Neuroanatomic Substances of Semantic Memory Impairment in Alzheimer's Disease: Patterns of Functional MRI Activation

To investigate the neuroanatomic basis of impairment in semantic processing & the differential impact on subtypes of semantic relations, patients with mild Alzheimer's disease & healthy controls underwent three functional MRI auditory stimulation tasks requiring semantic or phonological decisions (match-mismatch) about word pairs (category-exemplar, category-function, pseudoword). Patients showed a significant performance deficit only on the exemplar task. On vowel-based fMRI activation analyses, controls showed a clear activation focus in the left superior temporal gyrus for the phonological task; patients showed additional foci in the left dorsolateral prefrontal & bilateral cingulate areas. On the semantic tasks, predominant activation foci were seen in the inferior & middle frontal gyrus (left greater than right) in both groups but patients showed additional activation, suggesting compensatory recruitment of locally expanded foci & remote regions, for example, right frontal activation during the exemplar task. Covariance analyses indicated that exemplar task performance was strongly related to signal increase in bilateral medial prefrontal cortex. It is concluded that fMRI can reveal similarities & differences in functional neuroanatomical processing of semantic & phonological information in mild Alzheimer's disease compared to healthy elderly, & can help to bridge cognitive & neural investigations of the integrity of semantic networks in Alzheimer's disease. 2 Tables, 6 Figures, 83 References. Adapted from the source document