Is developmental dyslexia a disconnection syndrome?

TLDR

Adult developmental dyslexics exhibit phonological processing deficits across tasks such as spoonerisms, phonemic fluency, and digit naming, possibly linked to dysfunction of the left insula that normally bridges Broca’s area and temporal‑parietal language regions. The study proposes that dyslexia arises from weak connectivity between anterior and posterior language areas. Researchers employed visually presented rhyming and short‑term memory tasks to record PET brain activity in five compensated adult dyslexics. PET scans revealed that dyslexics activated Broca’s area during rhyming and temporo‑parietal cortex during short‑term memory, but never the left insula, and these regions were not co‑activated as in controls, supporting separate phonological representations.

Abstract

A rhyming and short-term memory task with visually presented letters were used to study brain activity in five compensated adult developmental dyslexics. Their only cognitive difficulty was in phonological processing, manifest in a wide range of tasks including spoonerisms, phonemic fluency and digit naming speed. PET scans showed that for the dyslexics, a subset only of the brain regions normally involved in phonological processing was activated: Broca's area during the rhyming task, temporo-parietal cortex during the short- term memory task. In contrast to normal controls these areas were not activated in concert. Furthermore the left insula was never activated. We propose that the defective phonological system of these dyslexics is due to weak connectivity between anterior and posterior language areas. This could be due to a dysfunctional left insula which may normally act as an anatomical bridge between Broca's area, superior temporal and inferior parietal cortex. The independent activation of the posterior and anterior speech areas in dyslexics supports the notion that representations of unsegmented and segmented phonology are functionally and anatomically separate.