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The term "synesthesia" comes from the Greek words syn, which means "together", and aisthesis, which means "sensation." Synesthesia is a perception in which stimulating one sensory or cognitive pathway causes experiences in another sense or cognitive pathway. In other words, a sense or concept is connected to a different sense or concept, such as smelling colors or tasting a word. The connection between pathways is involuntary and consistent over time, rather than conscious or arbitrary. So, a person experiencing synesthesia doesn't think about the connection and always makes the exact same relationship between two sensations or thoughts. Synesthesia is an atypical mode of perception, not a medical condition or neurological abnormality. A person who experiences synthesthesia over a lifetime is called a synesthete.
Types of Synesthesia
There are many different types of synesthesia, but they may be categorized as falling into one of two groups: associative synesthesia and projective synesthesia. An associate feels a connection between a stimulus and a sense, while a projector actually sees, hears, feels, smells, or tastes a stimulation. For example, an associator might hear a violin and strongly associate it with the color blue, while a projector might hear a violin and see the color blue projected in space as if it were a physical object.
There are at least 80 known types of synesthesia, but some are more common than others:
- Chromesthesia: In this common form of synesthesia, sounds and colors are associated with each other. For example, the musical note "D" may correspond to seeing the color green.
- Grapheme-color synesthesia: This is a common form of synesthesia characterized by seeing graphemes (letter or numerals) shaded with a color. Synesthetes don't associate the same colors for a grapheme as each other, although the letter "A" does appear to be red to many individuals. Persons who experience grapheme-color synesthesia sometimes report seeing impossible colors when red and green or blue and yellow graphemes appear next to each other in a word or number.
- Number form: A number form is a mental shape or map of numbers resulting from seeing or thinking about numbers.
- Lexical-gustatory synesthesia: This a rare type of synesthesia in which hearing a word results in tasting a flavor. For example, a person's name might taste like chocolate.
- Mirror-touch synesthesia: While rare, mirror-touch synesthesia is noteworthy because it can be disruptive to a synesthete's life. In this form of synesthesia, an individual feels the same sensation in response to a stimulus as another person. For example, seeing a person being tapped on the shoulder would cause the synesthete to feel a tap on the shoulder too.
Many other forms of synesthesia occur, including smell-color, month-flavor, sound-emotion, sound-touch, day-color, pain-color, and personality-color (auras).
How Synesthesia Works
Scientists have yet to make a definitive determination of the mechanism of synesthesia. It may be due to increased cross-talk between specialized regions of the brain. Another possible mechanism is that inhibition in a neural pathway is reduced in synesthetes, allowing multi-sensory processing of stimuli. Some researchers believe synesthesia is based on the way the brain extracts and assigns the meaning of a stimulus (ideasthesia).
Who Has Synesthesia?
Julia Simner, a psychologist studying synesthesia at of the University of Edinburgh, estimates at least 4% of the population has synesthesia and that over 1% of people have grapheme-color synesthesia (colored numbers and letters). More women have synesthesia than men. Some research suggests the incidence of synesthesia may be higher in people with autism and in left-handed people. Whether or not there is a genetic component to developing this form of perception is hotly debated.
Can You Develop Synesthesia?
There are documented cases of non-synesthetes developing synesthesia. Specifically, head trauma, stroke, brain tumors, and temporal lobe epilepsy may produce synesthesia. Temporary synesthesia may result from exposure to the psychedelic drugs mescaline or LSD, from sensory deprivation, or from meditation.
It's possible non-synesthetes may be able to develop associations between different senses through conscious practice. A potential advantage of this is improved memory and reaction time. For example, a person can react to sound more quickly than to sight or may recall a series of colors better than a series of numbers. Some people with chromasthesia have perfect pitch because they can identify notes as specific colors. Synesthesia is associated with enhanced creativity and unusual cognitive abilities. For example, synesthete Daniel Tammet set a European record for stating 22,514 digits of the number pi from memory using his ability to see numbers as colors and shapes.
- Baron-Cohen S, Johnson D, Asher J, Wheelwright S, Fisher SE, Gregerson PK, Allison C, "Is synaesthesia more common in autism?", Molecular Autism, 20 November 2013.
- Marcel Neckar; Petr Bob (11 January 2016). "Synesthetic associations and psychosensory symptoms of temporal epilepsy". Neuropsychiatric Disease and Treatment. National Institutes of Health (NIH). 12: 109-12.
- Rich AN, Mattingley JB (January 2002). "Anomalous perception in synesthesia: a cognitive neuroscience perspective". Nature Reviews Neuroscience (Review). 3 (1): 43-52.
- Simner J, Mulvenna C, Sagiv N, Tsakanikos E, Witherby SA, Fraser C, Scott K, Ward J (2006). "Synaesthesia: The prevalence of atypical cross-modal experiences". Perception. 35: 1024-1033.