| Synesthesia |
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Synesthesia
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.
Acquired Synesthesia
main article: Acquired Synesthesia
author: Michelle Li
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| Artistic rendition of synesthesia Retrieved March 26, 2014, from http://www.nytimes.com/2007/12/25/health/25brai.html |
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].
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.
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 http://www.spring.org.uk/2013/10/synesthesia-could-explain-how-some-people-see-auras.php |
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].
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: https://www.youtube.com/watch?v=dNy23tJMTzQ |
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.
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]
Synesthesia and Memory
main article: Synesthesia and Memory
author: Yu Feng
| Daniel Tammet |
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| Daniel Tammet's pi landscape, taken from www.daneltammet.net |
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.
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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| Image source: projetdimages.com |
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].
An interesting topic indeed. Small remark though, you guys might want to fix the Example Caption in the introduction picture. Well done otherwise!