Two different mechanisms for omission and substitution errors in neglect dyslexia.

Neglect dyslexia is a reading disorder often associated with right-sided brain lesions. In reading single words, errors are mostly substitutions or omissions of letters that occupy the left-sided positions. Typically, these errors have been thought to depend on a single mechanism. Conversely, we propose that they are due to different mechanisms. In particular, a visuo-spatial mechanism is responsible for omissions and a perceptual integration process for substitution errors. We measured the performance of six patients with both neglect and neglect dyslexia, analyzing their reading errors as a function of letter spacing. According to our conjecture, letter spacing should increase omissions by moving part of the string further in the unattended space, while it should reduce substitutions by restoring the integration processes. Furthermore, we predict that letter spacing should be more effective with pseudowords compared to words, in that in this latter case lexical effects are supposed to influence attentional and perceptual processes. Accordingly, we found that for pseudowords only the two types of errors are differently affected by this manipulation and only omissions correlate with the severity of the disorder in visuo-spatial tasks.