Typically, scientists study the effects of two contrasting environments. Some rats are raised alone in small cages with only food and water available. Other rats are raised in large, group cages that also contain novel objects and obstacles. Greenough calls these environments complex, rather than enriched. He points out that complex environments are enriched only in comparison with a rat's typical lab cage.
Neuroscientists use complex environments to mimic the rats' wild or natural environment.
They are not special, accelerated rodent learning environments. One should not think of them as high-quality infant care or Head Start for rats. In electron microscopic studies, Greenough and his colleagues found that young rats raised in complex environments have 25 percent more synapses per neuron in the visual areas of their brains than do rats raised in isolation. However, increases in synapses per neuron ratios do not occur to this extent in all brain areas, and some brain areas show no effects of complex rearing at all. Other studies in monkeys and humans have definitively established that the adult brain remains highly plastic and capable of extensive neural reorganization throughout life.