FIGURE 4.6 Experience affects brain development Mark Rosenzweig, David Krech, and their colleagues (1962) raised rats either alone in an environment without playthings, or with other rats in an environment enriched with playthings changed daily. In 14 of 16 repetitions of this basic experiment, rats in the enriched environment developed significantly more cerebral cortex (relative to the rest of the brain’s tissue) than did those in the impoverished environment.