Why are there different types of synesthete?

For people with synesthesia, sensations in two modalities are experienced when only one is stimulated (e.g., auditory stimuli might trigger colors and sounds). Synesthetes are manifestly different to the general population, but can also be different to each other. First, the condition is widely heterogeneous in that 60–150 different manifestations of synesthesia have been identified (e.g., auditory stimuli might trigger colors, shapes, flavors and so on; Cytowic and Eagleman, 2009). Second, synesthetes can differ on the quality of their synesthetic perceptions even within a given variant. Some experience their synesthetic percepts as being similar in quality to a real-world perceptions (e.g., synesthetic colors might be projected onto external objects and be difficult to dissociate from real-world colors) while other synesthetes experience less “veridical” percepts (see below). In this opinion piece I ask whether this particular difference— known as the “projector” vs. “associator” distinction—might fall out naturally from another, independent psychological quality. The projector/associator distinction was first phrased (by Dixon et al., 2004) in terms of grapheme-color synesthesia, in which colors are triggered by letters and/or digits. Some grapheme-color synesthetes report that their colors are experienced outside their own body space, projected into the world, and these are termed “projector” synesthetes. Ward and colleagues further divide this category into two: “surface-projectors” experience color projected onto the written type-face (or more generally, onto the inducing stimulus whatever that might be), and “spaceprojectors” project color onto some other externalized near-space. In contrast to this external projection, other synesthetes (termed “associators”) can be thought of as “non-projectors” in that their colors exist within their internal mental space. Those associators who claim to see colors in their mind’s eye have been termed “seeassociators,” while those who simply claim to know the colors of graphemes without any associated impression of “seeing” at all have been termed “know-associators” (Ward et al., 2007). In other words there is a four-way divide between synesthetes experiencing colors in a way resembling real-world experiences (projected onto the stimulus or into near-space) and those experiencing them “internally” (in the mind’s eye or as a type of propositional association). The claim put forward in this piece is a parsimonious one: that these differences might emerge from an otherwise unrelated individual difference, in the ability to form a visual mental image. Visual imagery—the mental construction of a scene-like object—is known to vary across individuals (e.g., Marks, 1973). At the upper extreme end there are individuals with “eidetic” imagery who report strongly evoked mental images with an almost veridical quality. At the opposite end of the healthy spectrum are those who report being unable to formmental images at all. Taking these latter into consideration, and assuming a prevalence of synesthesia of at least 4% (Simner et al., 2006), we might assume—all other things being equal—that 4% of people with poor or no imagery abilities will have synesthesia. The proposal here is that these individuals are precisely those termed “know associators.” Equally, those with extreme imagery abilities might be those we recognise as projectors, because their high imagery allows their synesthetic associations to become “scene-like” to an extreme extent. Note that this view (if it were correct) would rely on the assumption that imagery and synesthesia are independent qualities and I now compare this view with current thinking in the literature. Some researchers have proposed that synesthetes are in fact characterized, as a group, by superior mental imagery (e.g., Barnett and Newell, 2008; Price, 2009a,b), and some have gone so far as to ask whether some visual synesthesias may be nothing more than extreme visual imagery (Galton, 1880; Phillips, 1897; Price, 2009a). One well-phrased expression of a possible link between synesthesia and imagery is that normal cross-modal sensations (say, between numbers and space) may enter into consciousness only if the individual has heightened imagery (Eagleman, 2009; Price, 2009a), and this is precisely when he/she would become considered a “synesthete” (Simner, 2012). (The resultant synesthesia in this example would be sequence-space synesthesia— in which sequences such as numbers are consciously experienced in spatial arrays). In contrast with this view, others have suggested that synesthesia and imagery might be quasi-independent in that synesthetes might vary in their imagery ability, and hence some could have relatively weak imagery (see Grossenbacher and Lovelace, 2001, who put this view in passing). Questions about synesthesia and imagery have also been tested empirically. Barnett and Newell (2008) showed that (n = 38) synesthetes with colored language (e.g., grapheme-color synesthesia) report significantly stronger vivid everyday mental images than controls, in a self-report questionnaire (Vividness of Mental Imagery Questionnaire, VVIQ, Marks, 1973). Meier and Rothen (under review) show similarly for (n = 24) grapheme-color synesthetes using the