Neurocognitive signatures of phonemic sequencing in expert backward speakers

Abstract

Despite its prolific growth, neurolinguistic research on phonemic sequencing has largely neglected the study of individuals with highly developed skills in this domain. To bridge this gap, we report multidimensional signatures of two experts in backward speech, that is, the capacity to produce utterances by reversing the order of phonemes while retaining their identity. Our approach included behavioral assessments of backward and forward speech alongside neuroimaging measures of voxel-based morphometry, diffusion tensor imaging, and resting-state functional connectivity. Relative to controls, both backward speakers exhibited behavioral advantages for reversing words and sentences of varying complexity, irrespective of working memory skills. These patterns were accompanied by increased grey matter volume, higher mean diffusivity, and enhanced functional connectivity along dorsal and ventral stream regions mediating phonological and other linguistic operations, with complementary support of areas subserving associative-visual and domain-general processes. Still, the specific loci of these neural patterns differed between both subjects, suggesting individual variability in the correlates of expert backward speech. Taken together, our results offer new vistas on the domain of phonemic sequencing, while illuminating neuroplastic patterns underlying extraordinary language abilities. When visiting the local barber shop, a native of La Laguna, Spain, may be surprised by the speech of some fellow citizens. He might be greeted with the utterance nasbue chesno and fail to understand it at all. With time, however, he may realize that the phrase was a backward rendition of buenas noches (good evening) and that this peculiar way of speaking is quite widespread in this town. So much so, in fact, that a group of citizens demand that UNESCO acknowledge their linguistic extravaganza as intangible cultural heritage. Still, theirs might be a lost cause. Although word inversion is also part of other sociolects, such as Argentine lunfardo, the Canary Academy of Language has declared that this phenomenon has no scholarly value. Yet, that position is arguably shortsighted. Backward speech constitutes an extraordinary ability to quickly reverse words, pseudowords, and even sentences, which requires reordering phonemes while retaining their identity. Therefore, it offers a useful model to study phoneme sequencing as a key stage of phonological-phonetic encoding-i.e., the capacity to select, retrieve, and temporarily arrange phonemic information to set up an articulatory plan 1,2. Based on this premise, the present study offers a novel approach to phonemic sequencing mechanisms by examining behavioral, neuroanatomical, and neurofunctional signatures of expertise in backward speech. Pioneering behavioral research on phoneme sequencing was mainly concerned with speech errors in healthy 3-5 and brain-damaged 6-8 subjects, giving rise to serial associative, frame-based, and dynamic control signal psycholinguistic models 3. Although neuroimagin...

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