We used H215O positron emission tomography (PET) to measure age-related changes in regional cerebral blood flow (rCBF) during a verbal recognition memory task. Twelve young adults (20 to 29 years) and 12 older adults (62 to 79 years) participated. Separate PET scans were conducted during Encoding, Baseline, and Retrieval conditions. Each of the conditions involved viewing a series of 64 words and making a two-choice response manually. The complete reaction time (RT) distributions in each task condition were characterized in terms of an ex-Gaussian model (convolution of exponential and Gaussian functions). Parameter estimates were obtained for the mean of the exponential component (τ), representing a task-specific decision process and the mean of the Gaussian component (μ), representing residual sensory coding and response processes. Independently of age group, both μ and τ were higher in the Encoding and Retrieval conditions than in the Baseline condition, and τ was higher during Retrieval than during Encoding. Age-related slowing in task performance was evident primarily in μ. For young adults, rCBF activation in the right prefrontal cortex, in the Retrieval condition, was correlated positively with μ but not with τ. For older adults, rCBF changes (both increases and decreases) in several cortical regions were correlated with both μ and τ. The data suggest that the attentional demands of this task are relatively greater for older adults and consequently lead to the recruitment of additional neural systems during task performance.
