Synesthesia runs strongly in families.  1 in 23 people may be synesthetic.  

video credit Dir. Terri Timley / USA / 2009

Synesthesia "union of senses") - a neurological condition in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway -  is studied for the insights it may give into cognitive and perceptual processes  Research demonstrates that synesthetic experiences can have measurable behavioral consequences and can be identified by differences in brain pattern activity.