Subtitled speech: the neural mechanisms of ticker-tape synaesthesia.

Abstract

Reading acquisition modifies areas of the brain associated with vision, with language, and their connections. Those changes enable reciprocal translation between orthography, and word sounds and meaning. Individual variability in the pre-existing cerebral substrate contributes to the range of eventual reading abilities, extending to atypical developmental patterns, including dyslexia and reading-related synesthesias. The present study is devoted to the little-studied but highly informative ticker-tape synesthesia (TTS), in which speech perception triggers the vivid and irrepressible perception of words in their written form in the mind's eye. We scanned a group of 17 synesthetes and 17 matched controls with functional MRI, while they listened to spoken sentences, words, numbers, or pseudowords (Experiment 1), viewed images and written words (Experiment 2), and were at rest (Experiment 3). First, we found direct correlates of the TTS phenomenon: during speech perception, as TTS was active, synesthetes showed over-activation of left perisylvian regions supporting phonology, and of the occipitotemporal Visual Word Form Area (VWFA), where orthography is represented. Second, we brought support to the hypothesis that TTS results from atypical relationships between spoken and written language processing: the TTS-related regions overlap closely with cortices activated during reading, and the overlap of speech-related and reading-related areas is larger in synesthetes than in controls. Furthermore the regions over-activated in TTS overlap with regions under-activated in dyslexia. Third, during resting state, that is in the absence of current TTS, synesthetes showed increased functional connectivity between left prefrontal and bilateral occipital regions. This pattern may reflect a lowered threshold for conscious access to visual mental contents, and may implement a non-specific predisposition to all synesthesias with a visual content. Those data provide a rich and coherent account of TTS as a non-detrimental developmental condition created by the interaction of reading acquisition with an atypical cerebral substrate.