There is an old article by Jonah Lehrer in the Seed magazine regarding the historical process via which the fact of neurogenesis in the humna brain was discovered and established.
One of the findings related to the stress/depression and the-lack-of-neurogenesis linkage and the underlying mechanisms that are involved (including sertonergic triggering of cascade reactions that lead to increase in trophic factors). A corollary finding was that enriched environments also lead to more neurogenesis and can help heal the scars formed due to depression/stress by stimulating neurogenesis in the adult brain. How neurogenesis (in areas like hippocampus and dompaminergic neurons) leads to recovery from depression/ stress is still not clear.
To briefly summarize the findings (though it is highly recommended that you read the original article which is very well written):
- Neurogenesis happens in adult brains (rats, primates and even humans).
- Stress reduces neurogenesis.
- Depression and reduced neurogenesis have been found to co-occur.
- Enriched environments lead to increase in neurogeneisis. (in rats, marmoset monkeys)
- Sertonin-based antidepressants primarily work by increasing neurogeneisis.
Hence inductively it seems probable that Low IQ is caused by Lower SES. (OK, this may seem like a joke...but do go and read the article and Gould's views on the stress and poverty relationships- and I find her views (and her supporting experimental and observational facts) quite plausible.)
The scientists profiled in the article, at that time, were still wondering (and actively exploring) the exact mechanism between neurogenesis and depression/ stress.
My hypothesis of why depression leads to less nurogenesis in hippocampus would be related to the role of hippocampus in memory and learning and how, for example, repeated exposure to shocks in rats leads the rats to exhibit a phenomenon known as 'learned helplessness'. Once the memory of a shockful and distressing repetitive experience is entrenched in the rat's memory, in the hippocampal region, she may not try to explore the environment that much, to discover and learn what has changed regarding the environment, and whether the stressful conditions and environments are over. This may lead to reduced neurogenesis as the rat's brain resigns itself to fate. This inability-to-learn or 'learning helplessness' (my slightly changed term for the same behavioral description) may lead to a vicious downward cycle leading to depression.
Once the neurogenesis is re-triggered, either due to administration of prozac or other antidepressants, or due to Cognitive behavioral therapy (and it had been found using brain scans that these two approaches seem to converge- one working in a top-down fashion (expecations and beliefs), while the other on a molecular and bottom-down fashion ), then the increased neurogenesis leads to an enhanced ability to learn and adapt and thus overcome the depressive epsiode and get rid of the symptoms. In both cases, the brunt of effort to get out of depression is still borne by the individual who is affected.
The other piece of information that caught my fancy was that of the dopimenergic neurogenesis and the potential cure of parkinson's disease based on targetting this pathway. Whether neurogenisis is limited to hippocampal regions, or also happens in the substatntia nigra/ VTA region (where I guess all the dopaminergic neurons reside) is an important question and my lead to more insight as to which all areas of the brain (or all areas) are susceptible to neurogenesis.