Letter and letter-string processing in developmental dyslexia

This study evaluated letter recognition processing in Italian developmental dyslexics and its potential contribution to word reading Letter/bigram recognition (naming and matching) and reading of words and non words were examined A group of developmental dyslexics and a chronologically age matched group of skilled readers were examined Dyslexics were significantly slower than skilled readers in all tasks The rate and amount model (RAM Faust et al 1999) was used to detect global and specific factors in the performance differences controlling for the presence of over additivity effects Two global factors emerged One ( letter string factor) accounted for the performance in all (and only) word and non word reading conditions indicating a large impairment in dyslexics (more than 100% reaction time - RT increase as compared to skilled readers) All the letter/bigram tasks clustered on a separate factor ( letter factor) indicating a mild impairment (ca 20% RT increase as compared to skilled readers) After controlling for global factor influences by the use of the z score transformation specific effects were detected for the letter string (but not the letter) factor Stimulus length exerted a specific effect on dyslexics performance with dyslexics being more affected by longer stimuli furthermore dyslexics showed a stronger impairment for reading words than non words Individual differences in the letter and letter string factors were uncorrelated pointing to the independence of the impairments The putative mechanisms underlying the two global factors and their possible relationship to developmental dyslexia are discussed (C) 2009 Elsevier Sri All rights reserved