I have spoken about the use of fMRI (functional magnetic resonance imaging) and PET (positron emission tomography) analyses that have provided us a great deal of information about our brains- normal, impaired, and diseased.  It also has provided us information about its development as we age.

When I was growing up, we believed (that means we were taught) that, by adolescence, our brains were fully developed.  We knew the brain reached 90% of its full size by age 6, finally having 100 billion neurons by the time we are adults.  It turns out that is not really the whole story. Our hand/eye coordination and motor control are, indeed, completely developed by our early teens (good things for the manufacturers and vendors of Xbox and Wii games). But, that’s only part of the story.

Babies start out 2500 active synapses (the connections between neurons, nerve cells) in our brains. By the time we are three or four, that number has skyrocketed to 15000 or so.  That means our brains are “hard wired” to transmit and receive information.  As we age, grey matter (neurons, dendrites; the thinking portion of the brain) increases and white matter (myelin, the insulation that affects the ‘efficiency’ of the brain) decreases.  This is not the only surge in brain development, though.  Drs. Giedd, Blumenthal, Jeffries et. al reported results from a longitudinal study of teenage brain development about a decade ago (Nature Neuroscience, 1999).  Using MRI, they found (among 145 subjects) that there is a second phase where grey matter (the ‘thinking part of the brain) occurs (maybe due to sexual hormone activity) at age 11 in girls and age 12 in boys; the synapses slowly decay after that, if those portions of the brain do not continue to be employed by various activities (the reality behind the “use it or lose it” concept).

It also turns out that the prefrontal cortex (the front portion of the outer section of our brain, kind of like the “CEO” of the brain) does not reach its peak until the early to mid-20’s.  This is the portion of the brain critical for problem-solving and complex decision making.  The limbic system, the portion of the brain dealing with emotions, and dopamine levels (controlling pleasure and rewards) are hyper-developed in adolescents (Casey, Jones, Hare, 2008).  Dr.Yurgelun-Todd et. al. at Harvard (a series of studies effected at the McLean Hospital, MA, in the mid 200’s), using fMRI, found that some 50% of their teen subjects (ages 10 to 18) were not able to identify various emotional states in people.  More importantly, they found that teens employed the amygdala (memory and emotion processing,also part of the limbic system), rather than the prefrontal cortex, when making these decisions. Once we couple these three findings, we can see the capability for making some very “interesting” choices as teenagers.

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