Synesthesia
Synesthesia or synaesthesia is a perceptual phenomenon in which stimulation of one sensory or cognitive pathway leads to involuntary experiences in other sensory or cognitive pathways. Synesthesia can manifest as a bridge between the five traditional senses, though can also include other perceptions, such as nociception, thermoception, chronoception, and interoception. People with synesthesia are referred to as synesthetes.
Awareness of synesthetic perceptions varies from person to person with the perception of synesthesia differing based on an individual's unique life experiences and the specific type of synesthesia that they have. 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, or may appear as a three-dimensional map. Synesthetic associations can occur in any combination and any number of senses or cognitive pathways.
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.
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 Ancient Greek wikt:σύν syn 'together' and wikt:αἴσθησις aisthēsis 'sensation'.
Types
There are two overall forms of synesthesia:- Projective synesthesia: seeing colors, forms, or shapes when stimulated
- Associative synesthesia: feeling a very strong and involuntary connection between the stimulus and the sense that it triggers
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, where x is the "inducer" or trigger experience, and y is the "concurrent" or additional experience. For example, perceiving letters and numbers 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 → synesthesia.
While nearly every logically possible combination of experiences can occur, several types are more common than others.
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. Although it is one of the least common forms of synesthesia, a similar sensation can be evoked in many people known as ASMR.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."
Emotional synesthesia
Emotional synesthesia is a rarely documented form of synesthesia in which emotions trigger a response in another sensation. Emotional synesthesia is estimated to make up about 1% of the synesthete population.There is only one known case of acquired emotional synesthesia, which was caused by a focal thalamic lesion.
Grapheme–color synesthesia
In one of the most common forms of synesthesia, individual letters of the alphabet and numbers 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.Some authors have 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 rather than sensory. They have postulated that the term 'ideasthesia' is a more accurate description.
Kinesthetic synesthesia
Kinesthetic synesthesia is one of the rarest documented forms of synesthesia. This form of synesthesia is a combination of 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.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, referring 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.Mirror-touch synesthesia
This is a form of synesthesia where individuals feel the same/similar sensation as another person. 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. Some research suggests people with this type of synesthesia 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 some research links to empathy. However, other research finds no relationship between mirror-touch synesthesia and empathy.Number form
A number form is a mental map of numbers that automatically and involuntarily appears 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".Ordinal linguistic personification
Ordinal-linguistic 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. Although this form of synesthesia was documented as early as the 1890s, researchers have, until recently, paid little attention to it. This form of synesthesia was named "OLP" in the contemporary literature by Julia Simner and colleagues, although it is now also widely recognized by the term "sequence-personality" synesthesia. Ordinal linguistic personification normally co-occurs with other forms of synesthesia such as grapheme–color synesthesia.Spatial sequence synesthesia
Those with spatial sequence synesthesia 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, or perceive musical notes as occupying space in front and through them.Ticker-tape synesthesia
Those with ticker-tape synesthesia mentally see written words when they are heard, sometimes on imaginary strips of paper. It has been suggested that the name "subtitled synesthesia" would better describe the phenomenon. Research into people with ticker-tape synesthesia may help explain dyslexia.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 distinctive mode of perception in their childhood. Some have learned how to apply their ability in daily life and work. Synesthetes have used their abilities in memorization of names and telephone numbers, mental arithmetic, and more complex creative activities like producing visual art, music, and theater.
Despite the commonalities which permit the definition of the broad phenomenon of synesthesia, individual experiences vary in numerous ways. This variability was first noticed early in synesthesia research. Some synesthetes report that vowels are more strongly colored, while for others consonants are more strongly colored. Self-reports, interviews, and autobiographical notes by synesthetes demonstrate a great degree of variety in types of synesthesia, the intensity of synesthetic perceptions, awareness of the perceptual discrepancies between synesthetes and non-synesthetes, and the ways synesthesia is used in work, creative processes, and daily life.
Synesthetes are very likely to participate in creative activities. It has been suggested that individual development of perceptual and cognitive skills, in addition to one's cultural environment, produces the variety in awareness and practical use of synesthetic phenomena. Synesthesia may also give a memory advantage. In one study, conducted by Julia Simner of the University of Edinburgh, it was found that spatial sequence synesthetes have a built-in and automatic mnemonic reference. Whereas a non-synesthete will need to create a mnemonic device to remember a sequence, a synesthete can simply reference their spatial visualizations.