Neural activation during semantic processing in older adults

Neural activation on fMRI during semantic processing was examined in 38 older adults, 55–85 years of age, experiencing successfully cognitive aging, without mild cognitive impairment (MCI) or evidence of neurodegenerative disease. A mixed event-related block paradigm on which judgements were made as to whether semantic (abstract and concrete) and phonemic (rhyme) word-pairs were associated or unassociated during task-dependent fMRI. As hypothesized, greater activation intensity occurred during the processing of associated vs. unassociated, and to semantic vs. phonemic word-pairs in putative semantic hubs-control areas (angular gyrus, anterior middle temporal gyrus, and inferior frontal gyrus), and in secondary association areas. Several areas activated more to unassociated word-pairs, notably the hippocampus and thalamus. Cortical area exhibited similar activation intensity between semantic conditions, though several areas activated more during either abstract or concrete processing. Activation intensity of the left inferior frontal gyrus was greater during abstract processing, but not in the left angular or anterior temporal gyri. Greater age was associated with relatively few differences in cortical activation, though greater right homologous activation occurred in the angular gyrus, supramarginal gyrus, and the precuneus during the processing of strongly vs. weakly associated concrete word-pairs. Activation intensity did not differ across semantic conditions with greater age otherwise, suggesting that neural response of the semantic hubs, control and secondary cortical association areas is relatively preserved in older adults without MCI or neurodegenerative disease. Age-associated differences in neural activation are likely task-dependent, evident when there is greater demand for semantic control and generation among older adults.