ADULT AGE-DIFFERENCES IN REGIONAL CEREBRAL BLOOD-FLOW DURING VISUAL WORD IDENTIFICATION - EVIDENCE FROM (H2O)-O-15 PET

We used (H2O)-O-15 PET to investigate adult age differences in regiona l cerebral blood flow (rCBF) during the performance of a visual word i dentification task. The study participants were 20 healthy, right-hand ed men: 10 young adults between 18 and 27 years of age, and 10 older a dults between 63 and 75 years of age. The word identification task com prised six blocks of test trials representing four task conditions; su bjects responded manually. The task conditions varied with regard to w hether semantic retrieval was required (e.g., word/nonword discriminat ion vs simple response to each stimulus) and with regard to the diffic ulty of visual encoding (e.g., words presented normally vs words with asterisks inserted between adjacent letters). Each subject performed a ll six trial blocks, concurrently with each of six (H2O)-O-15 PET scan s. Analyses of quantitative CBE data obtained from the arterial time-a ctivity curve demonstrated a significant age-related decline in global CBF rate. Analyses of the changes in rCBF between task conditions ind icated that retrieval of semantic information sufficient to distinguis h words from nonwords is mediated by a ventral occipitotemporal cortic al pathway. Specific areas within this pathway were also associated wi th visual encoding processes. Several rCBF activations were significan tly greater for young adults than for older adults, indicating an age- related decline in processing efficiency within this ventral occipitot emporal pathway. Although the performance data demonstrated a greater age-related slowing for visual encoding than for semantic retrieval, t hese age-related performance changes were not associated with correspo nding changes in rCBF activation. (C) 1996 Academic Press, Inc.