Prefrontal alterations during memory processing in aging.

Studies of human amnesia provide evidence for a short-term memory store with information transfer to long-term memory occurring within 60 sec of encoding. Frontal cortical activation is critical for maintenance of the short-term store, and limbic structure are necessary for access to the long-term store. The P3 and N4 components of the event-related potential (ERP) are generated during memory processes mediated by these brain regions. The current study examines the effects of age on ERPs generated to correctly identified stimuli presented at delays of 1-150 sec in a visual recognition memory task. Consistently different evoked potentials and performance were obtained to stimuli repeated at 1.2 sec delay as opposed to stimuli repeated at delays of over 4 sec in all subjects. At the 1.2 sec delay, the performance and posterior P3 amplitudes generated by old and young subjects were comparable. At longer delays, the older subjects had impaired performance and decreased P3 amplitude at posterior scalp sites. In addition, fronto-central N4 activity was reduced at long delays in the elderly. Older subjects generated a sustained frontal positivity (50-800 msec) to both short and long delay stimuli that was not observed in the younger group. The late phase of the frontal positivity was enhanced at long delays in the elderly. The data provide evidence of intact rapid and impaired delayed recognition memory in aging. Alternations in frontal cortical control of posterior and limbic regions may contribute to the memory changes observed in aging.