Qualitatively different forms of pure alexia.

In this study we investigated two patients with pure alexia, F. C. and L. D. S., in order to make inferences about how processes and levels involved in the early stage of visual word recognition are organized and how they can be selectively damaged. Moreover, we investigated whether pure alexia can be caused by different functional deficits. F. C. and L. D. S. were presented with tasks of letter processing and tasks of orthographic integration. There was a clear double dissociation between the pattern of performance of F. C. and L. D. S. F. C. was able to process single letters rapidly and accurately, but was unable to group together the letters that he had correctly identified. By contrast, L. D. S. was slower and more impaired at letter identification, but she could use letter groups to assist reading. Thus, two different forms of pure alexia emerged: F. C. has a higher level deficit in integrating letters, whereas L. D. S. has a lower level deficit in letter processing. The results support the assumption of a functional organization of the reading process that involves a series of orthographic units ( i.e., single letters, sub-lexical letter groups, and the lexical unit), which can be selectively damaged. Finally, our data present difficulties for models of pure alexia that assume all patients to have a low-level processing deficit.