Chapter 6: Acquired Dyslexia

Acquired Dyslexia begins with a historical retrospective, detailing the landmark observations of Dejerine, who distinguished between "alexia with agraphia," where both reading and writing are lost due to parietal lobe damage, and "alexia without agraphia" (pure alexia), a disconnection syndrome where patients can write but not read their own writing. Central to the modern understanding of these disorders is the dual-route model of reading, which suggests that humans process written text through two distinct pathways: a lexical route that matches familiar words to a mental catalog of visual forms to access meaning and sound, and a nonlexical route that applies phonetic rules to "sound out" unfamiliar strings. Reading impairments are categorized into peripheral and central dyslexias. Peripheral dyslexias, including pure alexia, neglect dyslexia, and attentional dyslexia, stem from deficits in early visual processing or the allocation of spatial attention, often forcing patients into laborious letter-by-letter reading strategies. Central dyslexias involve disruptions to deeper linguistic or semantic functions. These include deep dyslexia, notable for semantic errors where a patient might read the word "castle" as "knight"; phonological dyslexia, which specifically impairs the reading of non-words while leaving real-word reading relatively intact; and surface dyslexia, where the ability to recognize irregular words is lost, causing patients to rely exclusively on phonetic pronunciation. The chapter also explores the latent reading capacity of the right hemisphere and utilizes advanced neuroimaging data to localize key functions, such as identifying the left fusiform gyrus as the site of the visual word form system. Finally, it addresses computational frameworks, such as connectionist and dual-route cascaded models, which provide a theoretical basis for clinical assessment and the development of targeted remediation strategies for those suffering from these complex neurological deficits.