Johnanna Bick (Research Fellow at Boston Children’s Hospital and Harvard Medical School) and Charles A. Nelson (Richard David Scott Chair in Pediatric Development Medicine, Boston Children’s Hospital; Professor of Neuroscience and Education, Harvard Medical School and Harvard Graduate School of Education), in an article for the June edition of Early Childhood Matters, provide an overview of recent research findings on the impact of early adverse experiences on the developing brain and on “the potential for recovery, both at the level of the brain and in behaviour, and implications for prevention and intervention.” They state, “recent advances in neuroimaging have led to a more nuanced and richer understanding of how the brain develops, starting from the first weeks after conception and continuing until the last years of life. We also know more about how the brain functions and have identified various neural systems that support higher-level emotional, cognitive, and behavioural functioning.”

Their overview of brain development, as summarized below, covers four principles describing the protracted development of the brain starting shortly after conception and proceeding through the first three decades of life to reach brain maturity.

Principle 1: Brain development is a protracted process

• The neural tube forms a few weeks after conception and shortly thereafter, cells begin to form, proliferate, and finally migrate to designed locations.
• “Around the 23rd week of gestation starts a massive overproduction of synapses, or neurochemical signaling points between neurons. This overabundance eventually becomes reduced through a process known as ‘pruning’, which is heavily based on input from the environment.”
• The final stage of brain development involves the myelination of axonal fibres. “As part of this process, fatty glial cells wrap around axons to insulate neurons, allowing for more efficient neuronal transmission and signaling…The timing of this process varies, with some areas (sensory and motor regions) becoming fully myelinated in the first five years of life and others (frontal regions of the brain) reaching full myelination during early adulthood.”

Principle 2: Brains develop within the context of experience

• While “genes establish the basic ‘blueprint’ of development, setting the foundation and basic structural plan for the brain” during prenatal brain development, “the actual ‘construction’ of this plan depends heavily upon signals from the environment.
• Two processes in brain development that are particularly experience-dependent are the absorption of dendrites and the pruning of synapses. “The density of dendritic branches depends on the amount of and intensity of input from other neurons”….” Synaptic connections that are used more often become strengthened, whereas those that are unused are retracted (a phenomenon referred to as ‘use it or lose it’).
• From an evolutionary perspective, this process of brain development offers advantages, however “this degree of ‘plasticiitiy’ comes at a cost if environmental exposures exceed that which brains are designed to handle. Exposures to extreme stress and/or early deprivation are examples of such adverse circumstances.”

Principle 3: Brain development occurs in a hierarchical fashion

• “Each phase of brain development sets the stage for the subsequent phase; accordingly, more advanced systems depend on the more basic”, with critical implications for development. “If adequate signals are not provided for the more basic systems, then the more complex systems, such as those that support emotion and cognitive control or language and memory, cannot develop to their full potential.”

Principle 4: The first years of life mark an especially sensitive point in brain development

• “The experiences during the first years of life have an especially powerful role in influencing the developing brain. Because brain regions vary in the maturation rates, they also vary in the point(s) at which they are maximally sensitive to the environment, or pass through ‘sensitive periods’. Despite varying time courses, the majority of sensitive periods arise during early childhood, making the input received (or not received) during this stage in development critical for ongoing development.

The authors stress that “healthy brain development depends on expected input from the environment in order to reach its full genetic potential….Species-atypical violations of these expected experiences have deleterious consequences for brain development.” They focus on two particular examples of early life stress that have received extensive study:

• Prolonged exposure to gluticosteroids, prompted by chronic exposure to “stress or excessively threatening stimuli, such as when children are reared in maltreating families or exposed to high levels of violence”…”can have adverse effects on regions of the brain that support memory and learning (the hippocampus), and stress regulation, fear response, and detection of threat (the amygdala).” “Human research also suggests that extreme childhood stress leads to alterations in the structural and functional development of portions of the prefrontal cortex, a brain region that supports emotional and cognitive control. (Hanson et al, 2010; Edmiston et al, 2011; De Brito et al, 2013)”
• Childhood neglect (psychosocial deprivation) can also interfere with brain development, especially when it occurs in early life. “Under neglecting circumstances, the brain does not receive adequate environmental input to carry out the normal course of neurodevelopment. This results in an ‘under-wired’ or ‘mis-wired’ brain, which confers risk for a number of cognitive, emotional and behavioural problems that persist throughout development. Studies on animal models have been paralleled in human findings. “For example, children reared in depriving circumstances show reductions in overall brain volume (Mehta et al, 2009; Sheridan et al, 2012) and reduced thickness in the cortex (McLaughlin et al, 2014) which may signal atypical trajectories of experience-dependent synaptic pruning. White matter changes are also observed in children exposed to institutional rearing, both on a global level (Sheridan et al, 2012) and in specific axonal bundles associated with emotional and cognitive control (Eluvathingalet al, 2006; Kumar et al, 2014), Bick et al, 2015) suggesting developmental delays in the degree to which neurons become myelinated across development.”

The authors end on a note of hope. Both animal and human studies have demonstrated that introduction of more complex environments to children who have suffered neglect can create significant improvement in brain development, “for example, institutionally reared children placed into enriching, responsive family settings show structural (Sheridan et al, 2012; Bick et al, 2015) and functional Vanderwert et al, 2010) improvements of the brain, and associated improvements in cognitive and emotional adjustment (Rutter, 1998; Nelson et al, 2007)”, especially if the children are able to be “removed from neglect and provided with an enriched environment at the earliest ages (Vanderwert et al, 2010; Rutter et al, 1998; Nelson et al, 2007).”

They conclude that, based on recent evidence, early intervention has the potential to lead to significantly improved outcomes in the long term.