The neurobiology of behavioral inhibition as a developmental mechanism

Humans are social creatures and have variable responses to novel social cues that range from cautious avoidance to eager approach. These trait differences in response to novelty have been defined as behavioral inhibition, a temperament that ranges from behaviorally inhibited on one extreme to behaviorally uninhibited at the other. For centuries temperament has been thought to reflect underlying differences in biology. With advances in neuroimaging methods, we now have a unique opportunity to identify the neurobiological basis of behavioral inhibition. In this chapter, we review the evidence that behavioral inhibition is associated with alterations in brain structure, function, and connectivity and present implications for understanding developmental trajectories. The emerging findings point to alterations in "bottom-up" mechanisms- heightened reactivity to novelty and failure to habituate-and "top-down" processes, failure of cognitive control and maladaptive anticipatory processing. We propose that the bottom-up mechanisms, which are present very early in childhood, contribute to the earliest observations of behavioral inhibition in children and shape early developmental trajectories. In contrast, the top-down mechanisms emerge in early adolescence as the prefrontal cortex begins rapid maturation. Developmental trajectories of behaviorally inhibited children likely diverge in adolescence based on prefrontal cortex development. Adolescents with early maturation or robust prefrontal cortical function will move toward a trajectory of normative development, while adolescents with delayed or deficient prefrontal cortical development will maintain their trajectory of extreme inhibition and risk for anxiety. Future research must systematically study behaviorally inhibited children across development to document developmental differences in brain structure, function, and connectivity and to further clarify the role of neurobiological mechanisms in shaping developmental trajectories.