This 2015 German review was of aging and activity in the context of adult neurogenesis:
“Adult neurogenesis might be of profound functional significance because it occurs at a strategic bottleneck location in the hippocampus.
Age-dependent changes essentially reflect a unidirectional development in that everything builds on what has occurred before. In this sense, aging can also be seen as continued or lifelong development. This idea has limitations but is instructive with regard to adult neurogenesis, because adult neurogenesis is neuronal development under the conditions of the adult brain.
The age-related alterations of adult neurogenesis themselves have quantitative and qualitative components. So far, most research has focused on the quantitative aspects. But there can be little doubt that qualitative changes do not simply follow quantitative changes (e.g., in cell or synapse numbers), but emerge on a systems level and above when an organism ages. With respect to adult neurogenesis, only one multilevel experiment including morphology and behavior has been conducted, and, even in that study, only three time points were investigated.
In old age, adult neurogenesis occurs at only a small fraction of the level in early adulthood. The decline does not seem to be ‘regulated’ but rather the by-product of many age-related changes of other sorts.
From a behavioral level down to a synaptic level, activity increases adult neurogenesis. This regulation does not seem to occur in an all-or-nothing fashion but rather influences different stages of neuronal development differently. Both cell proliferation and survival are influenced by or even depend on activity.
The effects of exercise and environmental enrichment are additive, which indicates that increasing the potential for neurogenesis is sufficient to increase the actual use of the recruitable cells in the case of cognitive stimulation. Physical activity would not by itself provide specific hippocampus-relevant stimuli that induce net neurogenesis but be associated with a greater chance to encounter specific relevant stimuli.
Adult hippocampal neurogenesis might contribute to a structural or neural reserve that if appropriately trained early in life might provide a compensatory buffer of brain plasticity in the face of increasing neurodegeneration or nonpathological age-related functional losses. There is still only limited information on the activity-dependent parameters that help to prevent the age-dependent decrease in adult neurogenesis and maintain cellular plasticity.
The big question is what the functional contribution of so few new neurons over so long periods can be. Any comprehensive concept has to bring together the acute functional contributions of newly generated, highly plastic neurons and the more-or-less lasting changes they introduce to the network.”
I’ve quoted quite a lot, but there are more details that await your reading. A few items from the study referenced in the first paragraph above:
“The hippocampus represents a bottleneck in processing..adult hippocampal neurogenesis occurs at exactly the narrowest spot.
We have derived the theory that the function of adult hippocampal neurogenesis is to enable the brain to accommodate continued bouts of novelty..a mechanism for preparing the hippocampus for processing greater levels of complexity.”
The role of the hippocampus in emotion was ignored as it so often is. The way to address many of the gaps mentioned by the author may be to Advance science by including emotion in research.
For example, from the author’s The mystery of humans’ evolved capability for adults to grow new brain cells:
“Adult neurogenesis is already effective early in life, actually very well before true adulthood, and is at very high levels when sexual maturity has been reached. Behavioral advantages associated with adult neurogenesis must be relevant during the reproductive period.”
When human studies are designed to research how “behavioral advantages associated with adult neurogenesis must be relevant” what purpose does it serve to exclude emotional content?
http://cshperspectives.cshlp.org/content/7/11/a018929.full “Activity Dependency and Aging in the Regulation of Adult Neurogenesis”