The Anatomic Basis Of Dyslexia

A variety of experimental techniques, including position emission tomography, functional magnetic resonance imaging (fMRI), and evoked potentials, have been employed to investigate the anatomic basis of reading in normal subjects. Although differences in experimental technique and design inevitably lead to variability in reported sites of activation, there appears to be at least relative agreement regarding the anatomic basis of several components of the reading system.

As previously noted, most accounts of reading postulate that after initial visual processing, familiar words are recognized by comparison to a catalog of stored representations that is often termed the visual word form system. A variety of recent investigations involving visual lexical decision with fMRI, viewing of letter, and direct recording of cortical electrical activity suggests that the visual word form system is supported by inferior occipital or inferior temporooccipital cortex.

Additional evidence for this localization comes from a recent investigation by Cohen et al. of five normal subjects and two patients with posterior callosal lesions. These investigators presented words and non-words for lexical decision or oral reading to either the right or the left visual fields. They found initial unilateral activation in what was thought to be V4 in the hemisphere to which the stimulus was projected. More importantly, however, for normal subjects activation in the left fusiform gyrus (Talairach coordinates —42, —57, and —6) that was independent of the hemisphere to which the stimulus was presented was observed. The two patients with posterior callosal lesions were impaired in the processing of letter strings presented to the right compared to the left hemisphere;

fMRI in these subjects demonstrated that the region of the fusiform gyrus described previously was activated in the callosal patients only by left hemisphere stimulus presentation. As noted by the investigators, these findings are consistent with the hypothesis that the hemialexia demonstrated by the callosal patients is attributable to a failure to access the visual word form system in the left fusiform gyrus.

Deriving meaning from visually presented words requires access to stored knowledge or semantics. Although the architecture and anatomic bases of semantic knowledge remain controversial, investigations involving semantic access for written words implicate cortex at the junction of the superior and middle temporal gyrus (Brodman areas 21,22, and 37).

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Breaking Bulimia

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