Synaesthetic visuo-spatial forms: Viewing sequences in space

A small sub-set of the population experiences an unusual phenomenon in which sequences (such as Arabic numerals, letters of the alphabet, days, months, etc.) are visualised as occupying particular spatial arrays, and these individuals are known as visuo-spatial synaesthetes. Their visuo-spatial forms depict sequenced units (e.g., January, February, March,.) laid out in set spatial arrangements which are open for conscious inspection, and which can be highly convoluted and idiosyncratic. Many examples of these spatial arrays are given in the following special section on Synaesthetic Visuo-spatial Forms (e.g., Figs. 1–6, Simner et al., 2009, this issue; Fig. 1, Jarick et al., 2009a, this issue) which explores the roots of this unusual phenomenon and its relationship to normal processing. Visuo-spatial synaesthetes may possess one or many visuospatial form(s), and these exist either in the peripersonal area outside the body, or in mental space within the mind’s eye. Although these spatial arrangements differ from synaesthete to synaesthete, they tend to be highly consistent within each individual over time, and many visuo-spatial synaesthetes are simply unaware that their life-long experience of sequences in space differs in any way from the experiences of the average person. Synaesthetic spatial forms can be induced by a range of different ordinal sequences, although forms triggered by time units (e.g., days or the week, months of the year) and by numbers and letters are particularly common (see Sagiv et al., 2006 for data on the prevalence of different visuo-spatial forms). Other rarer cases have involved forms for sequences such as shoe sizes, temperatures, historical eras, the Indian caste system, and even prime-time television schedules (Cytowic and Eagleman, 2009; Hubbard et al., 2005b; Sagiv et al., 2006; Seron et al., 1992; Seymour, 1980) and one particularly extra-ordinary case is reported in this special section by Hubbard et al. (2009, this issue). Their synaesthete, DG, has an astonishing set of at least 58 different forms, and these include spatial arrays not only for Arabic numerals, alphabet, and time units, but also for other numerical sequences (e.g., Roman numerals), different units of measure (e.g., kilograms, kilometres and degrees), financial series (stock prices, tax rates, etc.) and even the sequenced order of pure-bred dog naming conventions. In their empirical work, Hubbard and colleagues have focussed on DG’s form for Arabic numerals, and indeed, three papers in total in this special section investigate synaesthetic number lines (also Ward et al., 2009, this issue; Jarick et al., 2009a, this issue). The remaining papers examine synaesthetic forms encoding time units; specifically, forms for hours within a day (Jarick et al., 2009b, this issue), for days within a week (Price, 2009, this issue), for months within a year (Eagleman, 2009, this issue; Price, 2009, this issue; Jarick et al., 2009b, this issue) and for years within a century (Simner et al., 2009, this issue). As such, they cover a range of manifestations across a range of synaesthetic individuals, and their data converge to provide the most in-depth examination to date of this unusual phenomenon.