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Abstract
An investigation of the joint effects of orthographic neighbourhood size (N size) and of letter confusability in three letter-by-letter (LBL) dyslexics is reported. All three patients showed a facilitatory effect of increased N size with low letter-confusability words, but no N size effect with high confusability words. This exactly replicates previous observations by Arguin, Fiset, and Bub (2002) in another LBL dyslexic. A facilitatory N size effect requires parallel letter processing and the word recognition performance of normal readers is unaffected by letter confusability. The present findings therefore signal that the residual capacity for parallel letter processing in LBL dyslexia is blocked by letter similarity. This implies a deficit of letter encoding or identification, which appears to be a general feature of LBL dyslexia since it is exhibited by all of the four patients so tested.
Acknowledgments
We are grateful to DM, JL, and JT for their generous collaboration. This study was supported by a grant from the Canadian Institutes of Health Research. Martin Arguin is chercheur-boursier of the Fonds de la Recherche en Santé du Québec.
Notes
The letter confusability scores were obtained by averaging the letter confusion matrices published in Gilmore et al. (Citation1979), Loomis (Citation1982), Townsend (Citation1971), and Van der Heijden et al. (Citation1984). They correspond to the total error rates for each individual letter of the alphabet. These values range between .24 (for the letter L) and .71 (for the letter B), with an average of .47 and a standard deviation of .13.