A mesial-to-lateral dissociation for orthographic processing in the visual cortex

Area of Science:

• Neuroscience
• Cognitive Psychology
• Linguistics

Background:

• Efficient reading involves rapid conversion of written words into phonological (sound) and semantic (meaning) codes.
• The left occipitotemporal cortex is crucial for visual word recognition, but the precise roles of its subregions are debated.
• Existing models suggest separate pathways for lexico-semantic and phonological processing, potentially utilizing different levels of orthographic information.

Purpose of the Study:

• To investigate whether distinct subregions within the left occipitotemporal cortex specialize in processing orthographic codes for phonological versus semantic functions.
• To identify a potential grapheme-related area (GRA) involved in mapping multiletter graphemes to phonemes.
• To differentiate the functional roles and connectivity of the putative GRA and the visual word form area (VWFA).

Main Methods:

• Functional magnetic resonance imaging (fMRI) was used to scan 20 adults performing lexical decision and naming tasks.
• Participants processed words and pseudowords with manipulated letter coloring and spacing to facilitate or disrupt graphemic encoding (e.g., 'ch-ai-r' vs. 'c-ha-ir').
• Behavioral responses and brain activity modulations were analyzed in relation to graphemic encoding manipulations and task demands.

Main Results:

• Behavioral performance, particularly in pseudoword naming, was affected by the graphemic encoding manipulation.
• Brain activity was modulated in the left midfusiform sulcus, medial to the VWFA, suggesting a novel grapheme-related area (GRA).
• The GRA showed preferential functional connectivity with dorsal parietal areas involved in letter-by-letter reading, distinct from the VWFA's connections and its sensitivity to lexicality and spelling-to-sound regularity.

Conclusions:

• The findings suggest a partial functional dissociation within the left occipitotemporal cortex.
• A midfusiform GRA appears specialized for encoding orthographic information at a sublexical graphemic level, crucial for phonological processing.
• The lateral occipitotemporal VWFA likely contributes more directly to lexico-semantic access, supporting word meaning retrieval.