![]() Given that the fusiform gyrus has clusters of cells responsive to faces in addition to colors and graphemes, one might predict the existence of ‘face-color’ synesthesia in some individuals. ![]() Furthermore, the cross-activation of these sensory regions is mediated by increased white-matter connectivity in synesthetes, as assessed by diffusion tensor imaging ( Rouw & Scholte, 2007). For example, simple achromatic graphemes will spontaneously and inevitably activate grapheme-regions in the fusiform gyrus in tandem with area V4 ( Brang et al., 2010 Hubbard, Arman, Ramachandran, & Boynton, 2005 Sperling et al., 2006). Research into the neural basis of synesthesia has further confirmed the sensory cross-activation theory of many robust forms of synesthesia. Taken collectively, these observations support the early sensory cross-activation theory of synesthesia, although these do not negate powerful top-down influences (demonstrated using Navon figures Ramachandran & Hubbard, 2001b) and the existence of higher forms of synesthesia that are more ‘conceptually driven’ and harder to relate to physiology. Furthermore, synesthetically induced colors can drive apparent motion ( Kim, Blake, & Palmeri, 2006 Ramachandran & Azoulai, 2006) and the vividness of synesthetic colors can vary with eccentricity and field laterality ( Brang & Ramachandran, 2009). Synesthetically induced colors can lead to perceptual segregation ( Ramachandran & Hubbard, 2001a Ward, Jonas, Dienes, & Seth, 2010), different portions of a single letter can have multiple colors (Ramachandran & Brang, 2010), and some color anomalous synesthetes may even see synesthetic colors that are unique from those experienced in the real world (Martian colors Ramachandran & Hubbard, 2001b), despite the absence of the ‘necessary’ cone pigments. ![]() Several lines of evidence suggest that in most instances of synesthesia, the evoked color is a sensory experience as opposed to a high-level memory association (e.g., from having used colored refrigerator magnets as a child). Furthermore, this research has demonstrated that the phenomenon is relatively common (2–4% of the population Simner et al., 2006), is mediated by genetic factors ( Asher et al., 2009), and can influence aspects of everyday life ranging from memory ( Smilek, Dixon, Cudahy, & Merikle, 2002) to creativity ( Ward, Thompson-Lake, Ely, & Kaminski, 2008). Although synesthetic experiences were long disregarded as a rare curiosity, there has been a tremendous resurgence of interest in the last decade (e.g., Mattingley, Rich, Yelland, & Bradshaw, 2001 Ramachandran & Hubbard, 2001a, 2003). For example, an individual may experience a specific color for every given note, or every grapheme (printed number or letter) may be tinged with a specific hue (e.g., C-sharp or the number 5 evoking red). In the nineteenth century, Francis Galton observed that a certain proportion of the general population who were otherwise normal had a condition he dubbed ‘synesthesia’ a sensory stimulus presented through one modality spontaneously evoked a sensation in an unrelated modality ( Galton, 1883).
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