Synaesthesia, is a rare condition in which a stimulus received in one sense organ causes an experience in another. For example, in colored hearing, sound and vision may mingle: the different tones of words and letters can involuntarily evoke distinct and vivid colors.
Some researchers believe that synaesthesia is how we originally experienced the world. For example, neurologist Richard Cytowic (whose website is here):
"Cytowic has tracked the source of the synesthetic experience back to the limbic system, one of the oldest parts of the brain and a site at which emotions and memories are processed. Cytowic calls synesthetes "living cognitive fossils" because he believes this kind of multi-sensory perception is as ancient as the place in which it originates. According to his theory, synesthesia may well have been our primeval way of experiencing the world — until the more rational cortex involved and filled the senses into the individual compartments. "Synesthesia is a normal brain function in every one of us," Cytowic says, "but its workings only reach consciousness in a handful. It may well be a memory of how early mammals saw, heard, smelled, tasted and touched." "(copied from Time Magazine article available on Cytowic's website).
This view dovetails with panexperientialist theories which see the fundamental units of experience as affective in nature, which are then differentiated and canalised by the brain and sensory systems. A panexperientialist extension of Cytowic's theory would see the limbic system not as the primeval source of affect and sensa, but as the conduit through which emotional energy is channeled from the body and the outside world, prior to it's conversion and corellation with particular qualia in the higher centers of the brain.
Synaesthesia is currently an area of major research interest and speculation in brain and consiousness studies. It will be of great interest to see how knowledge in this area develops over the coming years.