Early Adversity and Critical Periods: Neurodevelopmental Consequences of Violating the Expectable Environment

Abstract

It is now widely recognized that children exposed to adverse life events in the first years of life are at increased risk for a variety of neural, behavioral, and psychological sequelae. As we discuss in this paper, adverse events represent a violation of the expectable environment. If such violations occur during a critical period of brain development, the detrimental effects of early adversity are likely to be long lasting. Here we discuss the various ways adversity becomes neurobiologically embedded, and how the timing of such adversity plays an important role in determining outcomes. We conclude our paper by offering recommendations for how to elucidate the neural mechanisms responsible for the behavioral sequelae and how best to model the effects of early adversity.

Keywords: adverse childhood experiences; brain development; critical periods; developmental programming; early adversity; neurobiological embedding.

Figure 1.. Relation between Adversity and Stress Experiences in Development.

This diagram illustrates the conceptual differences and overlap between experiences of adversity and stress in development. We define adverse experience to be a significant deviation from the expectable environment, independent of whether that experience triggers a response in the stress system (adverse exposures highlighted in red). Similarly, early stressors all impact the stress response system, independent of whether the experience reflects a severe deviation from the expectable environment (stress exposures highlighted in blue). Some experiences may be considered both stressful and adverse experiences (highlighted in purple). We provide examples of biological hazards (broken outline) and psychosocial hazards (dotted outline) in each category of adverse and stress experiences.

Figure 2.. Conceptual Models of Environmental Influence on Neurodevelopment.

These models differ in the dimensions of adversity they account for (e.g., duration, timing, number) and the underlying neurobiological mechanisms (i.e., experience-expectant or experience-dependent mechanisms). Adversity may have the most significant, long-lasting effects on the brain when it disrupts or abolishes expected experiences during critical or sensitive periods of development for encoding those experiences (cases at top right of figure). Adapted from [14].

Figure 3.

Multiple Critical and Sensitive Periods Occur in the First Years of Life. Sensitive and critical periods in early brain development exist across sensory, cognitive, and affective domains. There are multiple critical and sensitive periods both across and within domains, as illustrated here for language development. Adversity in early life may have particularly significant, lasting consequences if it disrupts these early critical and sensitive periods of brain development. Reproduced, with permission, from [26].

Figure 4.. Sequelae of Adverse Experiences.

Illustration of the consequences of early adverse experience across development (here for the psychosocial hazard type of adversity). Adverse psychosocial hazards in early life co-occurring with critical and sensitive periods to encode psychosocial experiences (e.g., language and caregiver attachment) interact with genetic profiles to produce biological and behavioral changes across development that together lead to a variety of detrimental outcomes persisting into adulthood. Adapted from [1].