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Six experiments traced the development of long-term memory of an escape response in the rat. The first showed 9 days to be the age at which rats first show an intersession improvement when sessions occur at 24-hr, intervals. The second showed the improvement to be a residual effect of training, and the third, that rats younger than 9 days display a shorter term retention. Experiments 4-6 provided a basis for referring to the 24-hr, retention as long-term memory. Collectively, they indicated that hypothermia selectively impairs 24-hr, retention, and that the degree of impairment is related to time between training and hypothermal treatment, findings which in research on adults are attributed to disruption of longterm memory processes.

The development of memory of an escape response in Swiss-Webster mice between 3-11 days of age was examined in four experiments. Mice were given 25 training trials in a straight-alley escape task and then retested at various retention intervals. Collectively, the results showed that 5-and 7day-old mice possess a retention capacity of less than 6 hr. At 9 days of age, however, retention capacity greatly increases to at least 96 hr., suggesting that a period of maturational development critical to long-term memory processes occurs at 9 days of age in the neonatal mouse.

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