Two different properties of short- and long-term memory.

Rats were trained in one-trial step-down inhibitory avoidance and tested 1.5-9 h later or 1-4 days later in order to evaluate short- and long-term memory respectively. No extinction was observed when the animals were tested repeatedly at 1.5, 3, 4.5 and 6 h; when they were tested repeatedly at 9, 24, 48, 72 and 96 h, there was partial extinction. Exposure to an open field for 2 min, 1 h after training did not affect retention scores of animals tested at 1.5 or 3 h, had a slight amnestic effect on groups tested at 4.5 or 6 h, and markedly reduced retention scores in animals tested at 9 or 24 h from training. Thus, there were two clear behavioural differences between memory measured in the first few hours or in the following few days, which suggests that both involve different mechanisms.