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.
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 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.
Synapses
Synaptogenesis
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.
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, as can the length, in young animals, of distal branches. Environmental enrichment rescues harmful effects of stress on dendritic complexity.
Activity and energy consumption 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. Glial cell numbers per neuron increase 12–14% The direct apposition area of glial cells with synapses expands by 19% The volume of glial cell nuclei for each synapse is higher by 37.5% The mean volume of mitochondria per neuron is 20% greater The volume of glial cell nuclei for each neuron is 63% higher Capillary density is increased. Capillaries are wider (4.35 μm compared to 4.15 μm in controls) Shorter distance exist between any part of the neuropil and a capillary (27.6 μm compared to 34.6 μm) These energy related changes to the neuropil are responsible for increasing the volume of the cerebral cortex (the increase in synapse numbers contributes in itself hardly any extra volume).
Motor learning stimulation Part of the effect of environmental enrichment is providing opportunities to acquire motor skills. Research on rats learning an "acrobatic" skill shows that such learning activity leads to increased synapse count.
Maternal transmission Environmental enrichment during pregnancy has effects upon the fetus, such as accelerating his or her retinal development.
Neurogenesis Environmental enrichment can also lead to the formation of neurons (at least in rats) and reverse both the loss of neurons in the hippocampus and memory impairment from chronic stress. However, its relevance has been questioned for the behavioral effects of enriched environments.
Mechanisms
Enriched environments affect the expression of genes that determine neuronal structure in the cerebral cortex and hippocampus. At the molecular level, this occurs through increased concentrations of the neurotrophins NGF, NT-3, and changes in BDNF. This alters the activation of cholinergic neurons, 5-HT, and beta-adrenolin. Another effect is to increase proteins such as synaptophysin and PSD-95 in synapses. Changes in Wnt signaling have also been found to mimic in adult mice the effects of environmental enrichment upon synapses in the hippocampus. Increase in neurons numbers could be linked to changes in VEGF.
Rehabilitation and resilience
Research in animals suggests that environmental enrichment aids recovery from certain neurological disorders and cognitive impairments. There are two mains areas of focus: neurological rehabilitation and cognitive reserve, the brain's resistance to the effects of exposure to physical, natural, and social threats. Although most of these experiments used animal subjects, mainly rodents, researchers have pointed to the affected areas of animal brains to which human brains are most similar and used their findings as evidence to show that humans would have comparable reactions to enriched environments. The tests done on animals are thus meant to represent human simulations for the following list of conditions.
Neurological rehabilitation
Autism
A study conducted in 2011 led to the conclusion that environmental enrichment vastly improves the cognitive ability of children with autism. The study found that autistic children who receive olfactory and tactile stimulation along with exercises that stimulated other paired sensory modalities clinically improved by 42 percent while autistic children not receiving this treatment clinically improved by just 7 percent. The same study also showed that there was significant clinical improvement in autistic children exposed to enriched sensorimotor environments, and a vast majority of parents reported that their child's quality of life was much better with the treatment. A second study confirmed its effectiveness. The second study also found after 6 months of sensory enrichment therapy, 21% of the children who initially had been given an autism classification, using the Autism Diagnostic Observation Schedule, improved to the point that, although they remained on the autism spectrum, they no longer met the criteria for classic autism. None of the standard care controls reached an equivalent level of improvement. The therapy using the methodologies is titled Sensory Enrichment Therapy.
Alzheimer's disease
Through environmental enrichment, researchers were able to enhance and partially repair memory deficits in mice between ages of 2 to 7 months with characteristics of Alzheimer's disease. Mice in enriched environments performed significantly better on object recognition tests and the Morris Water Maze than they had when they were in standard environments. It was thus concluded that environmental enrichment enhances visual and learning memory for those with Alzheimer's. Furthermore, it has been found that mouse models of Alzheimer's disease that were exposed to enriched environment before amyloid onset (at 3 months of age) and then returned to their home cage for over 7 months, showed preserved spatial memory and reduced amyloid deposition at 13 months old, when they are supposed to show dramatic memory deficits and amyloid plaque load. These findings reveal the preventive, and long-lasting effects of early life stimulating experience on Alzheimer-like pathology in mice and likely reflect the capacity of enriched environment to efficiently stimulate the cognitive reserve. A human study suggests that enriched gardens contribute to better cognitive function and independence in activities of daily living, compared to conventional sensory gardens.
Huntington's disease
Research has indicated that environmental enrichment can help relieve motor and psychiatric deficits caused by Huntington's disease. It also improves lost protein levels for those with the disease, and prevents striatal and hippocampal deficits in the BDNF, located in the hippocampus. These findings have led researchers to suggest that environmental enrichment has a potential to be a possible form of therapy for those with Huntington's.
Parkinson's disease
Multiple studies have reported that environmental enrichment for adult mice helps relieve neuronal death, which is particularly beneficial to those with Parkinson's disease. A more recent study shows that environmental enrichment particularly affects the nigrostriatal pathway, which is important for managing dopamine and acetylcholine levels, critical for motor deficits. Moreover, it was found that environmental enrichment has beneficial effects for the social implications of Parkinson's disease.
Stroke
Research done in animals has shown that subjects recovering in an enriched environment 15 days after having a stroke had significantly improved neurobehavioral function. In addition these same subjects showed greater capability of learning and larger infarct post-intervention than those who were not in an enriched environment. It was thus concluded that environmental enrichment had a considerable beneficial effect on the learning and sensorimotor functions on animals post-stroke. A 2013 study also found that environmental enrichment socially benefits patients recovering from stroke. Researchers in that study concluded that stroke patients in enriched environments in assisted-care facilities are much more likely to be engaging with other patients during normal social hours instead of being alone or sleeping.
Rett syndrome