Defining (trained) grapheme-color synesthesia

There is a current debate over the possibility and validity of synesthesia training experiments (Deroy and Spence, 2013; Rothen and Meier, 2014). In order to test whether a trainee should be considered to have acquired a trained form of synesthesia, a precise definition and specific diagnostic criteria of synesthesia are necessary. There is currently not one specific checklist available including all specific diagnostic methods and criteria, exacerbating the determination and interpretation of differences between (potentially) trained and genuine synesthesia. In order to facilitate communication surrounding these issues, we propose a practical guideline for diagnosing the specific characteristics that are typical of grapheme-color synesthesia (GCS). These guidelines can be applied to developmental cases of GCS, cases of trainees who may show synesthetic traits, and other types of acquired forms of GCS at the level of a single individual. Researchers have tried to train synesthesia since at least 1934 (Kelly, 1934), and several studies have been done in recent years (for reviews see: Deroy and Spence, 2013; Rothen and Meier, 2014). Leaving trained, acquired, or induced types of synesthesia aside, it is important to acknowledge the extent of interindividual differences between synesthetes (even within one sub-type), making a generally accepted definition difficult to reach. For example, an individual may report that letters automatically induced the conscious experience of color since childhood, however, these colors may not be consistently mapped to each letter. Such an individual would fail the “test of genuineness,” while he or she may still meet the other defining characteristics of synesthesia, such as the conscious experience of color in the absence of physical color. This is one example of why the definition of developmental synesthesia is a topic of active debate in the literature (Cohen Kadosh and Terhune, 2012; Eagleman, 2012; Simner, 2012). The need for a consensus on the specific defining characteristics of synesthesia becomes only more prominent when trying to compare trainees to synesthetes. Characteristics of GCS at the individual vs. group level must be distinguished. Group-level effects related to synesthesia cannot (yet) be used as diagnostic tools. If two groups differ relatively from each other on a certain score or measurement, this does not tell us at an absolute level whether “score X on task A” or “measurement Y in trait B” of an individual implies that he or she is necessarily a synesthete. The nature of such studies does not allow one to assume that a certain measurement is unique to synesthesia. For example, it has been shown that GCS is associated with increased structural connectivity compared to controls in the temporal cortex at the group-level (Rouw and Scholte, 2007). However, the absolute value measured for white-matter connectivity in a particular individual could be influenced by different unknown causes. Although measurements can made at the individual level (see Table 1), experiments designed to test group-level characteristics related to GCS by definition cannot be used as diagnostic criteria, such as memory benefits (Yaro and Ward, 2007; Rothen and Meier, 2010; Rothen et al., 2012; Pritchard et al., 2013; Terhune et al., 2013), differences in visual processing (Terhune et al., 2011; Brang et al., 2012; Banissy et al., 2013), the reported vividness of visual mental imagery (Barnett and Newell, 2008), distinct cognitive styles (Meier and Rothen, 2013) or differences in neuroanatomy (for a review see: Rouw et al., 2011). Similarly, idiosyncrasy (the fact that grapheme-color mappings differ between individuals) cannot be used as a marker at the individual level; if two synesthetes have nearly identical mappings, they are not excluded from being considered synesthetes (Witthoft and Winawer, 2013). Additionally, the low prevalence of synesthesia is a group-level characteristic that is not diagnostic at the individual level. Most neurobiological markers related to GCS (Rouw et al., 2011) and the presence of certain genetic markers (Asher et al., 2009; Tomson et al., 2011) are related to the group-level. Individual-level traits are sometimes confused with inclusive criteria for GCS. For example, GCS tends to be unidirectional at a conscious level (i.e., graphemes elicit color experiences, but color experiences do not elicit conscious grapheme experiences) and bidirectional at an unconscious level (Knoch et al., 2005; Cohen Kadosh and Henik, 2006; Cohen Kadosh et al., 2007; Gebuis et al., 2009; Weiss et al., 2009; Rothen et al., 2010). Still, an instance of conscious bidirectional GCS does not exclude an individual from being considered a synesthete when the other defining characteristics are met (e.g., Cohen Kadosh et al., 2007). Diagnosing synesthesia in a single individual is based on a variety of characteristics. Concerning diagnosing “trained” forms of synesthesia, Rothen and Meier (2014) state: “Hence, to confirm the hypothesis, that synesthesia can be induced via training, would require the trained inducers to (i) consistently and (ii) automatically elicit (iii) the associated concurrent experience with perceptual qualities on a subjective phenomenological basis (iv) for the great majority of the inducers’ occurrences (v) over an extended time period” (italics are the authors’ own). We propose to extend their definition with a diagnostic criteria checklist.