Early Adversity and the Neotenous Human Brain

Abstract

Human brain development is optimized to learn from environmental cues. The protracted development of the cortex and its connections with subcortical targets has been argued to permit more opportunity for acquiring complex behaviors. This review uses the example of amygdala-medial prefrontal cortex circuitry development to illustrate a principle of human development-namely, that the extension of the brain's developmental timeline allows for the (species-expected) collaboration between child and parent in co-construction of the human brain. The neurobiology underlying affective learning capitalizes on this protracted timeline to develop a rich affective repertoire in adulthood. Humans are afforded this luxuriously slow development in part by the extended period of caregiving provided by parents, and parents aid in scaffolding the process of maturation during childhood. Just as adequate caregiving is a potent effector of brain development, so is adverse caregiving, which is the largest environmental risk factor for adult mental illness. There are large individual differences in neurobiological outcomes following caregiving adversity, indicating that these pathways are probabilistic, rather than deterministic, and prolonged plasticity in human brain development may also allow for subsequent amelioration by positive experiences. The extant research indicates that the development of mental health cannot be considered without consideration of children in the context of their families.

Keywords: Amygdala; Brain development; Medial prefrontal cortex; Parents; Sensitive periods; Stress.

Figure 1.

Neotenous brain development permits increased opportunity to receive caregiver input. Altricial development (bottom two panels), unlike that of precocial species (top panel), requires caregiver input, which satisfies a species-expectation that the caregiving environment will scaffold the offspring’s developing neurobiology during periods of high developmental plasticity (green) before circuitry begins to take on adult characteristics (blue). Here, the example of amygdala-medial prefrontal cortex (mPFC) circuit development is used to illustrate how an expansion, and therefore protraction, of developmental processes enables significant influence from the caregiving environment on developing neurobiology in the case of the human (bottom panel). Dotted lines are meant to represent putative sensitive periods for the amygdala and its connections with mPFC. Interventions (e.g., changes in parenting, therapy) may have differential efficacy depending on when they occur (i.e., moments a,b,c), motivating the development and use of age-specific approaches. Note: ‘Developmental Time’ on x-axis is intended to be equated across the three species-types (top, middle, and bottom).