Preorthographical constraints on visual word recognition: evidence from a case study of developmental surface dyslexia.

We investigated the visual word recognition ability of M.T., a young boy with surface dyslexia, by means of a paradigm that measures performance as a function of the eye fixation position within the word, known as the "viewing-position effect" paradigm. In well-achieving readers, the viewing-position effect is mainly determined by factors affecting letter visibility and by lexical constraints on word recognition. We further quantified M.T.'s sensory limitations on letter visibility by computing visual-span profiles - that is, the number of letters recognizable at a glance. Finally, in an ideal-observer's perspective, M.T.'s performance was compared with a parameter-free model combining M.T.'s letter visibility data with a simple lexical matching rule. The results showed that M.T. did not use the whole visual information available on letter identities to recognize words and that preorthographical factors constrained his word recognition performance. The results can be best accounted for by a reduction of the number of letters processed in parallel.