synesthesia synesthesia synesthesia (american english) or synaesthesia (british english) is a perceptual phenomenon in which stimulation of one sensory or cognitive pathway leads to involuntary experiences in a second sensory or cognitive pathway. people who report a lifelong history of such experiences are known as synesthetes. awareness of synesthetic perceptions varies from person to person. in one common form of synesthesia, known as grapheme–color synesthesia or color–graphemic synesthesia, letters or numbers are perceived as inherently colored. in spatial-sequence, or number form synesthesia, numbers, months of the year, or days of the week elicit precise locations in space (e.g., 1980 may be "farther away" than 1990), or may appear as a three-dimensional map (clockwise or counterclockwise). synesthetic associations can occur in any combination and any number of senses or cognitive pathways. synesthesiahow someone with synesthesia might perceive certain letters and numbers. most synesthetes see characters just as others do (in whichever color actually displayed) but may simultaneously perceive colors as associated with or evoked by each one.specialtypsychology, psychiatry, neurology little is known about how synesthesia develops. it has been suggested that synesthesia develops during childhood when children are intensively engaged with abstract concepts for the first time. this hypothesis—referred to as semantic vacuum hypothesis—could explain why the most common forms of synesthesia are grapheme-color, spatial sequence, and number form. these are usually the first abstract concepts that educational systems require children to learn. the earliest recorded case of synesthesia is attributed to the oxford university academic and philosopher john locke, who, in 1690, made a report about a blind man who said he experienced the color scarlet when he heard the sound of a trumpet. however, there is disagreement as to whether locke described an actual instance of synesthesia or was using a metaphor. the first medical account came from german physician georg tobias ludwig sachs in 1812. the term is from the ancient greek σύν syn, 'together', and αἴσθησις aisthēsis, 'sensation'. types there are two overall forms of synesthesia: projective synesthesia: seeing colors, forms, or shapes when stimulated (the widely understood version of synesthesia) associative synesthesia: feeling a very strong and involuntary connection between the stimulus and the sense that it triggers for example, in chromesthesia (sound to color), a projector may hear a trumpet, and see an orange triangle in space, while an associator might hear a trumpet, and think very strongly that it sounds "orange". synesthesia can occur between nearly any two senses or perceptual modes, and at least one synesthete, solomon shereshevsky, experienced synesthesia that linked all five senses. types of synesthesia are indicated by using the notation x → y, where x is the "inducer" or trigger experience, and y is the "concurrent" or additional experience. for example, perceiving letters and numbers (collectively called graphemes) as colored would be indicated as grapheme-color synesthesia. similarly, when synesthetes see colors and movement as a result of hearing musical tones, it would be indicated as tone → (color, movement) synesthesia. while nearly every logically possible combination of experiences can occur, several types are more common than others. grapheme–color synesthesia main article: grapheme–color synesthesia from the 2009 non-fiction book wednesday is indigo blue in one of the most common forms of synesthesia, individual letters of the alphabet and numbers (collectively referred to as "graphemes") are "shaded" or "tinged" with a color. while different individuals usually do not report the same colors for all letters and numbers, studies with large numbers of synesthetes find some commonalities across letters (e.g., a is likely to be red). some authors had argued that the term synaesthesia may not be correct when applied to the so-called grapheme-colour synesthesia and similar phenomena in which the inducer is conceptual (e.g. a letter or number) rather than sensory (e.g. sound or color). they have postulated that the term ideasthesia is a more accurate description. chromesthesia main article: chromesthesia another common form of synesthesia is the association of sounds with colors. for some, everyday sounds can trigger seeing colors. for others, colors are triggered when musical notes or keys are being played. people with synesthesia related to music may also have perfect pitch because their ability to see and hear colors aids them in identifying notes or keys. the colors triggered by certain sounds, and any other synesthetic visual experiences, are referred to as photisms. according to richard cytowic, chromesthesia is "something like fireworks": voice, music, and assorted environmental sounds such as clattering dishes or dog barks trigger color and firework shapes that arise, move around, and then fade when the sound ends. sound often changes the perceived hue, brightness, scintillation, and directional movement. some individuals see music on a "screen" in front of their faces. for deni simon, music produces waving lines "like oscilloscope configurations – lines moving in color, often metallic with height, width, and, most importantly, depth. my favorite music has lines that extend horizontally beyond the 'screen' area." individuals rarely agree on what color a given sound is. composers franz liszt and nikolai rimsky-korsakov famously disagreed on the colors of musical keys. spatial sequence synesthesia those with spatial sequence synesthesia (sss) tend to see ordinal sequences as points in space. people with sss may have superior memories; in one study, they were able to recall past events and memories far better and in far greater detail than those without the condition. they can also see months or dates in the space around them, but most synesthetes "see" these sequences in their mind's eye. some people see time like a clock above and around them. number form a number form from one of francis galton's subjects (1881). note how the first 4 digits roughly correspond to their positions on a clock face. a number form is a mental map of numbers that automatically and involuntarily appear whenever someone who experiences number-forms synesthesia thinks of numbers. these numbers might appear in different locations and the mapping changes and varies between individuals. number forms were first documented and named in 1881 by francis galton in "the visions of sane persons". auditory–tactile synesthesia in auditory–tactile synesthesia, certain sounds can induce sensations in parts of the body. for example, someone with auditory–tactile synesthesia may experience that hearing a specific word or sound feels like touch in one specific part of the body or may experience that certain sounds can create a sensation in the skin without being touched (not to be confused with the milder general reaction known as frisson, which affects approximately 50% of the population). it is one of the least common forms of synesthesia. ordinal linguistic personification ordinal-linguistic personification (olp, or personification) is a form of synesthesia in which ordered sequences, such as ordinal numbers, week-day names, months, and alphabetical letters are associated with personalities or genders (simner & hubbard 2006). although this form of synesthesia was documented as early as the 1890s, researchers have, until recently, paid little attention to it (see history of synesthesia research). this form of synesthesia was named "olp" in the contemporary literature by julia simner and colleagues although it is now also widely recognised by the term "sequence-personality" synesthesia. ordinal linguistic personification normally co-occurs with other forms of synesthesia such as grapheme–color synesthesia. misophonia main article: misophonia misophonia is a neurological disorder in which negative experiences (anger, fright, hatred, disgust) are triggered by specific sounds. cytowic suggests that misophonia is related to, or perhaps a variety of, synesthesia. edelstein and her colleagues have compared misophonia to synesthesia in terms of connectivity between different brain regions as well as specific symptoms. they formed the hypothesis that "a pathological distortion of connections between the auditory cortex and limbic structures could cause a form of sound-emotion synesthesia." studies suggest that individuals with misophonia have a normal hearing sensitivity level but the limbic system and autonomic nervous system are constantly in a "heightened state of arousal" where abnormal reactions to sounds will be more prevalent. newer studies suggest that depending on its severity, misophonia could be associated with lower cognitive control when individuals are exposed to certain associations and triggers. it is unclear what causes misophonia. some scientists believe it could be genetic, others believe it to be present with other additional conditions however there is not enough evidence to conclude what causes it. there are no current treatments for the condition but could be managed with different types of coping strategies. these strategies vary from person to person, some have reported the avoidance of certain situations that could trigger the reaction: mimicking the sounds, cancelling out the sounds by using different methods like earplugs, music, internal dialog and many other tactics. most misophonics use these to "overwrite" these sounds produced by others. mirror-touch synesthesia this is a form of synesthesia where individuals feel the same sensation that another person feels (such as touch). for instance, when such a synesthete observes someone being tapped on their shoulder, the synesthete involuntarily feels a tap on their own shoulder as well. people with this type of synesthesia have been shown to have higher empathy levels compared to the general population. this may be related to the so-called mirror neurons present in the motor areas of the brain, which have also been linked to empathy. lexical–gustatory synesthesia main article: lexical–gustatory synesthesia this is another form of synesthesia where certain tastes are experienced when hearing words. for example, the word basketball might taste like waffles. the documentary 'derek tastes of earwax' gets its name from this phenomenon, in references to pub owner james wannerton who experiences this particular sensation whenever he hears the name spoken. it is estimated that 0.2% of the synesthesia population has this form of synesthesia, making it one of the rarest forms. kinesthetic synesthesia kinesthetic synesthesia is one of the rarest documented forms of synesthesia in the world. this form of synesthesia is a combination of various different types of synesthesia. features appear similar to auditory–tactile synesthesia but sensations are not isolated to individual numbers or letters but complex systems of relationships. the result is the ability to memorize and model complex relationships between numerous variables by feeling physical sensations around the kinesthetic movement of related variables. reports include feeling sensations in the hands or feet, coupled with visualizations of shapes or objects when analyzing mathematical equations, physical systems, or music. in another case, a person described seeing interactions between physical shapes causing sensations in the feet when solving a math problem. generally, those with this type of synesthesia can memorize and visualize complicated systems, and with a high degree of accuracy, predict the results of changes to the system. examples include predicting the results of computer simulations in subjects such as quantum mechanics or fluid dynamics when results are not naturally intuitive. other forms other forms of synesthesia have been reported, but little has been done to analyze them scientifically. there are at least 80 types of synesthesia. in august 2017 a research article in the journal social neuroscience reviewed studies with fmri to determine if persons who experience autonomous sensory meridian response are experiencing a form of synesthesia. while a determination has not yet been made, there is anecdotal evidence that this may be the case, based on significant and consistent differences from the control group, in terms of functional connectivity within neural pathways. it is unclear whether this will lead to asmr being included as a form of existing synesthesia, or if a new type will be considered. signs and symptoms some synesthetes often report that they were unaware their experiences were unusual until they realized other people did not have them, while others report feeling as if they had been keeping a secret their entire lives. the automatic and ineffable nature of a synesthetic experience means that the pairing may not seem out of the ordinary. this involuntary and consistent nature helps define synesthesia as a real experience. most synesthetes report that their experiences are pleasant or neutral, although, in rare cases, synesthetes report that their experiences can lead to a degree of sensory overload. though often stereotyped in the popular media as a medical condition or neurological aberration, many synesthetes themselves do not perceive their synesthetic experiences as a handicap. on the contrary, some report it as a gift – an additional "hidden" sense – something they would not want to miss. most synesthetes become aware of their