Neurobiological Bases of Behavioral Development in the Second Year

This review summarizes the temporal relations between selected psychological milestones in the first year of the human infant and theoretically relevant developmental neurobiological changes in the brain, supplemented where appropriate, with evidence from the non-human primate. The disappearance of the palmar grasp reflex and the decrease in endogenous smiling and spontaneous crying, which occur at 2-3 months, are correlated to emergent cortical inhibition of brainstem circuits. In addition, the improved ability to recognize an event experienced in the immediate past (recognition memory) is related to growth of the hippocampus and adjacent structures at this age. The behavioral developments at 7-10 months include an enhanced ability to retrieve stored representations of the past and to compare past and present (working memory), along with the emergence of the universal fears of strangers and separation from the caretaker. These milestones are correlated in time with maturational changes in the prefrontal and rhinal cortices and hippocampal formation, the integration of the limbic system and increased responsiveness of the hypothalamus-pituitary-adrenal axis. Knowledge of age-dependent correlations of brain and behavioral maturation is a basis for the investigation of causal relationships between brain development and behavior. A close collaboration of pediatricians, psychologists and neuroscientists is, therefore, necessary.