Trends in Neurosciences
ReviewSeries: Long-lasting Impact of Early-Life Stress and AdversityEarly Adversity and Critical Periods: Neurodevelopmental Consequences of Violating the Expectable Environment
Section snippets
A Conceptual Framework for Considering the Effects of Early Adversity on Brain Development
There is growing evidence that children exposed to adversity (see Glossary) early in life are at increased risk for atypical variations in brain development that in turn are associated with a variety of psychological, behavioral, and physical health sequelae [1., 2., 3.]. Adversity generally involves exposure to biological hazards (e.g., malnutrition, environmental toxins, chronic infection), psychological hazards (e.g., maltreatment, neighborhood or domestic violence), or both; and, although
What Do We Mean by ‘Adversity’?
Adversity has been used in a variety of ways. Some investigators have drawn analogies between early life stress and adversity [4., 5., 6.]. However, this can be misleading, as not all forms of adversity will be interpreted and/or encoded as stressful, depending on brain maturity and developmental history (e.g., an impoverished language environment, where a child is exposed to fewer words and less complex language, is likely a form of adversity but it is in and of itself not stressful and does
Adversity as a Violation of the Expectable Environment
For the purposes of this paper, we argue that adversity should be taken to reflect deviations in or disruptions of the expectable environment [22,23]; that is, experiences that are expected to occur (in order to confer survival and adaptation to the environment) either do not occur (e.g., lack of caregiving; lack of nutrition) or are atypical in some way (e.g., physical abuse). The reason an absence of an expected experience or the presence of an atypical experience matter can be attributed to
The Role of Critical Periods
Recently, Gabard-Durnam and McLaughlin [14] summarized several conceptual models that attempt to explain how adversity impacts neurodevelopment (Figure 2). These models emphasize different dimensions of adversity (e.g., timing, duration, type, number) or focus on how individual-level traits moderate the impact of an adverse experience [12,15., 16., 17., 18., 19.]. The conceptual models make assumptions not only about the most relevant features of environmental experience but also the underlying
Considering Critical Period Plasticity in the Context of Adversity
We wish to make several points to clarify the association between adversity and critical period plasticity. First, adversity is not itself an expectable experience that the brain prepares for. For example, the brain does not expect exposure to domestic violence. Adversity reflecting absent or impoverished specific expectable experiences clearly influences critical period inputs, but the adversity is not the expected substrate. Moreover, critical periods are inherently specific to particular
A High-Level Summary of the Effects of Adversity
We now consider the empirical evidence that early adversity can have enduring effects on human development. Countless studies have demonstrated an association between exposure to early, adverse life events and later maladaptive outcomes, with sequelae spanning a broad number of developmental domains. What follows is a cursory summary of some of the main findings.
Strategies to Parse the Effects of Adversity on Development
Given this robust literature linking early adversity with lasting impacts on development, a question that is currently receiving considerable attention empirically is how early adversity becomes neurobiologically embedded. Figure 4 complements the mechanisms highlighted in Figure 2 by illustrating a potential general model of biological embedding and sequelae across the lifespan. How may these conceptual models of biological embedding be translated into productive empirical strategies? At the
Concluding Remarks
Epidemiological studies dating back several decades advanced the idea that early adversity is associated with compromised neural and psychological outcomes. Recent work in neuroscience has begun to shed light on how a violation in experience-expected development during critical periods of brain development accounts for altered developmental outcomes. Not surprisingly, many questions remain unanswered (see Outstanding Questions). Moving forward, we advocate that this critical period approach
Acknowledgments
The writing of this paper was made possible by support to Charles A. Nelson from the Bill and Melinda Gates Foundation (OPP1111625), the National Institute of Mental Health (MH091363), and the Richard David Scott Chair in Pediatric Developmental Medicine Research, Boston Children’s Hospital; and to Laurel Gabard-Durnam from the University of Tokyo International Research Center for Neurointelligence.
Glossary
- Adversity
- a violation of the expectable environment that takes the form of biological hazards, psychosocial hazards, of complex exposures of both hazard types, with negative effects on development.
- Biological embedding
- the mechanisms through which environmental experiences impact neurobiology such that these experiences have enduring consequences on brain structure and function.
- Biological hazard
- adverse biological factors in the environment that have negative effects on development, such as
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