Sleep EEG, the Clearest Window Through Which to View Adolescent Brain Development

Abstract

It can be argued that the two periods of time in human development that show the most dramatic changes in the brain are from mid-gestation through age 6 years and during adolescence. While the prenatal changes involve both proliferative neurogenesis and apoptosis, the postnatal changes are not so much in the number of neurons, but in the number and form of the interconnections between them.

The human female pelvic girdle places limitations on the extent of brain development that can occur in utero. The rapid growth in brain and head size in the first two years of postnatal life are due to the rapid development of trillions of synaptic connections, providing young children with a rich interconnected network of neurons, as an ideal and highly plastic substrate to support learning. Thus children exposed to a bilingual environment from birth, have a greater facility with language, and a different organization of language areas in the brain compared to those of us drilled in a foreign language in high school. Nonetheless, there is a cost to running a highly interconnected network. While it has great potential, it is inefficient, can be slow, and has a high metabolic demand.

Starting in childhood, brain regions start losing plasticity and gain speed and efficiency, by pruning weaker synaptic connections and myelination of axons in those remaining. This use it or lose it pruning process starts in primary cortices (probably with the sensorimotor strip or occipital cortex), and then moves forward and outward eventually encompassing the whole brain. The last area to lose plasticity and to take on its adult form is the prefrontal cortex. Again, there are probably adaptive advantages to having this area remain plastic for as long as possible, but eventually it too must finish pruning, myelinate its axons, and take on its adult form.

The paper by Tarokh and colleagues in this issue of SLEEP highlights how longitudinal sleep EEG data is arguably the optimal functional measure of these structural brain changes. The use of longitudinal data removes much of the noise associated with individual differences and permits a more accurate determination of maturational effects on the brain. Together with data from other very recent papers, data from Tarokh et al. present evidence of decreased EEG power across a wide spectrum in NREM and REM sleep. The comparison of the data showing differences between 15-16 and 17-18 years, with those collected in a similar design but from younger adolescents highlights the fact that later adolescents are still showing dramatic changes in sleep EEG power, reflecting ongoing changes in brain development.