BACKGROUND: Some people can "feel" colors, not just see them; other people can taste, hear, or smell colors. This is the result of a rare neurological condition called synesthesia, in which two or more senses intertwine. Long dismissed as the product of an individual's over-active imagination, scientists in recent years have come to associate the phenomenon with a real neurological basis. The University of Waterloo is one of a small number of institutions where studies are being conducted on persons suffering from synesthesia. Anywhere between 1 in 200 and 1 in 2,000 people suffer from synesthesia of some sort.
TYPES OF SYNESTHESIA: There are many different types of synesthesia. For example, among those who associate letters and numbers with color, there are so-called "projector" synesthetes. In their perception, color can fill the printed letter or it can appear directly in front of their eyes as if projected on an invisible screen. In contrast, "associate" synesthetes see the colors in their "mind's eye" rather than outside their bodies. Another category is "perceptual" synesthetes, triggered by sensory stimuli like sights and sounds, while "conceptual" synesthetes respond to abstract concepts like time. For instance, a conceptual synesthete might describe the months of the year as a flat ribbon surrounding the body, each month being a distinct color.
WHAT'S THE CAUSE: There are several theories as the cause of synesthesia. One theory posits that irregular sprouting of new neural connections within the brain leads to a breakdown of the boundaries that normally exist between the senses. So synesthesia would be the collective "chatter" of sensory neighbors once isolated from each other. Another theory is that all infants begin life as synesthetes, born with immature brains that are highly malleable. Connections between different sensory parts of the brain exist that later become pruned or blocked as the infant matures. Still another theory states that synesthesia doesn't require extra connections, but arises when normally covert channels of communication between the senses are "unmasked" and cross into conscious awareness. None of these theories can be tested experimentally, because we do not yet have the technology to observe brain-connection changes in the living human brain, and determine how they relate to mental changes. Certain drugs, such as LSD or mescaline, can also induce synesthesia in some individuals.