Increased structural connectivity in grapheme-color synesthesia

Abstract

Diffusion tensor imaging allowed us to validate for the first time the hypothesis that hyperconnectivity causes the added sensations in synesthesia. Grapheme-color synesthetes (n = 18), who experience specific colors with particular letters or numbers (for example, 'R is sky blue'), showed greater anisotropic diffusion compared with matched controls. Greater anisotropic diffusion indicates more coherent white matter. Anisotropy furthermore differentiated subtypes of grapheme-color synesthesia. Greater connectivity in the inferior temporal cortex was particularly strong for synesthetes who see synesthetic color in the outside world ('projectors') as compared with synesthetes who see the color in their 'mind's eye' only ('associators'). In contrast, greater connectivity (as compared with non-synesthetes) in the superior parietal or frontal cortex did not differentiate between subtypes of synesthesia. In conclusion, we found evidence that increased structural connectivity is associated with the presence of grapheme-color synesthesia, and has a role in the subjective nature of synesthetic color experience.

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Figure 1: Increased anisotropy in synesthetes as compared with non-synesthetes.
Figure 2: Increased brain activation and increased anisotropy in the inferior temporal cortex in grapheme-color synesthetes.
Figure 3: Anisotropy differentiates subtypes of grapheme-color synesthesia.

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Acknowledgements

We thank I. Veer, M. Hillen, L. Zil, N. Rusiyanadi, F. van Klaveren and F. Binkhorst for their contribution to testing; all the synesthetic and control subjects for their enthusiastic collaboration; and B. Laeng and V. Lamme for their comments.

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Correspondence to Romke Rouw.

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Supplementary information

Supplementary Fig. 1

Main directions of white matter tracts in synesthetes and controls. (PDF 230 kb)

Supplementary Fig. 2

White matter tracts in synesthetes and controls. (PDF 848 kb)

Supplementary Table 1

Correlation matrix (nonparametric) of projector-associator score, FA values in the four clusters differentiating synesthetes from non-synesthetes, and BOLD signal in the four areas responding to synesthetic color in synesthetes. (PDF 57 kb)

Supplementary Table 2

Correlations between FA in inferior temporal cluster and BOLD signal in the anterior cluster in temporal cortex responding to synesthetic color, for synesthetes and non-synesthetes separately. (PDF 58 kb)

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Rouw, R., Scholte, H. Increased structural connectivity in grapheme-color synesthesia. Nat Neurosci 10, 792–797 (2007). https://doi.org/10.1038/nn1906

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