Introduction

Telephones ring, dogs bark, and computers hum, yet seeing any of these objects when their characteristic sound is absent does not automatically induce a compelling auditory hallucination of that sound. In olfaction, this type of automatic cross-modal hallucination can occur routinely. Certain odors, such as vanilla, are consistently reported as smelling sweet, yet sweetness is normally associated with the stimulation of another sense, that of taste. This phenomenon has been termed odor-taste synesthesia (Stevenson, Boakes, & Prescott, 1998), and its prevalence, reliability, validity, and origins are the focus of this chapter.

Because considerable confusion surrounds use of the terms taste and smell, the following definitions are used here. Taste refers to sensations induced primarily by stimulation of receptors located on the surface of the tongue (McLaughlin & Margolskee, 1994). The five basic taste sensations (along with their prototypical stimuli) are sweet (sucrose), salty (sodium chloride), sour (citric acid), bitter (quinine), and umami, or “meaty taste” (monosodium glutamate). On the other hand, odors (smells) are detected by receptors on the olfactory epithelium, which is located a short distance back from the bridge of the nose (Buck, 1996). There is a separate piece of olfactory epithelium for each nostril (Lanza & Clerico, 1995). An unusual feature of olfaction is that the receptors can be stimulated by two anatomically distinct routes: either via the nose, by sniffing (orthonasal olfaction), or via the mouth, as volatile chemicals rise up the nasopharynx during eating and drinking (retronasal olfaction; Pierce & Halpern, 1996). There appear to be about 1000 different types of olfactory receptors, with the result that olfactory sensations are far more diverse than the range of sensations produced by the relatively small set of taste receptors.