Dissecting the sublexical route for reading: Frontal and parietal networks support learned orthography-to-phonology mappings

Oral reading relies on lexical and sublexical processes with distinct neural mechanisms. Damage within the sublexical system causes phonological alexia, a blanket diagnosis describing acquired deficits in reading unfamiliar words. Improving the precision of alexia diagnosis requires understanding the neurocognitive basis of specific reading subprocesses. This study investigated the neural correlates of sublexical reading in 64 adults with chronic left-hemisphere stroke (LHS), focusing on lesions that impair the use of learned orthography-to-phonology (OP) mappings to read new words. Participants read aloud real words and three types of pseudowords varying in the number of plausible OP mappings at the level of the orthographic body: zero mappings (0M), one mapping (1M), and multiple mappings (MM). LHS participants exhibited phonological reading deficits with an exaggerated lexicality effect compared to 71 neurotypical controls. Across both groups, pseudowords with learned OP mappings were read more accurately than those without. Voxelwise and connectome-based lesion-symptom mapping revealed that relative lexical reading deficits were associated with lateral temporal lesions, while sublexical reading deficits were associated with lesions or disconnections of the left inferior frontal (IFG), supramarginal, and pre/postcentral gyri. Applying learned OP mappings relied on anterior IFG and frontoparietal connections, while resolving multiple plausible OP mappings relied on intraparietal connections. These results underscore the role of learned mutigraphemic OP mappings in sublexical reading, and demonstrate that disruptions of different sublexical reading subprocesses result in subtly different deficit patterns. Dissecting the neurocognitive basis of reading subprocesses may improve the precision of alexia diagnosis and point to new treatments.