The breakdown of parallel letter processing in letter-by-letter dyslexia

Two critical issues were examined regarding letter-by-letter (LBL) dyslexia: (1) What is the nature of the functional impairment responsible for the incapacity of LBL patients to overtly recognise words on the sole basis of parallel letter processing? (2) What is the purpose of sequential letter processing? Four experiments focusing on these issues were conducted in LH, an LBL dyslexic. Expt 1 showed facilitatory effects of increased phonographic neighbourhood size, lexical frequency, and imageability on the word naming performance of LH. These high-order effects reflect a modulation of parallel letter processing in LH and demonstrate that he is able to rapidly access phonological, lexical, and semantic knowledge during reading. Congruently, Expt 2 demonstrated that all three high-order effects are eliminated when words are presented one letter at a time, from left to right. Expt 3 showed that these high-level effects are also abolished if target words are made of letters that are highly confusable (i.e., visually similar) to other letters of the alphabet. These observations suggest that LBL dyslexia may rest on an impairment at the letter encoding level that causes an excessive level of background noise in the activation of higher-order representations (i.e., letter combinations) when letters are processed in parallel. An additional experiment (Expt 4) shows that the letter confusability effect is also eliminated when words are presented one letter at a time, from left to right. This latter finding suggests that compensatory sequential processing invoked by LBL dyslexics serves to avoid the confusion between visually similar letters, which is present with parallel letter processing, and to amplify the signal-to-noise ratio required to achieve overt word identification.