Synesthesia (also spelled synæsthesia or synaesthesia, plural synesthesiae or synaesthesiae)
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.
A common form of Synesthesia is the perception of letters or numbers as coloured, while months, and days of the week evoke different personalities. There is also sound synesthesia, involves hearing sounds in response to visual motion and flicker.
Sound → color synesthesia
Sound → color synesthesia: music, voice and assorted environmental sounds such as backfiring of cars and screeching of tires can trigger color and simple shapes that arise, move around, and then fade when the sound stimulus ends.
For some, the stimulus type is limited (For example, a certain key, or one sound in particular) for others, a wider 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.” – Wikipedia.org.