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Inter- and Intra- Hemispheric Interactions in Reading Ambiguous Words

Poster C128 in Poster Session C, Wednesday, October 25, 10:15 am - 12:00 pm CEST, Espace Vieux-Port

Tali Bitan^1,2, Nofar Mizrachi¹, Zohar Eviatar¹, Orna Peleg³; ¹University of Haifa, ²University of Toronto, ³Tel Aviv University

The present study investigates the functional connectivity between language regions in the two hemispheres, while processing written words with multiple meanings. Previous studies suggest that processing ambiguous words relies more heavily on the right hemisphere, and therefore on inter-hemispheric interactions, compared to processing unambiguous words. However, we suggest that this depends of the type of ambiguity. Specifically, we compared unambiguous words to two types of ambiguous words: homophonic homographs, in which a single orthographic representation is mapped into a single phonological representation with multiple meanings (e.g., ‘bank’); and heterophonic homographs, in which a single orthographic representation is mapped into different phonological representations with different meanings (e.g., ‘tear’). The later are very frequent in Hebrew orthography, in which many of the vowel are not represented in the script. We used effective connectivity analysis via Dynamic Causal Modeling (DCM) on previously published fMRI data (Bitan et al., Neuropsychology, 2017), to study inter- and intra- hemispheric connectivity among three bilateral regions. Two bilateral regions in the inferior frontal gyrus (i.e. pars opercularis and pars orbitalis) typically associated with phonological and semantic processing, respectively, and bilateral visual word form areas, known to be involved in orthographic processing. Our results for unambiguous words showed positive inter-hemispheric connections between the left and right frontal regions, which speaks against the hypothesis of transcallosal suppression during language processing. However, they show evidence of hemispheric competition at the level of orthographic processing. We also found direct connections from the VWFA to the left orbitalis, consistent with connectionist reading models suggesting direct orthographic to semantic activation. For heterophonic homographs, we observed increased top-down connectivity from the left opercularis to the VWFA within the left hemisphere, highlighting the importance of re-activation of orthographic representations by phonological ones for considering alternative meanings. Finally, processing of homophonic homographs, more than other words, relies on inter-hemispheric connectivity, which differentially affects the activation of the dominant and subordinate meanings. These findings provide novel insights into the different interactions among regions in the two hemispheres during the processing of ambiguous and unambiguous words.

Topic Areas: Reading, Phonology