FIGURE 3.8 Experience affects brain development In this experiment, some rats lived alone in an environment without playthings. Others lived together in an environment enriched with playthings that changed daily. In 14 of 16 repetitions of the experiment, rats in the enriched setting developed more cerebral cortex (relative to the rest of the brain’s tissue) than was found in those raised in the impoverished environment (Renner & Rosenzweig, 1987; Rosenzweig, 1984).