Neuronal bases of pure alexia: a study using magnetoencephalography and tractography

Patients with strokes have been a major source of knowledge about the relationship between cognitive dysfunction and the site of causative lesions. Recent neuroimaging techniques may confirm theories related to the neuronal bases of cognitive functions. We applied magnetoencephalography and tractography to explorations into disconnection hypothesis for pure alexia. Evoked magnetic responses, related to letter discrimination tasks presented in the left visual field, were recorded in 4 patients with alexia and 4 controls. In the control group, equivalent current dipoles (ECD) of the first and second peaks were estimated in the right visual cortices and the third peak were estimated in the higher visual cortices on both sides, while all ECDs up to 200 ms were estimated only in the right hemisphere in the patient group. These findings supported the disconnection theory for classic type of pure alexia, from intact right visual cortex to the left language areas. The tractography in a patient with alexia demonstrated the inferior longitudinal fasciculus and the posterior segment of the arcuate fasciculus only on the right side. Thus pathways from bilateral occipital lobes to the left parietal lobe seemed to be impaired, which was consistent with disconnection theory for non-classic type of pure alexia.