Bridging speech and sight: white matter anatomy in ticker-tape synaesthesia

Ticker-tape synaesthesia is an informative yet little studied developmental condition in which persons see automatically and vividly in their mind's eye the written form of the spoken words they are hearing. Ticker-tapers show an over-activation and a functional over-connectivity of core regions of the left-hemispheric reading system: the posterior superior temporal and supramarginal gyri (pSTG/SMG), where speech is processed and interfaced with vision, and the Visual Word Form Area (VWFA), which supports orthographic representations in the occipitotemporal cortex. We predicted that synesthetes should show increased anatomical connectivity between these regions. We scanned 17 synesthetes and 17 matched controls with diffusion-weighted MRI, and used probabilistic tractography to compare the density of streamlines between groups. We found that ticker-tapers had a higher streamline density in the white matter underlying the SMG, and connecting the SMG and the mid and posterior STG. We propose that those increased white matter connections at the temporoparietal junction and towards the VWFA boost the top-down influence of phonology on orthography, giving rise to the ticker-tape phenomenology. More generally, while atypical anatomical connectivity may be detrimental to the acquisition of culture-dependent abilities, as in dyslexia, it may also underlie a gain of function, as illustrated by ticker-tape synaesthesia.