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

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Chapter 1
experience is due to additional structural connectivity (i.e. structural differences),
malfunctions in cortical inhibition (i.e. functional but not structural differences) or a
combination of both (Bargary and Mitchell, 2008; Cohen Kadosh and Henik, 2007;
Cohen Kadosh and Walsh, 2008; Grossenbacher and Lovelace, 2001; Hubbard and
Ramachandran, 2005; Rouw and Scholte, 2007; Smilek, Dixon, Cudahy, and Merikle,
2001).
Supporting evidence for structural connectivity accounts is provided by a DTI
study which reports greater white matter coherence in grapheme-colour synaesthetes
compared to non-synaesthetic control subjects - grapheme-colour synaesthetes show
increased structural connectivity in inferior-temporal, parietal and frontal brain
regions when compared to non-synaesthetes (Rouw and Scholte, 2007). Some
authors have interpreted these findings to be consistent with accounts of synaesthesia
which argue in favour of aberrant connectivity between adjacent cortical regions
(Ramachandran and Hubbard, 2001; Hubbard, 2007). For example, given that the
brain regions involved in the visual recognition of graphemes (i.e. the putative visual
word form area; Cohen and Dehaene, 2004) lie adjacent to brain areas involved in
colour perception (i.e. human V4 - Wade et al., 2002), it has been suggested that
grapheme-colour synaesthesia may arise from direct cross-activation between these
regions as a result of either increased connectivity between adjacent brain regions or
reduced inhibition between adjacent regions. This local cross-activation account has
also been extended to explain sequence-space synaesthesia (i.e. number forms), in
terms of cross-activation between adjacent parietal regions (Ramachandran and
Hubbard, 2001), and may also be important for other variants of synaesthesia (e.g.
lexical-gustatory synaesthesia; Ward, Simner and Auyeung 2005). However, it is of
note that the generality of enhanced structural connectivity in grapheme-colour