Quoted: The fear of a decaying brain has inspired a mini-industry of products from dietary supplements to computer games.

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Glucocorticoids are steroids produced by bodies at times of stress. Prolonged stress(and exposure to the steroids) damages hippocampus neurons. PTSD and depression(ie chronic stress) can cause permanent damage to the hippocampus, slow neurogenesis and affect memory.

From studies of baboon clans, this steroid level-
(a) is generally low in high ranking males and high in low ranking males. They are high for high-ranking males when the societal hierarchy is under threat.
(b) is high when the personal threat is unpredictable and arbitrary.
(c) is higher when a baboon is unable to distinguish a real threat from a non-threat.
(d) is lower where baboons have a social outlet for anger.
(e) is lower when they are sociable and not alone.

Quoted: For people whose work is unstimulating, having mentally challenging hobbies, like learning a new language or playing bridge, can help maintain cognitive performance. But the belief that any single brain exercise program late in life can act as a quick fix for general mental function is almost entirely faith-based. One form of training, however, has been shown to maintain and improve brain health — physical exercise.

Quoted: The subconscious brain is more active, independent and purposeful than once thought. Sometimes it takes charge.