readings> the ever dynamic brain
A few years back, mothers were being warned their toddlers had to
“use it or lose it”. Massive synaptic pruning was
taken to mean that a child had until just age three to maximise its
brain potential. Then came the finding that brains undergo another
developmental lurch during the teenage years.
MRI scans revealed that
parietal and frontal lobe association areas suddenly bulk up with grey
matter at puberty and get pruned to shape over the following decade.
Researchers drew the obvious alarming conclusion: "You’re
hardwiring your brain in adolescence. So do you want to hardwire it for
sports and playing music and doing mathematics or for lying on the
couch in front of the television?"
And believe it or not, the US
Supreme Court is now seriously considering raising the age for the
death penalty to 18 because of the neurological argument that no
teenager is fully in control of their faculties until their prefrontals
come on-line to tame their unruly amygdalas.
Well happily the story keeps on changing with almost every MRI study;
the very latest making for a much more positive and balanced
developmental tale. The surprising truth appears to be that programmed
phases of growth continue well into our 40s and 50s. Certainly there is
no time when the brain finally achieves some fixed end state. Stability
is anathema. Instead the brain offers an eternally changing landscape.
A crucial shift in thinking is that researchers have recognised that
what looks like grey matter shrinkages can actually be a white matter
advance, myelin moving in progressively to firm up optimised pathways.
And studies show that white matter volumes don’t peak until
we are in our mid-40s. Indeed, the slowest maturing regions may well
still be undergoing myelinisation as we approach our 60s!
But even just
the development of grey matter presents a remarkably patchwork story.
As might be expected, low level cortical areas such as the cingulate,
frontal pole, entorhinal and the various primary sensory cortices,
settle early and shrink in gradual, steady, fashion over a lifetime.
The higher association areas show more variety. For dorsal areas
generally, grey matter volume shrinks by nearly a third between the
ages of 7 and 60, then holds steady. But given the fact that white
matter is still moving in, “maturity” for dorsal
parietal areas is reckoned now to come in our 40s. For dorsal frontal
cortex, it is our 50s.
The temporal and inferior parietal regions may
then have the lengthiest maturation schedules of all (as might be
expected for our object memory and vocabulary areas). The evidence is
that their grey matter volume is still on the rise in our 30s. So if
grey matter bulking up is equated to plasticity, then these areas are
keeping their options open longest.
This unexpectedly dynamic picture has quite naturally prompted quite a
bit of ingenious new speculation about brain disease. For example argue
some, schizophrenia could be a result of late teenage myelinisation
– a sudden increase in transmission rates delivering too much
information to faultily laid down nerve circuits. Alzheimer’s
might be connected with the slowing of the myelinisation process in our
mid-50; a tapering off in most people becoming active degeneration in
others. It seems to fit that the cortex areas which are the last to
myelinate are also among the first to decline.
And what about diseases
that appear in late childhood, then moderate somewhat, like
Tourette’s and hyperactivity? The root of these too could lie
in some malfunctioning of the normal maturational schedule.
Of course, it’s a big leap from an MRI volumetric scan to
knowing what is really happening in the growing brain. The
“use it or lose it” brigade have rightly been given
stick for not even being able to answer the basic question of which is
the real critical window of opportunity for the learning brain
– the initial grey matter bulking, the later synaptic
pruning, or the still later process of myelinisation (or
indeed, all three). So the ever expanding maturational story is making
their knee-jerk social policy pronouncements sound all the sillier.
There is the same danger for medical folk. The problem is that such is
the wealth of newly discovered neurodevelopmental landmarks that
researchers will always find some apparently significant event taking
place at precisely the moment of any disease onset.
But still
it’s good news that we are now seeing a developmental story
which is much more in keeping with a commonsense view of how the brain
might grow – that is, a steady journey from youthful
experimentation to mature calm wisdom, with serious mental decline in
old age only as the result of actual disease.