1 Frequency and Features of Synesthesia

The frequency of the incidence of synesthetic perceptions cannot be unequivocally determined. The results of various field studies vary between 0.05% and 4%, women being more frequently affected (estimations vary here from 1:1.1 to 1:6). It is presumed that synesthesia has a genetic component as it often appears within a single family.

Each type of synesthesia can be formally characterized by a set of activating features (e.g., music), called inducers, and activated features (e.g., colors), called concurrents, that are automatically coupled, which means they cannot be suppressed (Bergfeld-Mills, 1999). The inducing perceptions can be either complex (such as, e.g., music) or simple (such as, e.g., sounds), whereas the concurrent perceptions are for the most part simple (e.g., colors, simple geometric figures, individual sounds). An exception to this are so-called ordinal spatial sequences (Sagiv et al. 2005) and an only recently investigated form of synesthesia in which numbers, units of time, or letters are perceived as personalities, so-called ordinal-linguistic personification (OLP).

The most important criterion for determining synesthesia is the individual consistency of a coupling over extended periods of time. For example, one synesthete always perceives the number 1 as green, while for another it is always yellow. Thus, every synesthete has his or her individual constant couplings of activating and activated perceptions, or his or her own synesthetic reality, which clearly complicates the study of the synesthetic phenomenon.