pediatric Research. Res. 69, 48R–54R, doi:10.1203/PDR.0b013e3182130c54
4. Simmons D. R., Robertson A. E., McKay L. S., Toal E., McAleer P., Pollick F. E. (2009). Vision in autism spectrum disorders. Vis. Res. 49, 2705–2739, doi: 10.1016/j.visres.2009.08.005
5. Asher J. E., Lamb J. A., Brocklebank D., Cazier J. B., Maestrini E., Addis L., et al. (2009). A whole-genome scan and fine-mapping linkage study of auditory-visual synesthesia reveals evidence of linkage to chromosomes 2q24, 5q33, 6p12, and 12p12. Am. J. Hum. Genet. 84, 279–285.10.1016/j.ajhg.2009.01.012

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