Synaesthesia

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Synesthesia 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 synesthesia, some of which are more common than others. The exact prevalence of synesthesia is unknown, and different studies give different population figures. Synesthesia is fairly rare but is also likely undiagnosed. A person with synesthesia is called a synesthete.

Types of synesthesia

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

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

• Grapheme-color synesthesia: 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 synesthesia: the association of numerical sequences as points in space. One study showed that people with spacial sequence synesthesia 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 synesthesia thinks about numbers.
• Auditory-tactile synesthesia: 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 synesthesia: an individual with this type of synesthesia will feel the same sensation as another individual (such as touch). People with this type of synesthesia have higher empathy.
• Lexical-gustatory synesthesia: certain tastes are experienced when hearing words.

Many other forms of synesthesia 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 synesthesia.

Correlation with Autism

Research has suggested that synesthesia is more common among Autistic individuals than that of the general population.