Conditional learning in aged rats: evidence of hippocampal and prefrontal cortex impairment

Groups of normal old rats and young adult rats were administered a test of conditional discrimination learning in which different visual stimuli were associated with responses to different levers. Initially, rats were tested in a zero-delay condition in which they selected their responses in the presence of the conditional stimuli. They were later tested at 5- and 15-s delays between stimulus presentation and the appearance of the levers. Old rats were impaired in learning the basic conditional discrimination, a test thought to be sensitive to frontal lobe dysfunction. Age differences increased with the length of the interval, revealing a time-dependent memory loss that was attributed to impaired hippocampal function.