AGE-RELATED DIFFERENCES IN NEURAL ACTIVITY DURING MEMORY ENCODING ANDRETRIEVAL - A POSITRON EMISSION TOMOGRAPHY STUDY

Positron emission tomography (PET) was used to compare regional cerebr al blood flow (rCBF) in young (mean 26 years) and old (mean 70 years) subjects while they were encoding, recognizing, and recalling word pai rs. A multivariate partial-least-squares (PLS) analysis of the data wa s used to identify age-related neural changes associated with (1) enco ding versus retrieval and (2) recognition versus recall. Young subject s showed higher activation than old subjects (1) in left prefrontal an d occipito-temporal regions during encoding and (2) in right prefronta l and parietal regions during retrieval. Old subjects showed relativel y higher activation than young subjects in several regions, including insular regions during encoding, cuneus/precuneus regions during recog nition, and left prefrontal regions during recall. Frontal activity in young subjects was left-lateralized during encoding and right-lateral ized during recall [hemispheric encoding/retrieval asymmetry (HERA)], whereas old adults showed little frontal activity during encoding and a more bilateral pattern of frontal activation during retrieval. In yo ung subjects, activation in recall was higher than that in recognition in cerebellar and cingulate regions, whereas recognition showed highe r activity in right temporal and parietal regions. In old subjects, th e differences in blood flow between recall and recognition were smalle r in these regions, yet more pronounced in other regions. Taken togeth er, the results indicate that advanced age is associated with neural c hanges in the brain systems underlying encoding, recognition, and reca ll. These changes take two forms: (1) age-related decreases in local r egional activity, which may signal less efficient processing by the ol d, and (2) age-related increases in activity, which may signal functio nal compensation.