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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.
Early research
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 environents affected enzyme cholinesterase activity.[2] This work led in 1962 to the discovery that environmental enrichment increased cerebral cortex volume.[3] In 1964, it was found that this was due to increased cerebral cortex thickness and greater synapse and glial numbers.[4][5]
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.[6]
Synaptogenesis
Rats raised with environmental enrichment have thicker cerebral cortices (3.3-7%) that contain 25% more synapses.[5][7] This effect of environmental richness upon the brain occurs whether it is experienced immediately following birth,[8] after weaning,[5][7][9] or during maturity.[10] When synapse numbers increase in adults, they can remain high in number even when the adults are returned to improvised environment for 30 days[10] 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.[11][12] Stimulation affects not only synapses upon pyramidal neurons (the main projecting neurons in the cerebral cortex) but also stellate ones (that are usually interneurons).[13] It also can affect neurons outside the brain in the retina.[14]
Dendrite complexity
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,[13][15] as can the length, in young animals, of distal branches.[16]
Activity and energy consumption
Synapses in animals in enriched environments show evidence of increased synapse activation.[17] Synapses tend to also be much larger.[18] This increased energy consumption is reflected in glial and local capillary vasculation that provides synapses with extra energy.
- Glial cell numbers per neuron increase 12-14%[5][7]
- The direct apposition area of glial cells with synapses expands by 19%[19]
- The volume of glial cell nuclei for each synapse is higher by 37.5%[17]
- The mean volume of mitochondria per neuron is 20% greater[17]
- The volume of glial cell nuclei for each neuron is 63% higher[17]
- Capillary density is increased.[20]
- Capillaries are wider (4.35 μm compared to 4.15 μm in controls)[17]
- Shorter distance exist between any part of the neuropil and a capillary (27.6 μm compared to 34.6 μm)[17]
Motor learning stimulation
Part of the effect of environmental enrichment is providing opportunities to acquire motor skills. Research upon “acrobatic” skill learning in the rat shows that it leads to increased synapse numbers.[21][22]
Maternal transmission
Environmental enrichment during pregnancy has effects upon the fetus such as accelerating its retinal development.[23]Neurogenesis
Environmental enrichment can also lead to the formation of neurons (at least in rats)[24] and reverses the loss of neurons in the hippocampus and memory impairment following chronic stress.[25] However, its relevance has been questioned for the behavioral effects of enriched environments.[26]Mechanisms
Enriched environments affect the expression of genes in the cerebral cortex and the hippocampus that determine neuronal structure.[27] At the molecular level, this occurs through increased concentrations of the neurotrophins NGF, NT-3,[28][29] and changes in BDNF.[14][30] This alters the activation of cholinergic neurons,[29] 5-HT,[31] and beta-adrenolin.[32] Another effect is to increase proteins such as synaptophysin and PSD-95 in synapses.[33] Changes in Wnt signaling have also been found to mimic in adult mice the effects of environmental enrichment upon synapses in the hippocampus.[34] Increase in neurons numbers could be linked to changes in VEGF.[35]Resilience and rehabilitation
Research (as least upon rats) suggests that environment enrichment might reduce the effects or ameliorate the cognitive impairments caused by a diverse variety of conditions and neurological disorders.- Aging,[36] (also in dogs[37])
- Alzheimer’s disease[38]
- Huntington's disease[39]
- Parkinson's disease[40]
- Stroke[41]
- Chronic spinal cord injuries[42]
- Amblyopia[43]
- Rett syndrome[44]
- Autism[45]
- Prenatal and perinatal cocaine exposure[46][47][48]
- Fetal alcohol syndrome[49][50]
- Lead exposure[51][52]
- Prenatal and maternal separation stress[53][54][55]
- Child neglect[56]
- Sensorial deprivation[57]
Humans
Though environmental enrichment research has been mostly done upon rodents, similar effects occur in primates,[58] and are likely to affect the human brain. However, direct research upon human synapses and their numbers is limited since this requires histological study of the brain. A link, however, has been found between educational level and greater dendritic branch complexity following autopsy removal of the brain.[59]
Localized cerebral cortex changes
MRI detects localized cerebral cortex expansion after people learn complex tasks such as mirror reading (in this case in the right occipital cortex),[60] three-ball juggling (bilateral mid-temporal area and left posterior intraparietal sulcus),[61] and when medical students intensively revise for exams (bilaterally in the posterior and lateral parietal cortex).[62] Such changes in gray matter volume can be expected to link to changes in synapse numbers due to the increased numbers of glial cells and the expanded capillary vascularization needed to support their increased energy consumption.Institutional deprivation
Children that receive impoverished stimulation due to being confined to cots without social interaction or reliable caretakers in low quality orphanages show severe delays in cognitive and social development.[63] 12% of them if adopted after 6 months of age show autistic or mildly autistic traits later at four years of age.[64] Some children in such impoverished orphanages at two and half years of age still fail to produce intelligible words, though a year of foster care enabled such children to catch up in their language in most respects.[65] Catch-up in other cognitive functioning also occurs after adoption, though problems continue in many children if this happens after the age of 6 months[66]Such children show marked differences in their brains, consistent with research upon experiment animals, compared to children from normally stimulating environments. They have reduced brain activity in the orbital prefrontal cortex, amygdala, hippocampus, temporal cortex, and brain stem.[67] They also showed less developed white matter connections between different areas in their cerebral cortices, particularly the uncinate fasciculus.[68]
Conversely, enriching the experience of preterm infants with massage quickens the maturating of their electroencephalographic activity and their visual acuity. Moreover, as with enrichment in experimental animals, this associates with an increase in IGF-1.[69]
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