What is the taste of the letter "A," and the number 4? How do blue squares sound? These questions are meaningful for a small subset of human beings who have synesthesia. Synesthesia is neither an illness nor a disorder; rather, it is a perceptual condition in which stimulation in one sensory modality elicits a concurrent sensation in another---for example, hearing particular sounds might induce vivid experiences of color, taste or odor, as might the sight of visual symbols, such as letters or digits (Rich & Mattingley, 2002).
The purpose of this paper is to present an overview of the condition of synesthesia. The review looks at the prevalence and various types of this perceptual mode. It also presents a discussion of the relationship of neurobiology and psychology in synesthesia.
However, one point that must be made is that there are reasons to doubt the existence of synesthesia. For example, da Costa (1996) reports that the condition tends to be conceptualized using phenomenological rather than measurable, behavioral and more biophysical terms. Further, she points out that most studies of the condition are phenomenological without much in the way of physiological or anatomical substantiation. Indeed, without this substantiation, da Costa reports that the phenomenon of synesthesia will be relegated to the same category as tales of UFOs who have been visiting earth and abducting people. Therefore, this paper also presents an examination of the empirical support for the validity of the condition.
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 m...