Age-related changes in word retrieval: Frontal-executive vs. temporal-semantic substrates

People over the age of 65 frequently complain of word-finding difficulties. The current study tests two competing hypotheses to determine whether word retrieval problems in older adults result from changes in neural substrates in the frontal lobe subserving strategic processes to access lexical and semantic information or whether neural substrates of semantic information in the inferior temporal lobe change in function with normal aging. We investigate the role of category (living, nonliving) and visual attribute (global form, local details) in semantic representation in the fusiform gyrus. Forty adults (20 younger, 20 older) named pictures of animals, tools, and vehicles during FMRI. Results show that compared to younger adults, older adults activate a larger frontal network with decreased lateralization during word retrieval. In particular, older adults show greater activity in Broca's area homologue (BA 45) in the right hemisphere, an anterior region of the right inferior frontal gyrus, and the rostral cingulate zone and supplementary motor area bilaterally. The lack of an age-associated difference in the inferior temporal cortex indicates that older and younger adults do not differ in terms of how the presumed substrates for semantic functions operate. Results also support the role of visual attribute in the organization of semantic information. Collapsed across subjects, findings indicate that categories are processed in the lateral and medial regions of the fusiform gyrus according to whether they are living (animals) or nonliving (tools, vehicles), respectively. In contrast, visual attributes of global form (animals) are processed more by the right fusiform gyrus and local details (tools) are processed more by the left fusiform gyrus. When both attributes are relevant to processing (vehicles), cortex from both left and right fusiform gyri is active. The left-right distinction for visual features is not restricted to the fusiform gyrus but includes the lateral temporal cortex as well. Taken together, these findings suggest that a deterioration of executive functions underlies age-related changes in word retrieval. Additionally, results support the role of features (visual attribute) and category in semantic representation.