During the 20th century, the consensus was that lower brain and neocortical areas were immutable in structure after childhood, meaning learning only happens by changing of connection strength, whereas areas related to memory formation, such as the hippocampus and dentate gyrus, where new neurons continue to be produced into adulthood, were highly plastic. This belief is being challenged by new findings, suggesting all areas of the brain are plastic even after childhood.
Environmental enrichment concerns how the brain is affected by the stimulation of its information processing provided by its surroundings (including the opportunity to interact socially). Brains in richer, more stimulating environments, have increased numbers of synapses, and the dendrite arbors upon which they reside are more complex. This effect happens particularly during neurodevelopment, but also to a lesser degree in adulthood. With extra synapses there is also increased synapse activity and so increased size and number of glial energy support cells. Capillary vasculation also is greater to provide the neurons and glial cells with extra energy. The neuropil (neurons, glial cells, capillaries, combined together) expands making the cortex thicker. There may also exist (at least in rodents) more neurons.
Research in nonhuman animals finds that more stimulating environment could aid the treatment and recovery of a diverse variety of brain related dysfunctions, including Alzheimer’s disease and those connected to aging, whereas a lack of stimulation might impair cognitive development.
Research upon humans suggests that lack of stimulation (deprivation—such as in old-style orphanages) delays and impairs cognitive development. Research also finds that higher levels of education (which is both cognitively stimulating in itself, and associates with people engaging in more challenging cognitive activities) results in greater resilience (cognitive reserve) to the effects of aging and dementia.