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.

Types
There are two overall forms of synesthesia: 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".
 * projective synesthesia: people who see colors, forms, or shapes when stimulated (the widely understood version of synesthesia).
 * associative synesthesia: people who feel 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 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
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).

Chromesthesia
Another common form of synesthesia is the association of sounds with colors. For some, everyday sounds such as doors opening, cars honking, or people talking 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/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. B flat might be orange for one person and blue for another. 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 numerical sequences as points in space. For instance, the number 1 might be farther away and the number 2 might be closer. 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 also see months or dates in the space around them. Some people see time like a clock above and around them.

Number form
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". It is suggested that this might be caused by "cross activation" of the neural pathway that connects the parietal lobes and angular gyrus. Both of these areas are involved in numerical cognition and spatial cognition respectively.

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 approx 50% of the population. It is one of the least common forms of synesthesia.

Ordinal linguistic personification
Ordinal-linguistic personification (OLP, or personification for short) 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). For example, the number 2 might be a young boy with a short temper, or the letter G might be a busy mother with a kind face. Although this form of synesthesia was documented as early as the 1890s (Flournoy 1893; Calkins 1893) researchers have, until recently, paid little attention to this form (see History of synesthesia research). This form of synesthesia was named as 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
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
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 the rarest form.

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.

Canaan
In Canaan, Canaan herself is a synesthete. This not only allowed her to overcome the Ua virus, but also enhanced her abilities.

List of Notable Abilities
Enhanced Empathy: Canaan is able to perceive other people's emotions and intentions as colored auras. Cyberpathy: Canaan is able to use her synesthesia abilities to mentally hack into electronic machinery in order to obtain knowledge such as a password as seen in episodes 3 and 7. = Trivia =
 * Yellow: Yellow is described as Maria Osawa's color.
 * Green: Green indicates a person's fear.
 * Blue: Blue indicates a person's hatred whether it be the intent to kill or to harm. This helped Canaan identify who her enemies were in combat.
 * Canaan perceived the sun as blue in her youth (Episode 4).
 * Purple: Light purple in particular indicates a person's slobbish behavior (Episode 4).
 * Grey: Grey indicates a person's hunger.
 * White: White indicates a person's emotional detachment. Alphard gave off this color which Canaan understood as her heart being "dead" after Siam's death (Episode 13).
 * Canaan first saw this color as light brown, like Siam's, which can mean him not feeling for his enemies.
 * Colorless: Objects such as weapons and toys don't give off a color due to them being inanimate objects.
 * As far as scientists know about synesthesia, cyberpathy is not possible for a synesthete to do in reality. However, this can be rationalized as a fictional synthesis between neurons and electronics due to both using electricity.