" As adolescence begins, the second genetically programmed growth of synapses is followed by a long decade of brain remodeling that involves the programmed destruction of neurons and their connections that are not being used. The parcellation process in which connections are pruned is thought to be a 'use -it-or-lose-it' neural reshaping that is exacerbated by stress. Such a period of brain change is also marked by vulnerability , as underlying neural deficits, unrealized before adolescence, may becomes exposed during the pruning process. "

― Daniel J. Siegel , The Mindful Brain: Reflection and Attunement in the Cultivation of Well-Being