Environmental enrichment

Environmental enrichment is the stimulation of the brain by its physical and social surroundings. Brains in richer, more stimulating environments have higher rates of synaptogenesis and more complex dendrite arbors, leading to increased brain activity. This effect takes place primarily during neurodevelopment, but also during adulthood to a lesser degree. With extra synapses there is also increased synapse activity, leading to an increased size and number of glial energy-support cells. Environmental enrichment also enhances capillary vasculation, providing the neurons and glial cells with extra energy. The neuropil (neurons, glial cells, capillaries, combined) expands, thickening the cortex. Research on rodent brains suggests that environmental enrichment may also lead to an increased rate of neurogenesis.

Research on animals finds that environmental enrichment could aid the treatment and recovery of numerous brain-related dysfunctions, including Alzheimer's disease and those connected to aging, whereas a lack of stimulation might impair cognitive development. Moreover, this research also suggests that environmental enrichment leads to a greater level of cognitive reserve, the brain's resilience to the effects of conditions such as aging and dementia.

Research on humans suggests that lack of stimulation delays and impairs cognitive development. Research also finds that attaining and engaging in higher levels of education, environments in which people participate in more challenging cognitively stimulating activities, results in greater cognitive reserve.

Donald O. Hebb in 1947 found that rats raised as pets performed better on problem solving tests than rats raised in cages. His research, however, did not investigate the brain nor use standardized impoverished and enriched environments. Research doing this first was started in 1960 at the University of California, Berkeley by Mark Rosenzweig, who compared single rats in normal cages, and those placed in ones with toys, ladders, tunnels, running wheels in groups. This found that growing up in enriched environments affected enzyme cholinesterase activity. This work led in 1962 to the discovery that environmental enrichment increased cerebral cortex volume. In 1964, it was found that this was due to increased cerebral cortex thickness and greater synapse and glial numbers.

Also starting around 1960, Harry Harlow studied the effects of maternal and social deprivation on rhesus monkey infants (a form of environmental stimulus deprivation). This established the importance of social stimulation for normal cognitive and emotional development.

Rats raised with environmental enrichment have thicker cerebral cortices (3.3–7%) that contain 25% more synapses. This effect of environmental richness upon the brain occurs whether it is experienced immediately following birth, after weaning, or during maturity. When synapse numbers increase in adults, they can remain high in number even when the adults are returned to impoverished environment for 30 days suggesting that such increases in synapse numbers are not necessarily temporary. However, the increase in synapse numbers has been observed generally to reduce with maturation. Stimulation affects not only synapses upon pyramidal neurons (the main projecting neurons in the cerebral cortex) but also stellate ones (that are usually interneurons). It can also affect neurons outside the brain, such as those in the retina.

Environmental enrichment affects the complexity and length of the dendrite arbors (upon which synapses form). Higher-order dendrite branch complexity is increased in enriched environments, as can the length, in young animals, of distal branches. Environmental enrichment rescues harmful effects of stress on dendritic complexity.

Animals in enriched environments show evidence of increased synapse activation. Synapses tend to also be much larger. Gamma oscillations become larger in amplitude in the hippocampus. This increased energy consumption is reflected in glial and local capillary vasculation that provides synapses with extra energy.