Hippocampal Age Differences Reoccur after Modification of Stimulus Configurations in Rabbit Jaw Movement Conditioning

Rabbits of two age groups, young (3–7 months; n = 7) and aging (40–49 months; n = 7), were implanted with chronic recording electrodes in area CA1 of dorsal hippocampus and trained in a delay conditioned jaw movement (CJM) paradigm (after previously reaching criterion in a trace CJM paradigm with the same conditioned stimulus and unconditioned stimulus). Unlike results from the trace experiment, there was no significant difference in trials to behavioral criterion between the two age groups. There were large, but temporary, differences in the magnitude of conditioning-related hippocampal neural responses early in delay training. Aging rabbits’ hippocampal responses were significantly smaller on Day 2 of delay training than corresponding responses of young rabbits (p = 0.0008). It is important to note that neural differences were not observed on Days 1 and 3 of delay training or at criterion performance in the prior trace conditioning. These results are interpreted in terms of age-related differences in hippocampal responsiveness to changes in biologically significant stimulus configurations.