Fifty years ago, Mark Rosenzweig and coworkers described environmental effects on brain chemistry and gross brain weight. William Greenough then used stereological tools, electron microscopy, and the Golgi stain to demonstrate that enrichment led to dendritic growth and synapse addition. Together these forms of plasticity accounted for cortical expansion and a reduction in cell density. In parallel with other investigators, Greenough demonstrated that these effects were not limited to the rodent, the cortex, or development, but instead generalize to many species, brain regions, and life stages. Studies of the anatomical effects of enrichment foreshadowed the recent empirical evidence for cortical volumetric increases after environmental experience and training in humans. Since research in humans is limited to regional effects, the analysis of the cellular and synaptic effects of enrichment, and their contribution to volumetric increases can inform us of the potential cellular and subcellular plasticity the leads to volume change in humans.
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