Some people say they "see red" when angry, or they talk about "sharp" cheese, "cool" jazz, or "loud" ties. But do you see pink triangles on hearing music, or feel yourself on a prickly bed of nails when eating cherry pie? Do ordinary words, names, and voices have specific colors, peculiar shapes - even distinctive flavors?
If this sounds perfectly normal, then you may be one of a handful of individuals with synesthesia, the cross-wiring of the senses that is experienced in the first-person not as imagination, but as an external and involuntary sensation.
Neurologically, synesthesia involves only the left hemisphere, and relies on limbic structures for its expression. While we usually regard the cortex as the home of sensory perception, there is, during the multisensory synesthetic experience, a counter-intuitive collapse of cortical metabolism, without effecting subjects' higher intelligence or normal behavior.
The implications of synesthesia lead to a multiplex model of brain organization that rejects the hierarchical supremacy of neocortex and the usual emphasis on objectivity. This multiplex model stresses limbic regulation, subjective experience, and non-verbal knowledge.