Some Effects of Severe Hypothermia on Learning and Retention

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Abstract
Two experiments have been conducted to assess the effects of extreme hypothermia on learning and retention. In the first experiment two groups of rats were subjected to identical treatments except that the deep body temperature of one group was lowered to between + I° and o° C. before both groups began learning a serial problem solving task. The second experiment included three groups of animals: a control and two experimental groups, the deep body temperature of animals in one of the latter being reduced to between + 13–4° and + 18–5° C. and that of the other to between + I° and o° C. Except for the “hypothermia” phase of the design all groups in the second experiment received identical treatments in the following order: Training on a maze, hypothermia, tests for retention of the maze habit, training on the serial-problem-solving task, tests for speed of locomotion. Animals whose body temperature had been lowered to between + I° and O°C. showed a significant impairment in learning performance when compared with control animals. On the other hand, no significant differences were found between the performances of control animals and those cooled to between + 13.4° and + 18.5°C. The results also suggest that the effects of severe hypothermia may be temporary, since the percentage of impairment decreased as the interval between body cooling and learning increased. The differences in retention of the maze habit among experimental and control groups were very small and in no instance were they statistically significant, although there was a consistent trend towards poorer retention following hypothermia. These small differences may be functions of the technique used to reduce deep body temperature rather than of the effects of hypothermia per se. These results are based upon observations of the behaviour of non-hibernating, homoiothermic animals. With such animals, extreme hypothermia, such as that employed in the present study, results in complete arrest of heart beat, circulation, and respiration. It also suppresses electrical activity in the brain and in our animals cerebral activity may have been arrested for as long as 13 to z hours. Within the limits of our experimental procedure we have failed to find evidence that arrest of these vital metabolic processes as a result of hypothermia produces any very serious, permanent effects on the animal's behaviour once it has been successfully reanimated. If, as previous writers have suggested, severe hypothermia can be used to “… stop all nerve impulses in the brain momentarily …” (Gerard, 1953), our results are difficult to explain if long-term memory is dependent upon the continuous activity of the brain.

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