Increased left fusiform surface area: a structural predisposition to ticker-tape synesthesia

Ticker-tape synesthesia (TTS) is a developmental variant in which individuals perceive in their mind's eye the written form of heard speech vividly and automatically. Previous work documented functional over-activation of the reading network during speech perception, increased functional connectivity between the Visual Word Form Area (VWFA) and left temporoparietal regions and increased anatomical connectivity through the arcuate fasciculus in synesthetes. Here, we investigated cortical morphology in 17 synesthetes and 17 matched controls. Synesthetes showed significantly increased cortical surface area in the left fusiform gyrus, centred on the mid-fusiform gyrus and extending towards the VWFA. No group difference was found in cortical thickness or curvature. This establishes cortical morphology as a further line of evidence implicating the left ventral occipitotemporal cortex in TTS. Three arguments suggest that this area increase reflects an innate predisposition rather than a consequence of the synesthetic experience: cortical surface area, unlike thickness, is more closely related to early neurodevelopmental processes; reduced area in the same region characterizes developmental dyslexia and the surface increase extends beyond the reading network. We propose that an innate expansion of fusiform cortex provides a richer substrate for orthographic representations, excessively activated by top-down phonological input, giving rise to the involuntary visualization of speech.