This hypothesis-referred to as semantic vacuum hypothesis-could explain why the most common forms of synesthesia are grapheme-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 grapheme–color synesthesia or color–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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