Physical Evidence of Synaesthetic Colours : Structural and Neurophysiological Differences in Grapheme-Colour Synaesthetes

Abstract

The present study combined a synaesthetic Stroop task with EEG, in addition to diffusion tensor imaging (DTI) and surface based morphometry (SBM) to study functional and structural aspects of grapheme-colour synaesthesia in 12 synaesthetes and 12 non-synaesthetes. Results from the synaesthetic Stroop task indicated a significant Stroop effect in a subgroup of the synaesthetes. The effect consisted in significantly slower response time and lower accuracy for “strong” synaesthetes in incongruent conditions relative to congruent conditions. This difference was significantly larger than the one observed in “weak” synaesthetes and controls. These “strong” synaesthetes also showed differences in ERPs stimulus-locked to the synaesthetic Stroop task, when compared to “weak” synaesthetes and controls. “Strong” synaesthetes had a significantly more positive deflection at central and parietal electrodes 400-600 ms post-stimulus compared to “weak” synaesthetes and controls. Analysis of DTI data revealed a frontal cluster with significantly larger fractional anisotropy (FA) in “strong” synaesthetes relative to non-synaesthetic controls. No other FA differences were found between the groups. SBM was used to measure cortical thickness. The analysis showed an area of significantly thinner cortex in “strong” synaesthetes relative to “weak” synaesthetes, but not relative to controls. Regression analysis of the variable representative of the ERP-effect, as well as values from the significant clusters in the FA and cortical thickness analyses was performed. Both ERP amplitude and FA in the significant cluster contributed significantly in explaining the group differences. The contributions were independent, indicating separate contributions from structural and functional mechanisms to the synaesthetic effect observed.