Taking the Sublexical Route: Brain Dynamics of Reading in the Semantic Variant of Primary Progressive Aphasia.

Reading aloud requires mapping an orthographic form to a phonological one. The mapping process relies on sub-lexical statistical regularities (e.g., “oo” to |u:|) or on learned lexical associations between a specific visual form and a series of sounds (e.g., yacht to /jɑt/). Computational, neuroimaging, and neuropsychological evidence suggest that sub-lexical and lexical processes rely on partially distinct neural substrates: a dorsal (occipito-parietal) and a ventral (occipito-temporal) route, respectively. Here, we investigated the spatiotemporal features of sub-lexical processes capitalizing the time resolution of magnetoencephalography and the unique clinical model offered by patients with semantic variant of Primary Progressive Aphasia (svPPA). Behaviorally, svPPA patients manifest marked lexico-semantic impairments including difficulties in reading words with exceptional orthographic to phonological correspondance (irregular words). Moreover, they present with focal neurodegeneration in the anterior temporal lobe (ATL), affecting primarily the ventral, occipito-temporal, lexical route. Therefore, this clinical population allows for testing of specific hypotheses on the neural implementation of the dual-route model for reading, such as whether damage to one route can be compensated by over-reliance on the other. To this end, we reconstructed and analyzed time-resolved whole-brain activity in 12 svPPA patients and 12 healthy age-matched controls while reading irregular words (e.g., yacht) and pseudowords (e.g., pook). Consistent with previous findings that the dorsal route is involved in sub-lexical processes, in control participants we observed enhanced neural activity over dorsal occipito-parietal cortices for pseudowords, when compared to irregular words. This activation was manifested in the beta-band (13-20 Hz), ramping up slowly over 500 ms after stimulus onset and peaking at ~800 ms, around response selection and production. This temporal pattern of neural activity was not observed in svPPA patients. Furthermore, a direct comparison of neural activity between patients and controls revealed a dorsal spatiotemporal cluster during irregular word reading. These findings suggest that the sub-lexical route is involved in processing both irregular and pseudo- words in svPPA. Together these results provide further evidence supporting a dual-route model for reading aloud - mediated by the interplay between lexical (ventral) and sub-lexical (dorsal) neuro-cognitive systems. When the lexical route is damaged, as in the case of neurodegeneration affecting the ATL, partial compensation appears to be possible by over-recruitment of the slower, serial attention-dependent, sub-lexical one.