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

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1.2 Prevalence and characteristics of synaesthesia
Chapter 1
Synaesthesia is typically considered as having three defining features; 1)
experiences are conscious perceptual or percept-like experiences; 2) experiences are
induced by an attribute not typically associated with the conscious experience; 3)
these experiences occur automatically (Ward and Mattingley, 2007). Further, the
synaesthetic percept tends to co-exist with the percept of the inducing stimulus rather
than over-riding it – for example in lexical-gustatory synaesthesia written or heard
words are recognised but also result in a simultaneous subjective sensation of taste in
the mouth and tongue area (Ward and Simner, 2003). Note that throughout the thesis
the terminology of referring to different types of synaesthesia in terms of inducer-
concurrent pairs separated with a hyphen is used. As such, touch-colour synaesthesia
refers to tactile inducers eliciting a concurrent experience of colour, and vision-touch
synaesthesia refers to a visual inducer eliciting a tactile experience.
Cases of synaesthesia can be either developmental or acquired, with
developmental cases thought to be dependent upon genetic and environmental factors
and acquired cases reflecting synaesthesia following specific environmental
influences (e.g. following brain injury or drug ingestion). Developmental forms of
synaesthesia have been shown to run in families and previous research suggested that
the condition may be more common in women than men, which may reflect an X-
linked dominant mode of inheritance (Baron-Cohen, Burt, Smith-Laittan, Harrison,
and Bolton, 1996). More recent research indicates that synaesthesia may be equally
common within males and females and that previous methodologies may have led to
an over inflated male-female ratio (Ward and Simner, 2005; but see Barnett et al.,
2008). Similarly, reports of twins discordant for synaesthesia (Smilek, Mofatt,
Pasternak, White, Dixon, and Merilke, 2002), as well as evidence that synaesthesia