Stimulation of one sensory or cognitive passageway leads to automatic, involuntary experiences in a second sensory or cognative passageway.  Some synesthetes hear, smell, taste or feel pain in color. Others taste shapes, and still others perceive written digits, letters and words in color. Some, who possess what researchers call “conceptual synesthesia,” see abstract concepts, such as units of time or mathematical operations, as shapes projected either internally or in the space around them. And many synesthetes experience more than one form of the condition.