Impaired memory registration and speed of reasoning caused by low body temperature.

Volunteers' body core temperatures were lowered by immersion in water at 15 degrees C. Aspects of cognitive function were subsequently tested after rewarming had been started in water at 41 degrees C when their skin was warm and they felt comfortable but their body core temperature remained low. Memory registration was found to be impaired progressively when core temperature fell from about 36.7 degrees C; at core temperatures of 34-35 degrees C the impairment caused loss of approximately 70% of data that could normally be retained. However, recall of previously learned data was not impaired at these core temperatures. On a two-digit calculation test, speed of performance was impaired by about 50% at a core temperature of 34-35 degrees C, but provided enough time was available, accuracy of performance was not reduced.