Sidney Perkowitz in Nautilus:
When I was about 6, my mind did something wondrous, although it felt perfectly natural at the time. When I encountered the name of any day of the week, I automatically associated it with a color or a pattern, always the same one, as if the word embodied the shade. Sunday was dark maroon, Wednesday a sunshiny golden yellow, and Friday a deep green. Saturday was interestingly different. That day evoked in my mind’s eye a pattern of shifting and overlapping circular forms in shades of silver and gray, like bubbles in a glass of sparkling water.
Without knowing it, I was living the unusual mental state called synesthesia, aptly described by synesthesia researcher Julia Simner as a “condition in which ordinary activities trigger extraordinary experiences.” More exactly, it is a neurological event where excitation of one of the five senses arouses a simultaneous reaction in another sense or senses (the Greek roots for “synesthesia,” also spelled “synaesthesia,” translate as “joined perception”). Some 4 percent of the population experiences this kind of cross-sensory linking, and studies have shown it’s more prevalent in creative people. Artists who’ve reported extraordinary experiences of synesthesia range from 19th-century composer Nikolai Rimsky-Korsakov to contemporary artist David Hockney to pop music star Lady Gaga.
For me, the words “Sunday,” “Monday,” and so on, generated internal visions of color and pattern. Most synesthetic reactions also involve color in response to lexical stimuli—words written or spoken (“word-color” synesthesia), and letters, numbers, and symbols (“grapheme-color” synesthesia)—or to music and sound (“colored-hearing” synesthesia). Researchers have also observed dozens of other types of stimulus-reaction combinations: taste evoking a visual image, such as the flavor of chicken producing a 3-D shape; physical touch inducing the sensation of smell; and somehow the most extraordinary pairing, words generating the sensation of taste, such as “jail” creating the flavor of bacon.
More here.