Types of synesthesia are indicated by using the notation x в†’ y, where x is the 'inducer' or trigger experience, and y is the 'concurrent' or additional experience. 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.

associative synesthesia: feeling a very strong and involuntary connection between the stimulus and the sense that it triggersДЇor 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: seeing colors, forms, or shapes when stimulated (the widely understood version of synesthesia).

There are two overall forms of synesthesia: The term is from the Ancient Greek ПѓПЌОЅ syn, 'together', and О±бјґПѓОёО·ПѓО№П‚ aisthД“sis, ' sensation'. The first medical account came from German physician Georg Tobias Ludwig Sachs in 1812. However, there is disagreement as to whether Locke described an actual instance of synesthesia or was using a metaphor. 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. A more accurate but less common term may be ideasthesia. Many different phenomena have been included in the term synesthesia (based on the Greek words ПѓПЌОЅ and О±бјґПѓОёО·ПѓО№П‚ meaning 'union of the senses'), and in many cases the terminology seems to be inaccurate. These are usually the first abstract concepts that educational systems require children to learn.Дifficulties have been recognized in adequately defining synesthesia. This hypothesisВ вЂ“ referred to as semantic vacuum hypothesisВ вЂ“ could explain why the most common forms of synesthesia are graphemeвЂ“color, spatial sequence, and number form. It has been suggested that synesthesia develops during childhood when children are intensively engaged with abstract concepts for the first time. Little is known about how synesthesia develops. Synesthetic associations can occur in any combination and any number of senses or cognitive pathways. 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).

In one common form of synesthesia, known as graphemeвЂ“color synesthesia or colorвЂ“graphemic synesthesia, letters or numbers are perceived as inherently colored. Awareness of synesthetic perceptions varies from person to person. People who report a lifelong history of such experiences are known as synesthetes. 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. Most synesthetes see characters just as others do (in whichever color actually displayed) but may simultaneously perceive colors as associated with or evoked by each one. How someone with synesthesia might perceive certain letters and numbers.

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