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

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Chapter 9
the mirror-touch synaesthete; anatomical subtype). The first reported case of the
condition was provided in a single case fMRI study, which linked the condition to
heightened neural activity in a network of brain regions which are also activated in
non-synaesthetic control subjects when observing touch to others (the mirror-touch
system, comprised of the primary and secondary somatosensory cortex, premotor
cortex, intraparietal sulcus and superior temporal sulcus; Blakemore et al., 2005).
Chapter 2 examined the prevalence and characteristics of the condition.
In the first experiment reported in chapter 2 the prevalence of mirror-touch
synaesthesia was assessed by screening a large population of undergraduate students
for the presence of mirror-touch synaesthesia and determining the validity of reported
cases with a synaesthetic stroop task (Banissy and Ward, 2007). In the task,
participants were asked to indicate the site of touch on their own body while
observing touch to another person. Participants were asked to report the site of
veridical touch and ignore any synaesthetic tactile experience induced. For
synaesthetes, but not for controls, veridical touch could be in the same (congruent) or
different (incongruent) location to observed / synaesthetic touch (congruency was
determined according to each synaesthete’s self reports). Synaesthetes performed
slower in the incongruent condition and produced more errors linked to their
synaesthesia. Nine mirror-touch synaesthetes (from 567 participants screened) were
confirmed, which provides an estimated prevalence rate of 1.6%. This places mirror-
touch synaesthesia as one of the most common variants of synaesthesia, alongside
grapheme-colour synaesthesia (estimated prevalence rate of 2%; Simner et al., 2006)
and day-colour synaesthesia (estimated prevalence rate of 2.8%; Simner et al., 2006).
By combining cases of mirror-touch synaesthesia from the prevalence study
with cases of mirror-touch synaesthesia from self-referrals, the findings from chapter