Classical conditioning in 3-, 30-, and 45-month-old rabbits: behavioral learning and hippocampal unit activity.

Evidence has accumulated to suggest that the hippocampus is impaired by aging processes. Hippocampal multiple unit activity was examined in 3-, 30-, and 45-month-old rabbits during classical conditioning in the trace paradigm. Animals were trained daily with 126 paired trials of tone and airpuff. The 3-month-olds learned to a criterion of 8/9 CR's in a mean of 3.2 days, while the 30-month-olds took a mean of 9.4 days, and the 45-month-olds took a mean of 11.75 days. Neural unit activity in the hippocampus modeled the behavioral eyelid response but took much longer to develop in the older animals. Results suggest that rabbits as young as 2 1/2 years old show behavioral and correlated neural response deficits.