One concept in developmental neuroscience that

One concept in developmental neuroscience that resonates with stage approaches common in developmental psychology is that of sensitive or critical maturational periods. In this approach, ddr1 and behavioral organization are considered particularly amenable to specific types of information during specific developmental windows (Nelson and Panksepp, 1998; Bell et al., 2010; Meaney, 2010; Nelson and Guyer, 2011; Crone and Dahl, 2012; Takesian and Hensch, 2013; Nelson et al., 2014). Sensitive period models of brain development generally regard neuronal circuit maturation as a confluence of endogenous maturation and experiential sculpting. When neural systems begin to mature, they are weakly responsive to a wide scope of stimuli, and generate diffuse patterns of activation within and between other circuits. However, this pattern changes as experience with categorical exemplars accumulate. Brain responses become stronger, more efficient, and automatic as the boundaries of relevant stimuli becomes narrower, while responses to non-experienced stimuli are dampened (Greenough et al., 1987; Knudsen, 2004; Stiles, 2008; Leppanen and Nelson, 2009; Werker and Hensch, 2015). Interestingly, while the timing of the opening and closing of sensitive windows has generally been considered an internally mediated phenomenon with pre-determined timing parameters, under some conditions, environmental experiences can also affect the timing and pace of the period of heightened sensitivity. For example, in animal models, complete light restriction prolongs the closing of the window for organization of the visual cortex (Johnson, 2005), while restricted physical exposure to conspecifics can delay the closing of sexual imprinting (Bischof et al., 2002; Bischof, 2007) and delay the narrowing of face perception capabilities (Sugita, 2008). In humans, several factors, including body weight and life stress, influence the timing of puberty onset (Ellis et al., 2011; Lee and Styne, 2013). Moreover, recent findings suggest that the timing of neural circuit organization during human development may also be susceptible to extreme differences in environmental conditions, whereby brain maturation may be accelerated in order to facilitate social goals in the face of adversity (Gee et al., 2013). Although the extent to which environmental influences affect timing parameters of brain development has not been extensively investigated, particularly in humans, the existing data are suggestive of this being an important factor to consider in future studies. Advances in research on molecular contributions to developmental plasticity may inform our understanding of factors that contribute to the onset and offset of sensitive periods during both early life and later phases, as cortical organization continues to occur (Blakemore, 2014). For instance, recent studies indicate that changes in the plasticity of local circuits are dependent on the maturation of local inhibitory connections, which may regulate sensitive periods at a molecular level by shifting the local excitatory–inhibitory balance within local circuits (Takesian and Hensch, 2013; Werker and Hensch, 2015). Isolating regionally specific markers for heightened plasticity across sensitive periods in development may help us understand how neural organizational sculpting influences specific social functions during different phases of development. In addition it may be possible to re-open a closed critical period with targeted pharmacological approaches (Gervain et al., 2013). The difference between these mechanistic approaches to developmental periods and more traditional holistic stage-based approaches in developmental psychology may be simply a matter of semantics or scale. An important point of emphasis in both approaches, however, is that experiences are not uniformly effective in inducing change across maturation. Rather the degree to which different experiences can affect developmental trajectory depends to a large extent on when in the trajectory the experiences occur.