New Scientist published a look at recent research into synesthesia, a neurologically based condition of mixed sensations. For synesthetes, a stimulus in one sensory modality (e.g., vision) automatically elicits a sensation or experience in another modality (e.g., hearing). It can also manifest as a perception of a form (like a letter or phoneme) inducing an unusual perception in the same modality (like a color).
What is happening in the brains of synesthetes? One hypothesis is that synesthesia occurs because of cross-activation between nearby brain areas. The cross-wiring could be due to a failure of synaptic pruning, a normal developmental process that refines and eliminates connections in the brain. Neuronal pruning is necessary in the months after birth, as sensory experiences strengthen some brain connections and let others wither. Research by Daphne Maurer and Catherine Mondlach at McMaster University in Ontario, Canada, suggests all infants may start out as synesthetes; those few that retain synaptic connections between sensory areas will remain synesthetic adults.
This article focuses on the connection between synesthesia and the origins of human language. It all started with an experiment designed by Wolfgang Kohler. When people were asked to choose which of two shapes is called bouba and which is kiki, 95-98% chose bouba for the rounded shape and kiki for the angular one. A similar effect has been found in non-English speakers and in 2.5-year-old children who could not yet read. Results such as these, and newer research highlighted in the link, suggest most humans may associate sounds with other senses to some degree. Some sounds are pointy, others smooth. Is this a leftover of our synesthetic early development?
 Ramachandran VS and Hubbard EM (2001). “Synaesthesia: A window into perception, thought and language.” (PDF). Journal of Consciousness Studies 8 (12): 3–34.