Abstract: Delayed conditioned responses were performed in four dogs in a maze.The effect of ionizing radiation of thorium on learning and memory wasobserved. The results showed that after 62 doses (10mg/kg. a dose, delivered oncea day) of chronic injections of thorium nitrate intravenously, theaccumulated dosage was 8.68g on the average, which exceedes themaximum permissible dose by 350 times, no obvious deterioration of thedelayed conditioned response was observed. This means that the immediatememory in dogs is rather invulnerable to ionizing radiation of thorium.

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