Temporal dynamics of letter processing revealed by multivariate pattern analysis of EEG data

Letters are the primitives of reading expertise. Single letter recognition relies on a hierarchy of processing stages, in which early visual features gradually evolve into abstract letter representations, but the temporal organization of these stages remains poorly understood. To address it, we applied multivariate pattern analysis (MVPA) to electroencephalography (EEG) data recorded while adult readers (n = 35) performed a one-back repetition detection task on single letters and pseudoletters. Traditional event-related potential (ERP) analyses revealed differences between letters and pseudoletters in the N1 (140-170 ms), P2 (210-270 ms), and P3 (300-500 ms) components. Multivariate temporal generalization analyses showed that neural patterns distinguishing letters from pseudoletters were highly generalizable from approximately 140 to 600 ms after stimulus onset. A spatiotemporal searchlight analysis indicated that, despite this temporal generalization, the topographic configuration of EEG channels contributing to classification changed along this window, suggesting that neural representations in later processing stages were transformed from earlier perceptual stages. These findings indicate that letter recognition unfolds as a cascade of continuous and interacting processes rather than via discrete stages. Early perceptual letter-specific activity, indexed by the N1 component, remains engaged throughout later, increasingly abstract, orthographic processing stages to jointly support letter identification.