"Sundays are white"
"December is at the bottom of the year"
"This song looks like a white brain"
As a child, I used to puzzle my parents with expressions like these. I couldn't understand what they thought was so strange about this. They couldn't understand what I meant. As I grew older, it became clear that almost nobody else shared these experiences. I wrote it off, stopped speaking of it and hid it inside my mind.
Not until I was an adult, almost 30 years later, did I first hear of synesthesia. My mother had heard it mentioned on TV, and reminded me: "Do you remember that Sundays were white? You should look at this..."
I experience most of the forms of synesthesia that are discussed in the Wikipedia article, but the strongest association is the one that shows sounds and smells as visual images. When a word has a colour, I don't see the colour strongly when I look at the letters of the word, but I see it when it is spoken out loud. The shapes are usually in 3D.
The song that looked like a white brain was an upbeat, big band version of a famous jazz song. A well known radio show in my country uses it as its intro. I can't find this specific version of it online, but I have found several others. Interestingly enough, they look different. Where the big band recording looks like a white brain, a slower version with the main melody played on a clarinet looks more like a dark green fractal tree. The general "winding" shape is the same, but the faster version has a lighter and more compact visual.
Smells have even stronger associations than sounds. I have learned that the sense of smell has a strong connection with the primitive, subconscious part of the brain, especially with old memories. I can often visualise a smell before my conscious mind can come up with the name for it. The smell of blood, for example, is a semi transparent iron grey blob.
I experience synesthesia stronger when my mind isn't fully alert. When I am very sleepy, intoxicated, or sick, the associations become stronger. This is especially an issue with (lucid) dreaming. The connection between a word, its sound, and its shape and colour is sometimes so strong that it makes the dream hard to type up.
One peculiar thing I have noticed is the fact that the lower quality the sound recording is, the less colourful and detailed is the image, unless the recording is of a sound I know very well. It is as if the brain fills in the blanks, and gives me the image I expect.
The article also links synesthesia with misophonia. I don't know if that is universal, but I definitely have misophonia too. Some sounds translate to fear and/or aggression. I wonder if that works in the same way as the connection between smells, memories and emotions.
Wikipedia article about synesthesia
A good article on Google+