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 pre-orthographical factors constrained his word recognition performance. The results can be best accounted for by a reduction of the number of letters processed in parallel.