The neuroanatomy of grapheme–color synesthesia

Abstract

Grapheme-color synesthetes perceive particular colors when seeing a letter, word or number (grapheme). Functional neuroimaging studies have provided some evidence in favor of a neural basis for this type of synesthesia. Most of these studies have reported extra activations in the fusiform gyrus, which is known to be involved in color, letter and word processing. The present study examined different neuroanatomical features (i.e. cortical thickness, cortical volume and cortical surface area) in a sample of 48 subjects (24 grapheme-color synesthetes and 24 control subjects), and revealed increased cortical thickness, volume and surface area in the right and left fusiform gyrus and in adjacent regions, such as the lingual gyrus and the calcarine cortex, in grapheme-color synesthetes. In addition, we set out to analyze structural connectivity based on fractional anisotropy (FA) measurements in a subsample of 28 subjects (14 synesthetes and 14 control subjects). In contrast to the findings of a recent neuroanatomical study using modern diffusion tensor imaging measurement techniques, we did not detect any statistically significant difference in FA between synesthetes and non-synesthetes in the fusiform gyri. Our study thus supports the hypothesis of local anatomical differences in cortical characteristics in the vicinity of the V4 complex. The observed altered brain anatomy in grapheme-color synesthetes might be the anatomical basis for this particular form of synesthesia but it is also possible that the detected effects are a consequence (rather than the primary cause) of the life-long experience of grapheme-color synesthesia.

Neural Basis of Individual Differences in Synesthetic Experiences

“…We tested for differences between synesthetes and controls in a number of areas reported in previous VBM studies (Jäncke et al, 2009;Weiss and Fink, 2009). We therefore performed a post hoc analysis on the left and right anterior intraparietal sulcus (both hIP1 and hIP2) from the Juelich Histological Atlas (Choi et al, 2006) and bilaterally in the fusiform gyrus; occipital fusiform gyrus, temporal occipital fusiform cortex, temporal fusiform cortex (anterior division), and temporal fusiform cortex (posterior division) from the Harvard-Oxford cortical structural atlas.…”

“…Previous studies found activation (Gray et al, 2006) or increased white matter integrity (Jäncke et al, 2009) in the hippocampal region, but examined only the contrast between synesthetes and nonsynesthetes. In cognitive (neuro)psychology literature, the hippocampus is mostly known for its implication in memory functions, including episodic and declarative memory (Scoville and Milner, 1957;Squire, 1992;Eichenbaum, 2004), but also spatial memory (O'Keefe and Nadel, 1978;Maguire et al, 2000).…”

“…Another factor that might influence significance level is variability between subjects, as different patterns of variation between individuals are found in different brain areas (Mechelli et al, 2005). Jäncke et al (2009) found increased gray matter in the fusiform gyrus and adjacent regions in grapheme-color synesthetes. In addition to the fusiform gyrus, they found differences in the precuneus, superior occipital cortex, orbitofrontal cortex, vicinity of the central sulcus, insula, and hippocampus.…”

“…Familial studies and a genetic association study support a genetic predisposition for synesthesia (Baron-Cohen et al, 1996;Simner et al, 2006;Asher et al, 2009). Synesthetes, compared with nonsynesthetes, show structural brain differences in white matter (WM) and gray matter (GM) properties (Rouw and Scholte, 2007;Hänggi et al, 2008;Jäncke et al, 2009;Weiss and Fink, 2009). These structural differences are obtained in modality-specific regions as well as in other brain areas.…”

Social responsiveness to inanimate entities: Altered white matter in a ‘social synaesthesia’

“…These include, for example, studies associating posterior tract reductions in FA with visual and other hallucinations (Ashtari et al, 2007;Fujiwara et al, 2007 (Melero et al, 2013;Hänggi et al, 2008;and Whitaker et al, 2014 respectively). Yet other researchers may have been in a position to find reduced FA but stated a priori they would not to explore this direction within their results (Jäncke et al, 2009). Moreover, at least one functional study of synesthetes found a deactivation of certain areas in response to synesthesia-inducing stimuli (Paulesu et al, 1995).…”