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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 other sensory or cognitive pathways.

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 (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. 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.

Projective synesthesia: seeing colors, forms, or shapes when stimulated (the widely understood version of synesthesia) 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. 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. Although it is one of the least common forms of synesthesia, a similar sensation can be evoked in many people known as ASMR.

Emotional synesthesia is a rarely documented form of synesthesia in which emotions trigger a response in another sensation (such as smell, taste, color, and texture).

Kinesthetic synesthesia is one of the rarest documented forms of synesthesia. This form of synesthesia is a combination of different types of synesthesia. It is estimated that 0.2% of the synesthesia population has this form of synesthesia, making it one of the rarest forms.

Ordinal linguistic personification normally co-occurs with other forms of synesthesia such as grapheme–color synesthesia.

This study also concluded that one common form of synesthesia – grapheme–color synesthesia (colored letters and numbers) – is found in more than one percent of the population, and this latter prevalence of graphemes–color synesthesia has since been independently verified in a sample of nearly 3,000 people in the University of Edinburgh. Prevalence refers to both how common is synesthesia (or different forms of synesthesia) within the population, or how common are different forms of synesthesia within synesthetes. So within synesthetes, forms of synesthesia that trigger color also appear to be the most common forms of synesthesia with a prevalence rate of 86% within synesthetes. A study exploring whether individuals with misophonia reported chromesthesia, grapheme–colour synesthesia, number form synesthesia, or other kinds of synesthesia reported 9–17% experienced each type; although the study did not include a comparison group of people without misophonia, the study authors noted this prevalence would be higher than previously found in the general population.

Research into synesthesia proceeded briskly in several countries, but due to the difficulties in measuring subjective experiences and the rise of behaviorism, which made the study of any subjective experience taboo, synesthesia faded into scientific oblivion between 1930 and 1980. These rapidly grew into international organizations such as the American Synesthesia Association, the UK Synaesthesia Association, the Belgian Synesthesia Association, the Canadian Synesthesia Association, the German Synesthesia Association, and the Netherlands Synesthesia Web Community.

Quick Facts

• In one common form of synesthesia, known as grapheme–color synesthesia or color–graphemic synesthesia, letters or numbers are perceived as inherently colored.
• These rapidly grew into international organizations such as the American Synesthesia Association, the UK Synaesthesia Association, the Belgian Synesthesia Association, the Canadian Synesthesia Association, the German Synesthesia Association, and the Netherlands Synesthesia Web Community.
• 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.
• Prevalence refers to both how common is synesthesia (or different forms of synesthesia) within the population, or how common are different forms of synesthesia within synesthetes.

Source material: Wikipedia - "Synesthesia". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.