So I am sititng in a movie theater the other day, and some teenagers sitting behind me are talking. Of course, they are talking. They are ALWAYS talking behind me. And what particularly irks me is that it is a Tuesday night during the school year, and I only come to movies at 10 pm on Tuesday nights during the school year for the slim chance of avoiding talking teenagers.
Why, I ask you? Surely, there is some explanation for this behavior?
One theory is that teenagers are actually from a separate barbarian race. However, I suspect that there is also an underlying neurological reason for this barbaric behavior that has to do with the different rates of brain maturation in the human cortex.
The neurological changes that happen in the human brain over adolescence are described in a great article by Kendall Powell in Nature. Read the whole thing because it is really interesting:
The teenage years turn out to be a complicated time in the brain, with cells fighting it out for survival and the connections between different regions being rewired and upgraded. Some abilities, such as quashing offensive behaviour and empathizing with others, keep maturing well into the twenties. The passage from childhood to adulthood is not straightforward: some researchers now see the teenage remodelling as analogous to the ‘developmental window’ that allows the brain to be moulded by experience in infancy. There are ways in which teenage brains perform quite differently from either childish or adult ones.
No one is saying that a desire to yak endlessly on the phone, sneak out of the house or throw yourself off the side of a mountain on a snowboard in the gnarliest way possible is purely a function of neural architecture. But the brain changes tracked by neuroscientists offer parents, teachers and any other caring adult insights in how to help teenagers avoid too much danger. And such studies look likely to enrich scientific explanations of teenage vulnerability to depression, addiction, eating disorders and schizophrenia.
By around the age of 12, a child’s brain has the size, folding, weight and regional specialization of an adult’s. But a decades-long study conducted by the US National Institute of Mental Health (NIMH) in Bethesda, Maryland, has shown that such a brain still has a long way to go to reach adulthood. Begun in 1991, the study has followed 2,000 people who were then aged from 3 to 25, taking brain scans every 2 years. The study uses magnetic resonance imaging (MRI) to measure the water-to-fat content of tissues: in the brain, this distinguishes grey matter, which is mostly water-filled nerve cell bodies, from white matter, which is mostly made up of the nerve connections sheathed in fatty insulation called myelin.
The NIMH research team, led by Jay Giedd, has made a movie of normal brain changes from ages 5 to 20 (ref. 3). It reveals that the grey matter thickens in childhood but then thins in a wave that begins at the back of the brain and reaches the front by early adulthood (see graphic, below). The process completes itself sooner in girls than in boys. This corresponds to a long-held assumption that adolescence sees the prefrontal cortex regions that handle executive functions ‘waking up’ and to the conventional wisdom that girls mature faster in this respect.
The prefrontal cortex mentioned in the article is basically the part of the brain that keeps you from doing what you desperately want even though you know it is inappropriate — whether that be strangling your boss or me turning around and methodically stuffing an entire popcorn bucket into those teenager’s wagging jaws. (I assert that the fact that I have not been arrested for doing this is a testament to my fully functioning prefrontal cortex.) Research shows that the prefrontal cortex is one of the very last places for the grey matter (the part with the neurons) to mature.
To illustrate this one of the papers mentioned in the article created a video of gray matter maturation in the human brain that I think is super cool.
Gogtay et al from PNAS May 2004 created a video of this gray matter maturation by taking serial MRIs of people from age 4-21 and generating a statistical model to represent the degree of maturation. They compiled their results in a video shown below from two angles (more videos here). Blue represents increasing gray matter maturation according to their model.
Here is an image that may be somewhat easier to follow (click to enlarge):
Here is there description of what they did:
Here we present a study of cortical GM development in children and adolescents by using a brain-mapping technique and a prospectively studied sample of 13 healthy children (4-21 years old), who were scanned with MRI every 2 years for 8-10 years. Because the scans were obtained repeatedly on the same subjects over time, statistical extrapolation of points in between scans enabled construction of an animated time-lapse sequence (“movie”) of pediatric brain development. We hypothesized that GM development in childhood through early adulthood would be nonlinear as described before and would progress in a localized, region-specific manner coinciding with the functional maturation. We also predicted that the regions associated with more primary functions (e.g., primary motor cortex) would develop earlier compared with the regions that are involved with more complex and integrative tasks (e.g., temporal lobe).
The result is a dynamic map of GM maturation in the pre- and postpubertal period. Our results, while highlighting the remarkable heterogeneity, show that the cortical GM development appears to follow the functional maturation sequence, with the primary sensorimotor cortices along with frontal and occipital poles maturing first, and the remainder of the cortex developing in a parietal-to-frontal (back-to-front) direction. The superior temporal cortex, which contains association areas that integrate information from several sensory modalities, matured last. Furthermore, the maturation of the cortex also appeared to follow the evolutionary sequence in which these regions were created.
The interesting part about all of this is that there is a tightly choreographed ballet of brain development over a persons young life. Important landmarks in this development of the brain correlate quite well with important behavioral developments — like the ability for plan over the long term, balance checkbooks, etc.
And gray matter development isn’t the only kind of development that happens in the brain. There is another part of the brain called the white matter which contains cells called oligodendrocytes. Oligodendrocytes form the myelin around axons. Axons are long processes that connect different neurons in different parts of the brain to one another. If axons are wires, then the myelin is sort of like insulation. Without the insulation the wires do not function properly.
The interesting part is that oligodendrocyte and myelin maturation proceeds much longer over the life an adult than grey matter maturation. And this white matter maturation is occuring in parts of the brain that are the same parts we were just talking about in teenagers — like the prefrontal cortex. For example, Bartzokis et al followed white matter volume in the prefrontal cortex and temporal lobes over time and found that increases in white matter volume are occuring well into men’s 50s.
Here is a figure of those changes (click to enlarge). The white matter volume in the prefrontal cortex and temporal cortex vs. age is shown:
There is an important caveat to all of this, however, and I am glad that Powell makes it. The development of the human brain is not set in stone, either for the short time scale of adolescence or the long one of a lifetime. Development in the brain is shaped by experience and more importantly by the richness of experience. Thus, while we may stand in awe of the elaborateness of the biological plan, we should always remember that children without education, without a rich environment, do not benefit fully from that plan.
In Powell’s words:
Tomás caron Paus, director of the Brain and Body Centre at the University of Nottingham, UK, adds a further note of caution, warning that researchers should be careful not to simplify the brain-behaviour relationship into a one-way street. Behaviour almost certainly influences final brain structure, too; indeed, this is what Giedd’s current research is looking at. But Paus agrees the research has shown that the “brain continues to mature way beyond the first 3 to 5 years of life”. Referring to a popular pre-school ‘enrichment’ programme in the United States, he says: “We sorely need a Head Start programme at the beginning of adolescence. Things are not carved in stone by that age. Our field’s observations show that you can induce long-lasting changes that are entered into the brain.”