The deficit of letter processing in developmental dyslexia: combining evidence from dyslexics, typical readers and illiterate adults.

To clarify the link between anomalous letter processing and developmental dyslexia, we examined the impact of surrounding contours on letter vs. pseudo-letter processing by three groups of children - phonological dyslexics and two controls, one matched for chronological age, the other for reading level - and three groups of adults differing by schooling and literacy - unschooled illiterates and ex-illiterates, and schooled literates. For pseudo-letters, all groups showed congruence effects (CE: better performance for targets surrounded by a congruent than by an incongruent shape). In contrast, for letters, only dyslexics exhibited a CE, strongly related to their phonological recoding abilities even after partialling out working memory, whereas the reverse held true for the pseudo-letter CE. In illiterate adults, the higher letter knowledge, the smaller their letter CE; their letter processing was immune (to some extent) to inference from surrounding information. The absence of a letter CE in illiterates and the positive CE in dyslexics have their origin in different aspects of the same ability, i.e. phonological recoding.