Abstract
The ability to identify letters and encode their position is a crucial step of the word recognition process. However and despite their word identification problem, the ability of dyslexic children to encode letter identity and letter-position within strings was not systematically investigated. This study aimed at filling this gap and further explored how letter identity and letter-position encoding is modulated by letter context in developmental dyslexia. For this purpose, a letter-string comparison task was administered to French dyslexic children and two chronological age (CA) and reading age (RA)-matched control groups. Children had to judge whether two successively and briefly presented four-letter strings were identical or different. Letter-position and letter identity were manipulated through the transposition (e.g., RTGM vs. RMGT) or substitution of two letters (e.g., TSHF vs. TGHD). Non-words, pseudo-words, and words were used as stimuli to investigate sub-lexical and lexical effects on letter encoding. Dyslexic children showed both substitution and transposition detection problems relative to CA-controls. A substitution advantage over transpositions was only found for words in dyslexic children whereas it extended to pseudo-words in RA-controls and to all type of items in CA-controls. Letters were better identified in the dyslexic group when belonging to orthographically familiar strings. Letter-position encoding was very impaired in dyslexic children who did not show any word context effect in contrast to CA-controls. Overall, the current findings point to a strong letter identity and letter-position encoding disorder in developmental dyslexia.
Highlights
Developmental dyslexia is characterized by a written word identification disorder in children who otherwise exhibit normal intellectual efficiency and have been normally exposed to printed words and literacy instruction
Results show that (1) the performance pattern of dyslexic children is impaired relative to CA-control children but is similar to that of RA-controls; (2) letter strings are better discriminated when two letters are substituted rather than transposed, in the dyslexic children as in the control groups; (3) A substitution advantage is found in CA-controls whatever stimulus type but the effect is limited to words and pseudo-words in RAcontrols and is only found for words in dyslexic children; and (4) the impact of condition on stimuli effect depends on group: the substitution condition yielded a Word Superiority Effect” (WSE) (W > NW) in CA-controls and dyslexic children
LETTER-IDENTITY PROCESSING The current findings show that dyslexic children as beginning readers (e.g., RA-controls) are less prone than CA-controls at detecting substitutions across strings
Summary
Developmental dyslexia is characterized by a written word identification disorder in children who otherwise exhibit normal intellectual efficiency and have been normally exposed to printed words and literacy instruction. Our purpose in this study was to explore letter identity and letter-order encoding ability in developmental dyslexia and how letter processing is modulated by letter-string context Such encoding problems may be related to specific mechanisms within the framework of current reading models. Adelman et al (2010) showed that, in adult readers, letter identity encoding starts during the first 25 ms of printed word processing on all string positions simultaneously He showed that processing was initiated over the whole word letter string in parallel, all letters were not identified at the same level of efficiency so that multiple fixations are usually observed when reading longer (e.g., Radach et al, 2004) or less frequent (Ferrand and New, 2003; Valdois et al, 2006) words. Reading acquisition evolves from serial encoding of sub-lexical orthographic units (i.e., slow letter-by-letter decoding) to a more efficient parallel processing of the entire word sequence
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