Mirror-touch synaesthesia: the role of shared ... - UCL Discovery

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Chapter 1
can skip generations (Hubbard and Ramachandran, 2003), and that the proportion of
sons or daughters born to synaesthete mothers does not significantly differ (Barnett et
al., 2008; Ward and Simner, 2005), suggest that an X-linked dominant mode of
inheritance may be an over simplified account of the genetic mechanisms which
underlie developmental forms of the condition (Asher et al., 2009).
Current estimates on the prevalence of developmental synaesthesia indicate
that the condition has a prevalence rate of at least 4% and a female to male ratio of 1:1
(Simner et al., 2006; Ward and Simner, 2005). Although, depending on whether one
includes cases of ordinal linguistic personification (in which individuals attribute
genders or personalities to letters or numbers; Simner and Holenstein, 2007) or spatial
number forms (Sagiv, Simner, Collins, Butterworth, and Ward, 2006), the prevalence
rate of 4% is likely to be much higher (Simner et al., 2006).
A trend of all studies of the prevalence of synaesthesia is to report a higher
proportion of synaesthetes who experience colour evoked by letters or other linguistic
stimuli (e.g. grapheme-colour / day-colour synaesthesia; Baron-Cohen, Burt, Smith-
Laittan, Harrison, and Bolton, 1996; Rich, Bradshaw, and Mattingley, 2005; Simner
et al., 2006). It is perhaps not surprising that this variant of synaesthesia has been the
topic of much research amongst synaesthesia researchers. Research into this variant
of the condition has highlighted a number of interesting individual differences
between synaesthetes. For example, distinctions have been made between projector
and associator synaesthetes; which distinguishes between synaesthetes whose locus of
experienced colour is projected to a specific spatial location (projector synaesthetes)
and synaesthetes whose concurrent is perceived internally or in the ‘minds eye’
(associator synaesthetes) (Dixon, Smilek, and Merilke, 2004; see also Ward, Li, Salih,
and Sagiv, 2007). Similarly, Ramachandran and Hubbard (2001) have categorised