Not all synesthetes are alike: spatial vs. visual dimensions of sequence-space synesthesia

Sequence-space synesthesia (SSS) is a common condition in which ordinal sequences such as months, numbers or the letters of the alphabet are perceived to occupy spatial locations in the mind's eye or peripersonal or extrapersonal space (e.g., Price and Mentzoni, 2008; Jonas and Jarick, 2013). For example, thinking about a month may elicit the visuospatial impression of a circular arrangement of the months, or hearing a numeral may elicit a specifically shaped number line. These “spatial forms” are typically thought to be consistent over time within an individual (e.g., Smilek et al., 2007), though they can actually evolve (Price and Pearson, 2013; Gould et al., 2014; Price, 2014; see also Simner, 2012; Meier et al., 2014). They are also idiosyncratic, with synesthetes reporting many different shapes of varying complexity (Galton, 1880; Phillips, 1897) that are experienced outside the body (i.e., projected) or in the mind's eye (i.e., associated; Dixon et al., 2004; Smilek et al., 2007; Ward et al., 2007). SSS can vary along several dimensions, including the projector-associator distinction, automaticity, visual salience, and type of spatial transformation that can be applied to the spatial form (e.g., Price, 2013; Price and Mattingley, 2013). Since SSS could be considered a variety of visuospatial mental imagery, these individual differences may reflect known fractionation of imagery processes and skills (Price, 2013; Price and Pearson, 2013). However, a thorough and empirically grounded taxonomy for individual differences in SSS is missing (Price, 2014). We explore here a possible systematization for one area of these individual differences—the visual and spatial qualities of SSS. We further suggest this can help to classify synesthetes as experimental participants and perhaps explain some inconsistencies in published data.