Sharing a root with anesthesia, meaning ˇ°no sensation,ˇ± synesthesia means ˇ°joined sensation.ˇ± Two or more senses are automatically and involuntarily coupled such that a voice, for example, is not only something that is heard, but also felt, seen, or tasted.
Synesthesia is a physical experience of the brain, not the product of imagination or learning. It differs from metaphor and artistic contrivances such as colored music or son et lumiere that deliberately link sensations. Some couplings occur more often than others: sound?sight synesthesia (colored hearing), is common whereas unions involving taste and smell are rare. The most frequent synesthesia is colored letters and numbers.
Some form of synesthesia may occur in 1 in 200~2,000 people. It runs strongly in families, and is inherited as an X-linked dominant trait, which is why women outnumber men by at least 3:1. Synesthesia is a trait, not a disease.
Like most quirks of nature, synesthesia is a window onto the workings of ordinary brains.