Synesthesia is usually inherited (called congenital synesthesia), but exactly how people inherit it is unknown.
Synesthesia is sometimes reported by people using psychedelic drugs, after a stroke, or during an epileptic seizure. It is also reported to be a result of blindness or deafness. Synesthesia that comes from events unrelated to genes is called adventitious synesthesia. This synesthesia results from some drugs or a stroke but not blindness or deafness. It involves sound being linked to vision or touch being linked to hearing.
Synesthesia was investigated a lot in the 19th and early 20th centuries, but in the middle of the 20th century, it was less studied. Only recently has it been studied again in much detail.
Some musicians and composers have a form of synesthesia that allows them to "see" music as colors or shapes. This is called chromethesia. Mozart is said to have had this form of synesthesia. He said that the key of D major had a warm "orangey" sound to it, while B-flat minor was blackish. A major was a rainbow of colors to him. This may explain why he wrote some of his music using different colors for different music notes, and why much of his music is in major keys.
Another composer who had color-hearing was the Russian composer Alexander Scriabin. In 1907, he talked with another famous composer, Nikolai Rimsky-Korsakov, who had synesthesia, and they both found that some musical notes made them think of certain colors. Scriabin worked with a man named Alexander Mozer who made a color organ.
The same type of synesthesia may have different effects (pronounced and less pronounced) on different people.
Synesthetes often say that they did not know their experiences were unusual until they found out that other people did not have them. Others report feeling as if they had been keeping a secret their entire lives. Most synesthetes consider their experiences a gift—a "hidden" sense. Most synesthetes find out in their childhood that they have synethesia. Some learn to apply it in daily life and work. For example, they might use their gift to memorize names and telephone numbers or do mental arithmetic. Many people with synesthesia use their experiences to help them be more creative, for example, in making drawings and music.
- Grapheme–color synesthesia: Letters or numbers are seen to have colors of their own.
- Ordinal linguistic personification: Numbers, days of the week, and months of the year create the personalities of the person.
- Spatial-sequence: Numbers, months of the year, and/or days of the week are located in specific places in space. For example, 1980 may be "farther away" than 1990. Or a year may be seen three-dimensionally as a map.
- Visual motion → sound synesthesia: Hearing sounds in response to seeing motion.
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