I learned much of my neuroscience at tea time in Hubel and Wiesel's laboratory at Harvard Medical School during my post-doc days in the 1960's, as we discussed their discovery of critical periods during the development of ocular dominance columns in the visual cortex, and the apparent immutability of the adult pathways, once formed. Everything now has changed. We know our brains maintain their ability to make new nerve cells and connections throughout life. Greifzu et. al. add a new chapter to the plasticity story in their recent work showing how important enriched environments are in maintaining a younger brain that has not been locked into place by the increased inhibitory interactions characteristic of adult brains. Specifically, they show that ocular dominance columns can remain plastic in adult mice in enriched, but not ordinary cage, environments, and recover from stroke-induced damage or monocular deprivation.
Experimental animals are usually raised in small, so-called standard cages, depriving them of numerous natural stimuli. We show that raising mice in an enriched environment, allowing enhanced physical, social, and cognitive stimulation, preserved a juvenile brain into adulthood. Enrichment also rejuvenated the visual cortex after extended periods of standard cage rearing and protected adult mice from stroke-induced impairments of cortical plasticity. Because the local inhibitory tone in the visual cortex of adult enriched mice was not only significantly reduced compared with nonenriched animals but at juvenile levels, the plasticity-promoting effect of enrichment is most likely mediated by preserving low juvenile levels of inhibition into adulthood and thereby, extending sensitive phases of enhanced neuronal plasticity into an older age.
