The Neural Bases of Grapheme–Color Synesthesia Are Not Localized in Real Color-Sensitive Areas

Abstract

The subjective experience of color by synesthetes when viewing achromatic letters and numbers supposedly relates to real color experience, as exemplified by the recruitment of the V4 color center observed in some brain imaging studies. Phenomenological reports and psychophysics tests indicate, however, that both experiences are different. Using functional magnetic resonance imaging, we tried to precise the degree of coactivation by real and synesthetic colors, by evaluating each color center individually, and applying adaptation protocols across real and synesthetic colors. We also looked for structural differences between synesthetes and nonsynesthetes. In 10 synesthetes, we found that color areas and retinotopic areas were not activated by synesthetic colors, whatever the strength of synesthetic associations measured objectively for each subject. Voxel-based morphometry revealed no white matter (WM) or gray matter difference in those regions when compared with 25 control subjects. But synesthetes had more WM in the retrosplenial cortex bilaterally. The joint coding of real and synesthetic colors, if it exists, must therefore be distributed rather than localized in the visual cortex. Alternatively, the key to synesthetic color experience might not lie in the color system.

Are synesthetes exceptional beyond their synesthetic associations? A systematic comparison of creativity, personality, cognition, and mental imagery in synesthetes and controls

“…These associations are arbitrary, idiosyncratic, involuntary, and automatic: they are not chosen, contrary to metaphors, are not evoked at will, and usually cannot be suppressed (for discussions on the evolving definition of synesthesia, see e.g., Hupé, Bordier, & Dojat, 2012; Simner, 2012). Synesthete P.B.…”

Grapheme-color synesthetes show peculiarities in their emotional brain: cortical and subcortical evidence from VBM analysis of 3D-T1 and DTI data

“…The first area is the fusiform gyrus (FG) where hV4 lies. The color-selective area has shown differential activation in some grapheme-color functional experiments (Hubbard et al 2005;Sperling et al 2006;van Leeuwen et al 2011;Specht and Laeng 2011), but not in others (Hupé et al 2011;Rich et al 2006;Scholte 2007, 2010;Weiss et al 2005). The other focus of interest for anatomical investigation has been the intraparietal sulcus (IPS), which is believed to mediate the hyperbinding of the inducer (e.g., a grapheme) and the concurrent (e.g., one specific color).…”

“…Additionally, a recent paper on grapheme-color anatomical peculiarities (gray and white matter volume) applied FWE correction for multiple comparisons and reported increased white matter volume in the retrosplenial cortex and the superior temporal sulcus only (Hupé et al 2011). This can be due to the fact that the authors used a different method of analysis.…”

“…Different analyses of white matter (FA vs. volume) provide different information about its structure (highly directional diffusion vs. higher density of connectivity fibers, respectively). Thus, both white matter volume results (Hupé et al 2011) andFA data (Jancke et al 2009;Rouw and Scholte 2007) are informative in synesthesia research, and the combination of both (Hanggi et al 2008) increases the detection power of anatomical studies (Gazdzinski et al 2010). Moreover, another type of data analysis, based on connectivity matrices derived from region-wise cortical thickness correlations, has demonstrated widespread connectivity alterations in graphemecolor synesthetes (Hanggi et al 2011).…”

Synaesthesia in Chinese characters: The role of radical function and position

“…Around 80-90% of known synaesthesias involve colour triggered by language ; for example, in grapheme-colour synaesthesia, sensations of colour are triggered by letters or digits. During the last decade, the neural basis of synaesthesia has been examined in both functional imaging studies (e.g., Aleman, Rutten, Sitskoorn, Dautzenberg, & Ramsey, 2001;Hubbard, Arman, Ramachandran, & Boynton, 2005;Nunn et al, 2002;Sperling, Prvulovic, Linden, Singer, & Stirn, 2006;Tomson et al, 2013) and in structural imaging studies (e.g., Hanggi, Beeli, Clechslin, & Jande, 2008;Hupé, Bordier, & Dojat, 2012;Rouw & Scholte, 2007;Weiss & Fink, 2009). Together these show that synaesthetic experiences are characterised by atypical patterns of brain activity when compared with non-synaesthetes, and differences in white matter and grey matter structure.…”