Word reading in alphabetic languages involves letter identification, independently of the format in which these letters are written. This process of letter 'regularization' is sensitive to word context, leading to the recognition of a word even when numbers that resemble letters are inserted among other real letters (e.g., M4TERI4L). The present study investigates the electrophysiological correlates of number-to-letter regularization by means of the masked priming paradigm: target words (MATERIAL) were preceded by fully alphabetic primes (MATERIAL), primes with letter-like numbers (M4T3R14L), or primes with unrelated numbers (M7T6R28L). ERPs revealed three subsequent effects. Around 150 ms the unrelated numbers condition elicited a positive effect, compared to the other two conditions, in the occipital electrodes. Then, target words preceded by primes with numbers elicited a more negative N200 in the same electrodes compared to the fully alphabetic condition. Finally, both alphabetic primes and letter-like numbers elicited a posterior positive component peaking around 260 ms compared to unrelated numbers. Source analysis for each electrophysiological effect revealed a similar early increase of activity in the left occipito-temporal pathway for alphabetic primes and primes with letter-like numbers. Around 200 ms, the orthographic interference due to the numerical values correlated with an increase of activity in parietal areas; finally, a recursive effect in the left occipital cortex was found, reflecting abstract letter activation. These results indicate that direct feedback interaction from word units strongly influences the activation of the letter units at a format-independent abstract level.
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