Patterned (single) alternation in infant rats after combined or separate lesions of hippocampus and amygdala.

The role of the developing hippocampus and the amygdala on patterned (single) alternation (PA) in the infant rat was investigated in 4 experiments. In Experiments 1 and 2, pups were given 2 bilateral electrolytic hippocampal lesions or sham surgeries at 10 or 11 days of age and were trained 6 days later in a straight runway. In Experiment 1, there were 120 trials in 1 day, with an 8-, a 15-, or a 30-s intertrial interval (ITI). PA learning occurred in lesion and sham pups at the 8- and 15-s ITIs, but it was reduced in both groups at the 30-s ITI. In Experiment 2, training was extended to 240 trials over 2 days, with a 30- or 60-s ITI. Sham and lesion pups showed PA at the 30-s ITI, but the emergence of PA was delayed in the lesion pups at the 60-s ITI. In Experiment 3, amygdaloid lesions had no effect on PA learning at the 8-s ITI. However, when pups with hippocampal and amygdaloid lesions were trained at the 8-s ITI, the emergence of PA was delayed, and its size was reduced (Experiment 4). The results of these experiments argue for a role of the hippocampus in PA learning at long ITIs and suggest that, even in 16-day-old pups exposed to an 8-s ITI, the combined hippocampal and amygdaloid lesion produces a deficit greater than either the hippocampal or the amygdaloid lesion. The results are discussed in relation to current theories that distinguish between 2 levels of memory function.