Aging and recognition memory: Changes in regional cerebral blood flow associated with components of reaction time distributions

We used (H2O)-O-15 positron emission tomography (PET) to measure age-relate d changes in regional cerebral blood now (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 Encodin g, Baseline, and Retrieval conditions. Each of the conditions involved view ing a series of 64 words and making a two-choice response manually The comp lete reaction time (RT) distributions in each task condition were character ized in terms of an ex-Gaussian model (convolution of exponential and Gauss ian functions). Parameter estimates were obtained for the mean of the expon ential component (tau), representing a task-specific decision process and t he mean of the Gaussian component (mu), representing residual sensory codin g and response processes. Independently of age group, both mu and tau were higher in the Encoding and Retrieval conditions than in the Baseline condit ion, and tau was higher during Retrieval than during Encoding. Age-related slowing in task performance was evident primarily in mu. For young adults, rCBF activation in the right prefrontal cortex, in the Retrieval condition, was correlated positively with mu but not with tau. For older adults, rCBF changes (both increases and decreases) in several cortical regions were co rrelated with both mu and tau. The data suggest that the attentional demand s of this task are relatively greater for older adults and consequently lea d to the recruitment of additional neural systems during task performance.