We see it happen, but it is still amazing- the mental powers that come on line during child development. Neurobiologists are starting to look inside and see what is happening mechanistically- in anatomical connectivity, activity networks, and brain wave patterns. Some recent papers used fMRI and magnetoencephalography to look at activity correlations and wave patterns over adolescent development. While the methods and analyses remain rather abstruse and tentative, it is clear that such tendencies as impulsivity and cognitive control can be associated with observations about stronger brain wave activity at higher frequencies, lower activity at lower frequencies, and inter-network integration.
An interesting theme in the field is the recognition that not only is the brain organized physically in various crinkles, folds, nodules, etc., and by functional areas like the motor and sensory cortexes or Broca's area, involved in speech production, but that it is also organized in connectivity "networks" that can cross anatomical boundaries, yet show coherence, being coordinately activated inside much more densely than outside the network. An example is the default mode network (DMN, or task-negative network), which happens when adults are just resting, not attending to anything in particular, but also not asleep. This is an example of the brain being "on" despite little conscious mental work being done. It may be our unconscious at work or play, much like it is during sleep on a much longer leash. As one might imagine for this kind of daydreaming activity, it is strongly self-focused, full of memories, feelings, social observations, and future plans. Anatomically, the DMN extends over much of the brain, from the frontal lobes to the temporal and parietal lobes, touching on regions associated with the functions mentioned, like the hippocampus involved in memory, temperoparietal areas involved in sociality/ theories of mind, etc. There are roughly twenty such networks currently recognized, which activate during different mental fuctions, and they provide some answers to the question of how different brain areas are harnessed together for key functions typical of mental activity.
Two networks relevant to this current work are the salience network (SN) and the cingulo-opericular network (CN or CO). The latter is active during chronic attention- our state of being awake and engaged for hours at a time, termed tonic alertness. (This contrasts with phasic alertness, which is much shorter-term / sporadic and reactive). It is one of several task-positive networks that function in attention and focus. The salience network spans cortical (anterior insula an dorsal anterior cingulate) and subcortical areas (amygdala and central striatum) binding together locations that play roles in salience- assigning value to new events, reacting to unusual events. It can then entrain other brain networks to take control over attention, behavior, thoughts, etc.
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Here we get to brain waves, or oscillations. Superimposed on the constant activity of the brain are several frequencies of electrical activity, from the super-slow delta waves (~ 1Hz) of sleep to the super-fast gamma waves (~50 Hz) which may or may not correlate with attention and perception. The slower waves seem to correlate with development, growth, and maintenance, while the faster waves correlate with functions such as attention and behavior. Delta waves are thought to function during the deepest sleep in resetting memories and other brain functions, and decline sharply with age, being pervasive in infants, and disappearing by old age. Faster waves such as theta (5-9 Hz), alpha (8-12), and beta (14-26 Hz) correlate with behavior and attention, and are generally thought to help bind brain activities together, rather than transmitting information as radio waves might. Attention is a clear example, where large brain regions are bound by coordinated waves, depending on what is being attended to. Thus the "spotlight of attention" is characterized both by the activation of selected relevant brain areas, and also by their binding via phase-locked neural oscillations. These are naturally highly variable and jumbled as time goes on, reflecting the saccadic nature of our mental lives.
One of the papers above focused on theta and beta waves, finding that adolescents showed a systematic move from lower to higher frequencies. While fMRI scans of non-oscillatory network activity showed greater integration with age, studies of oscillations showed that the main story was *de-coupling mainly at the lower frequencies. What this all seems to add up to is a reduction of impulsivity, via reduced wave/phase coupling between especially between the salience and other networks, at the same time as control over other networks is more integrated and improved, via increased connectivity. So control by choice goes up, while involuntary reactivity goes down. It is suggested that myelination of axons, as part of brain development along with pruning extra cells and connections, makes long-range connections faster, enabling greater power in these higher frequency binding/coordination bands.
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| Brain wave phase coordination between all areas of the brain, measured by frequency and age. Low frequencies associated with basal arousal, motor activity, and daydreaming are notably less correlated in adults, while beta-range frequencies about 25 Hz, associated with focused attention, are slightly more correlated. |
Is this all a little hand-wavy at this point? Yes indeed- that is the nature of a field just getting to grips with perhaps the most complicated topic of all. But the general themes of oscillations as signs/forms of coordination and binding, and active sub-networks as integrating units of brain/mental activity on top of the anatomical and regional units are interesting developments that will grow in significance as more details are filled in.
