Mnemonics often have synesthetic qualities so I thought it would be useful to do some brief research on the subject. Synesthesia is a neurologically-based condition in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway.
Grapheme → colour synesthesia
In one of the most common forms of synesthesia, individual letters of the alphabet and numbers (collectively referred to as graphemes), are "shaded" or "tinged" with a colour. While different individuals usually do not report the same colors for all letters and numbers, studies with large numbers of synesthetes find some commonalities across letters (e.g., A is likely to be red).
As a child, Pat Duffy told her Dad, "I realized that to make an R all I had to do was first write a P and draw a line down from its loop. And I was so surprised that I could turn a yellow letter into an orange letter just by adding a line."
Another grapheme synesthete says, "When I read, about five words around the exact one I'm reading are in color. It's also the only way I can spell. In elementary school I remember knowing how to spell the word 'priority' [with an "i" rather than an "e"] because ... an 'e' was out of place in that word because e's were yellow and didn't fit."
Sound → color synesthesia
Cytowic calls sound → color synesthesia "something like fireworks": voice, music, and assorted environmental sounds such as clattering dishes or dog barks trigger color and simple shapes that arise, move around, and then fade when the sound stimulus ends. For some, the stimulus type is limited (e.g., music only, or even just a specific musical key); for others, a wide variety of sounds triggers synesthesia.
Sound often changes the perceived hue, brightness, scintillation, and directional movement. Some individuals see music on a "screen" in front of their face. Deni Simon, for whom music produces waving lines "like oscilloscope configurations—lines moving in color, often metallic with height, width and, most importantly, depth. My favorite music has lines that extend horizontally beyond the 'screen' area."
Individuals rarely agree on what color a given sound is however, synesthetes show the same trends as non-synesthetes do. For example, both groups say that loud tones are brighter than soft tones and that lower tones are darker than higher tones.
Number form synesthesia
A number form is a mental map of numbers, which automatically and involuntarily appears whenever someone who experiences number-forms thinks of numbers.
Personification
Ordinal-linguistic personification (OLP, or personification for short) is a form of synesthesia in which ordered sequences, such as ordinal numbers, days, months and letters are associated with personalities.
For example, one synesthete says, "T’s are generally crabbed, ungenerous creatures. U is a soulless sort of thing. 4 is honest, but… 3 I cannot trust… 9 is dark, a gentleman, tall and graceful, but politic under his suavity." Likewise, Cytowic's subject MT says, "I [is] a bit of a worrier at times, although easy-going; J [is] male; appearing jocular, but with strength of character; K [is] female; quiet, responsible...."
Lexical → gustatory synesthesia
In the rare lexical → gustatory synesthesia, individual words and the phonemes of spoken language evoke taste sensations in the mouth. According to James Wannerton, "Whenever I hear, read, or articulate (inner speech) words or word sounds, I experience an immediate and involuntary taste sensation on my tongue. These very specific taste associations never change and have remained the same for as long as I can remember."
Jamie Ward and Julia Simner have extensively studied this form of synesthesia, and have found that the synesthetic associations are constrained by early food experiences. For example, James Wannerton has no synesthetic experiences of coffee or curry, even though he consumes them regularly as an adult. Conversely, he tastes certain breakfast cereals and sweets that are no longer sold.
Additionally, these early food experiences are often paired with tastes based on the phonemes in the name of the word (e.g., /I/, /n/ and /s/ trigger James Wannerton’s taste of mince) although others have less obvious roots (e.g., /f/ triggers sherbet). To show that phonemes, rather than graphemes are the critical triggers of tastes, Ward and Simner showed that, for James Wannerton, the taste of egg is associated to the phoneme /k/, whether spelled with a "c" (e.g., accept), "k" (e.g., York), "ck" (e.g., chuck) or "x" (e.g., fax). Another source of tastes comes from semantic influences, so that food names tend to taste of the food they match, and the word "blue" tastes "inky."
