What happens to the brain as we age? Today, we know that the brain as a whole becomes smaller, it atrophies. As the brain develops, we know from studies like the ones performed by Sowell et al. that regions such as the primary sensory and motor areas mature early in the lifespan, while areas such as the prefrontal cortex mature very late.
Now, tying the evidence from development and ageing together, we can see how the brain changes over the lifespan. This is, of course, only partly true. We can study people at different ages at one time to get an estimate of how large brains and regions are in age-, gender- and educationally comparable groups people. Such cohort studies have the drawback that they do not tell us anything about how the individual brain changes over time. For that we have longitudinal studies. These studies very clearly shows how the brain changes over a few years. Together, these studies show much the same picture: brain changes dramatically as we age. We can put this into a few points:
Neural ageing is region specific: If we look at different area of the brain, we can see that the regions differ in how they age. Some structures mature early and degenerate late. Others mature late and degenerate early.
Neural ageing is non-linear: Previous models of ageing used linear models to understand the changes occuring in the brain over age spans. More recent studies now demonstrate that non-linear models fit better to the data.
These points can be seen in the following images. They show the size of the hippocampus (top) and nucleus accumbens (bottom). This is to demonstrate three points: 1) that the changes according to age are non-linear; 2) that the changes are region specific, and; 3) that there is a great variability in the relative size of each structure.
Now to my main point. Can we assume that the individual differences are "stable", in the way that people with larger-than-mean hippocampi will always stay larger than the mean? Or is the opposite true; that there are very individual trajectories in how the brain develops and ages? IMO, whether one model is true and than other false has potential high impact on our treatment in different settings. This problem is illustrated by a figure by Terry Jernigan at our lab that I reproduce here. The example shows age changes in dorsolateral prefrontal cortex. The mean is shown as the green line. The other lines indicate hypothetical trajectories for individuals. In other words, the PfC may change in a very individual manner.
Think, for example, about teaching in schools. In a country such as Denmark, teaching is rather homogenous, the same curriculum is applied in every school, the same tests etc. And your belonging to one particular class is defined by your chronological age. As I see it, this is based on the assumption that our mental capacities follow the same trajectory: those with a good learning ability will always stay high, those who are low will stay low. As a mean approach, we use age as the least common demoninator to define your group belonging.
So the question arises here: what if it turns out that development (and ageing in general) is very individual, non-linear and not stable relative to the mean? What if a person with a large prefrontal cortex at age 8 will have a below mean size at age 14, and then end up by a larger structure at 25? Would this make it harder or easier to do teaching, training or other interventions with people? Does it mean that we should re-consider our (Danish/Scandinavian) ways of teaching and assigning kids into age-dependent classes? I'm just asking. Feel free to comment.