Synaesthesia

Synaesthesia is a perceptual phenomenon in which stimulation of one sensory or cognitive pathway leads to involuntary experiences in a second sensory or cognitive pathway. There are many different types of synaesthesia, some of which are more common than others. The exact prevalence of synaesthesia is unknown, and different studies give different population figures. Synaesthesia is fairly rare but is also likely undiagnosed. A person with synaesthesia is called a synaesthete.

Types of synaesthesia
There are two main categories of synaesthesia: projective synaesthesia and associative synaesthesia. People with projective synaesthesia see colors, shapes, and forms when stimulated. By contrast, people with associative synaesthesia only feel a very strong and involuntary connection between a stimulus and the sense it triggers. No two synaesthetes have the same experience.

Some of the more well-known types of synaesthesia include:


 * Grapheme-color synaesthesia: individual letters and numbers are associated with different colors. While different individuals will not report the same exact colors for each letter and number, studies involving large groups of people show some commonalities.
 * Chromesthesia: the association of sounds with colors. People with chromesthesia are more likely to have absolute pitch due to their ability to more easily identify notes by color.
 * Spacial sequence synaesthesia: the association of numerical sequences as points in space. One study showed that people with spacial sequence synaesthesia have superior memory to those who don't.
 * Number form: a mental map of numbers that automatically and involuntarily appears whenever someone with said form of synaesthesia thinks about numbers.
 * Auditory-tactile synaesthesia: certain sounds can produce sensations in the body.
 * Ordinal linguistic personification: ordered sequences (ordinal numbers, days of the week, months of the year, individual letters, etc) are associated with personalities and genders.
 * Mirror-touch synaesthesia: an individual with this type of synaesthesia will feel the same sensation as another individual (such as touch). People with this type of synaesthesia have higher empathy.
 * Lexical-gustatory synaesthesia: certain tastes are experienced when hearing words.

Many other forms of synaesthesia have been reported, but they are extremely rare and therefore little research has been done to analyze them. There are at least 80 different types of synaesthesia.

Correlation with Autism
Research has suggested that synaesthesia is more common among Autistic individuals than that of the general population.