http://en.wikipedia.org/wiki/Synesthesia from the Ancient Greek σύν (syn), "together," and αἴσθησις (aisthēsis), "sensation"—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.
Over 60 types of synesthesia have been reported by people, but only a fraction have been evaluated by scientific research. Even within one type, synesthetic perceptions vary in intensity and people vary in awareness of their synesthetic perceptions.
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1. Synesthesia is involuntary and automatic.
2. Synesthetic perceptions are spatially extended, meaning they often have a sense of "location." For example, synesthetes speak of "looking at" or "going to" a particular place to attend to the experience.
3. Synesthetic percepts are consistent and generic (i.e., simple rather than pictorial).
4. Synesthesia is highly memorable.
5. Synesthesia is laden with affect.

Dedicated regions of the brain are specialized for given functions. Increased cross-talk between regions specialized for different functions may account for the many types of synesthesia.
The Gift by Vladimir Nabokov.