Abstract
It has been suggested that dorsal and ventral pathways support distinct aspects of language processing. Yet, the full extent of their involvement and their inter-regional connectivity in visual word recognition is still unknown. Studies suggest that they might reflect the dual-route model of reading, with the dorsal pathway more involved in grapho-phonological conversion during phonological tasks, and the ventral pathway performing lexico-semantic access during semantic tasks. Furthermore, this subdivision is also suggested at the level of the inferior frontal cortex, involving ventral and dorsal parts for lexico-semantic and phonological processing, respectively. In the present study, we assessed inter-regional brain connectivity and task-induced modulations of brain activity during a phoneme detection and semantic categorization tasks, using fMRI in healthy subject. We used a dynamic causal modeling approach to assess inter-regional connectivity and task demand modulation within the dorsal and ventral pathways, including the following network components: the ventral occipito-temporal cortex (vOTC; dorsal and ventral), the superior temporal gyrus (STG; dorsal), the dorsal inferior frontal gyrus (dIFG; dorsal), and the ventral IFG (vIFG; ventral). We report three distinct inter-regional interactions supporting orthographic information transfer from vOTC to other language regions (vOTC -> STG, vOTC -> vIFG and vOTC -> dIFG) regardless of task demands. Moreover, we found that (a) during semantic processing (direct ventral pathway) the vOTC -> vIFG connection strength specifically increased and (b) a lack of modulation of the vOTC -> dIFG connection strength by the task that could suggest a more general involvement of the dorsal pathway during visual word recognition. Results are discussed in terms of anatomo-functional connectivity of visual word recognition network.
Highlights
Despite the significant amount of knowledge of the cerebral networks involved in visual word recognition (e.g., Price, 2012), yet little is known on the functional connections between the regions belonging these networks
Our results showed that an activated language network was located exclusively in the left hemisphere and included: (a) frontal regions: supplementary motor area (SMA, Brodmann’s area (BA) 6), precentral gyrus (BA 4); dorsal part of the inferior frontal gyrus (IFG, BA 44) and the ventral part of the IFG (BA 45/46); (b) temporal regions: superior/middle temporal gyrus (STG, MTG, BA 22/21)
These studies suggested that the dorsal pathway is involved in grapho-phonological conversion during phonological tasks, whereas the ventral pathway is rather involved in lexico-semantic access during semantic tasks
Summary
Despite the significant amount of knowledge of the cerebral networks involved in visual word recognition (e.g., Price, 2012), yet little is known on the functional connections between the regions belonging these networks. In this study we were interested in better specifying the ventral and dorsal pathways involved in visual word recognition and assessing their modulation by lexicosemantic and phonological processes. We were interested in the specific involvement of the ventral and dorsal inferior frontal gyrus (IFG) in relation of the lexico-semantic and phonological processing, respectively. Phonological and lexicosemantic operations play a critical role in both spoken and written language and experimental data suggest that they are processed and relayed along two cerebral pathways. A metaanalysis by Jobard et al (2003) indicates that visual word recognition involves a dual-route process including a dorsal grapho-phonological stream or “surface” reading route, and a ventral lexico-semantic stream, or “deep” reading route (Jobard et al, 2003; Price, 2012; see Richardson et al, 2011; Taylor et al, 2012)
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