Cytowic (1995) notes that synesthesia can take diverse forms, although the most common form is known as colored hearing which is when a visual stimulus elicits not only the sight but also some form of color. Cytowic points out that research conducted on this form of synesthesia has actually shown that there are two subtypes. The first sub-type consists of seeing colors only when hearing words or letters, and not when hearing music or other sounds; while the second subtype consists of seeing color for all sounds. Other types of synesthesia are much rarer and consist of shape-tasting, sound-smelling and sight-smelling.
As to the prevalence of synesthesia, Easton (2004) states that about one in every 25,000 individuals have some form of synesthesia. It is usually more common in women with a ratio of three women for every one man with synesthesia in the United States. Easton further informs that synesthesia:
...seems to be more common in those individuals who are left-handed, with as many as 6 out of 7 synesthetes being left-handed...It is also not uncommon to see high numbers of individuals with synesthesia in the arts fields, as synesthetes tend to score higher on tests involving creativity...This prevalence of synesthesia, in particular colored hearing, in the artistic community has been well documented since the end of the nineteenth century. (p. 1)
Relationship of Neuro-Biology and Psy