The word "village" tastes like sausage, and Cincinnati like cinnamon rolls. The letter "A" is bright yellow, and female. The key of C sounds green, and irritating. The smell of peaches is spherical. The days of the week are each a different color, arrayed in a twisting shape with a smooth surface, superimposed on the scene ahead. A rainbow is a song.
These examples of apparent cross-sensory cross talk are different manifestations of synesthesia, which has long been observed but only recently attracted serious scientific attention. Richard Cytowic has been a dogged pioneer in this study, and this book is the best I've seen at describing and analyzing the evidence. Just following the scientific process is half the fun.
It's an intriguing subject with a fascinating history. For a long time science was skeptical that people really saw with their ears or tasted colors, unless they were mentally ill. The phenomenon got associated with psychedelics ("listen to the color of your dreams") and was otherwise thought to be inaccurate reporting: People didn't really hear colors, they were just waxing poetic. That famous cases were artists and writers (Kandinsky, Nabokov) made it seem exotic. Yet common cross-sensory metaphors (hot music, loud colors) suggest that everyone has some synesthesiac understanding.
Cytowic and other researchers found that various kinds of synesthesia are truly experienced by otherwise normal people, who comprise a higher proportion of the population than previously believed. To some extent, synesthesia is a common experience, especially in childhood.
The authors analyze some 40 different types of synesthesia (including sounds that have temperatures or colored shapes, and personalities expressed as smells), which are often experienced in combination. They conclude that these sensations are real, concrete (not metaphorical), automatic, involuntary and conscious, with emotional feelings attached. They come to the startling conclusion that synesthesia is the result of normal brain activity, but this sensory cross talk is usually inhibited in most people.
That may be why it's both weird and familiar, and why some artists can express it. (Those who see music say that Disney's Fantasia is pretty accurate.) It may also be why we can experience it under certain conditions, although chances are better in deep meditation than from hallucinogens. It could also be why some of our technology is designed to replicate it. For instance, to design his armor in the Iron Man movie, Robert Downey as scientist Tony Stark uses his hand to move transparent information around that's projected in space by computer. This is comparable to how some people keep their monthly calendar, without any technological or pharmacological help: just their brains on synesthesia.