Parallel processing blocked by letter similarity in letter by letter dyslexia: A replication.

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.