INS Topical Seminar – Diane Lazard (Paris)
“fMRI studies of cortical reorganization in postlingual deafness: modification of the left hemispheric dominance for speech”
Similarly to certain brain injuries, post-lingual deafness imposes central reorganization to adapt to the loss of easy and instinctive oral communication. Occurring in mature brains, shaped from childhood, plasticity cops with not so available brain resources such as those encountered in developing brains. Using fMRI in post-lingual deaf adults and matched normal hearing controls, we address the question of late deafness-induced reorganization by exploring phonological processing from written material and relate the different adopted strategies to speech recognition scores obtained after cochlear implantation (CI).
During easy to more difficult written rhyming tasks, the involvement of right temporal areas during deafness prior to CI is a consistent marker of poor speech outcome after auditory rehabilitation. The recruitment of these right areas, not usually involved in phonological processing but in paralinguistic processing, is a marker of the central reorganization some subjects develop to palliate the difficulties of losing oral communication. Depending on the skills of audio-visual fusion acquired in early childhood, two neurobiological profiles emerge: i) some subjects are able to maintain a left hemispheric dominance anchored to oral interactions through left audio-visual loops, maintained by lip-reading. They will later become proficient CI users, ii) some subjects, less balanced, will develop a greater reliance on visual inputs, based on a functional interaction between early visual cortex and the right superior temporal areas. This greater right involvement favors and speeds up written processing, easing social interactions when lip-reading is impossible. This shift in hemispheric dominance, leaving the visual cortex less available for left audio-visual interactions, seems a reliable predictor of poorer CI outcome.
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