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A mesial-to-lateral dissociation for orthographic processing in the visual cortex
Edited by Michael I. Posner, University of Oregon, Eugene, OR, and approved September 6, 2019 (received for review March 22, 2019)

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Significance
The visual word form area (VWFA) is essential to reading, yet the exact representations that it harbors are unclear. We tested the hypothesis that a subregion of ventral visual cortex may be specialized for the perception of multiletter graphemes such as “ch” or “ou.” Indeed, the selective disruption of graphemic information impaired reading behavior and modulated brain activity in a mesial region around the midfusiform sulcus. In contrast, the VWFA was modulated by lexical variables. Those findings suggest a medial-to-lateral division of labor within the ventral occipitotemporal cortex. The existence of a “grapheme area,” its functional dissociation from the VWFA, and their collaboration bear important implications for the understanding of reading acquisition and of interindividual differences in reading skills.
Abstract
Efficient reading requires a fast conversion of the written word to both phonological and semantic codes. We tested the hypothesis that, within the left occipitotemporal cortical regions involved in visual word recognition, distinct subregions harbor slightly different orthographic codes adapted to those 2 functions. While the lexico-semantic pathway may operate on letter or open-bigram information, the phonological pathway requires the identification of multiletter graphemes such as “ch” or “ou” in order to map them onto phonemes. To evaluate the existence of a specific stage of graphemic encoding, 20 adults performed lexical decision and naming tasks on words and pseudowords during functional MRI. Graphemic encoding was facilitated or disrupted by coloring and spacing the letters either congruently with multiletter graphemes (ch-ai-r) or incongruently with them (c-ha-ir). This manipulation affected behavior, primarily during the naming of pseudowords, and modulated brain activity in the left midfusiform sulcus, at a site medial to the classical visual word form area (VWFA). This putative grapheme-related area (GRA) differed from the VWFA in being preferentially connected functionally to dorsal parietal areas involved in letter-by-letter reading, while the VWFA showed effects of lexicality and spelling-to-sound regularity. Our results suggest a partial dissociation within left occipitotemporal cortex: the midfusiform GRA would encode orthographic information at a sublexical graphemic level, while the lateral occipitotemporal VWFA would contribute primarily to direct lexico-semantic access.
Footnotes
- ↵1To whom correspondence may be addressed. Email: florence.bouhali{at}gmail.com.
Author contributions: F.B., Z.B., S.D., and L.C. designed research; F.B. and Z.B. performed research; F.B. and Z.B. analyzed data; and F.B., Z.B., S.D., and L.C. wrote the paper.
The authors declare no competing interest.
This article is a PNAS Direct Submission.
Data deposition: Raw anonymized data from this study are available in BIDS format on OpenNeuro, https://openneuro.org/datasets/ds002155.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1904184116/-/DCSupplemental.
Published under the PNAS license.
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