Behaviorally induced changes in length of cerebrum in rats

In normal rats, growth in the width of the cerebral hemispheres ceases about 21 days of age, but growth in length continues at a reduced, steady rate in the young‐adult. Three behavioral treatments were used to determine whether this late phase in cerebral growth is sensitive to environmental influences. Rats that were trained on a complex visual discrimination task for 4 months had significantly longer cerebral hemispheres than their passive littermates. Rats that were raised in enriched or social environments had longer cerebral hemispheres than rats raised in isolation. Finally, the cerebrum of unhandled rats was shorter than that of rats handled during infancy; that of the latter was shorter than the cerebrum of trained or enriched animals. No changes were observed under any of these conditions in cerebral width. These results indicate that the length of the cerebrum characteristic of normal rats does not represent a genetically determined, fixed level. This level is not attained under certain rearing conditions and it can be surpassed when the complexity of the environment is increased. The morphological bases of these changes were not determined.