As testosterone affects the entire body (often by enlarging; men have bigger hearts, lungs, liver, etc.), the brain is also affected by this "sexual" advancement; the enzyme aromatase converts testosterone into estradiol that is responsible for masculinization of the brain in a male fetus.
There are some differences in a male and female brain (the result of different testosterone levels). A clear difference is the size: the male human brain is, on average, larger; however, in females (who generally do not have as high testosterone levels) the corpus callosum is proportionally larger. This means that the effect of testosterone is a greater overall brain volume, but a decreased connection between the hemispheres.[10]
A study conducted in 1996 found no immediate short term effects on mood or behavior from the administration of supraphysiologic doses of testosterone for 10 weeks on 43 healthy men.[11]
The literature suggests that attention, memory, and spatial ability are key cognitive functions affected by testosterone in humans. Preliminary evidence suggests that low testosterone levels may be a risk factor for cognitive decline and possibly for dementia of the Alzheimer’s type,[12] a key argument in Life Extension Medicine for the use of testosterone in anti-aging therapies. Much of the literature, however, suggests a curvilinear or even quadratic relationship between spatial performance and circulating testosterone,[13] where both hypo- and hypersecretion of circulating androgens have negative effects on cognition and cognitively-modulated aggressivity, as detailed above.
Contrary to what has been postulated in outdated studies and by certain sections of the media, aggressive behaviour is not typically seen in hypergonadal men who have their testosterone replaced adequately to the eugonadal/normal range. In fact aggressive behaviour has been associated with hypogonadism and low testosterone levels and it would seem as though supraphysiological and low levels of testosterone and hypogonadism cause mood disorders and aggressive behaviour, with eugondal/normal testosterone levels being important for mental well-being. Testosterone depletion is a normal consequence of aging in men. One consequence of this is an increased risk for the development of Alzheimer’s Disease (Pike et al, 2006, Rosario 2004).
