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. 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. 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). 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. Environmental enrichment during pregnancy has effects upon the fetus, such as accelerating his or her retinal development. 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. 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. 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.