Adult age differences are frequently observed in the performance of memory
tasks, but the changes in neural function mediating these differences are l
argely unknown. We used (H2O)-O-15 positron emission tomography (PET) to me
asure changes in regional cerebral blood flow (rCBF) during Encoding, Basel
ine, and Retrieval conditions of a recognition memory task. Twelve young ad
ults (20-29 years) and 12 older adults (62-79 years) participated. During e
ach task condition, participants made a two-choice manual response to each
of 64 words. Analyses of the performance data yielded evidence of age-relat
ed slowing of encoding and retrieval processes, and an age-related decline
in the accuracy of yes/no recognition (d'). The rCBF activation associated
with both encoding and retrieval was greater for older adults than for youn
g adults, but this pattern was more clearly evident for memory retrieval. F
or young adults, rCBF activation during retrieval occurred primarily in rig
ht prefrontal cortex, whereas older adults exhibited a more bilateral patte
rn of prefrontal activation. Regression analyses predicting reaction time i
n the memory task from regional PET counts confirmed that the neural system
mediating memory retrieval is more widely distributed for older adults tha
n for young adults. Both age groups exhibited some decrease in rCBF activat
ion in the second half of the test session, relative to the first half. The
practice-related decrease in rCBF activation was more prominent for young
adults, suggesting that the older adults' recruitment of additional neural
systems reflects a more continual allocation of attention to support task p
erformance. (C) 1999 Wiley-Liss, Inc.