Much of this experience-dependent control of brain development relies upon the experiences either increasing or decreasing the neural activity of a cell. For example, unused neurons (neurons with little neural activity) will die, while used neurons will survive. This is a normal process that occurs in the developing brain—too many cells are born and are then pruned. While new neurons are born in the brain throughout life, the enormity of early life growth is never replicated in later life. The implications of this process for custodial decisions in very early life are enormous—early life deprivation fails to activate neurons, which means that a greater number of neurons will die. Equally important, neurons that would typically die under “normal” conditions could be retained under deprivation or conditions of abuse. In either situation, brain function for the typical social environment in our Western culture might be compromised. For example, Romanian orphans reared in extreme physical and social isolation have smaller brains, and adopted orphans from Romania and China have a larger amygdala than their non-adopted counterparts.5 The amygdala is a brain area concerned with emotion and fear, and a larger amygdala would suggest altered emotion and fear processing.
Next, more refined control of brain development is accomplished by changing the activity of specific connections between neurons. Activity patterns between neurons can cause some neurons to grow more dendritic branches and synapses but prune others, and so particular types of information processing are enhanced. Importantly, a specific level of neural activity is needed because both too much and too little activity has been shown to be suboptimal. Equally important, the optimal types and intensities of experiences will vary at each stage of development. For example, while rough and tumble play or watching a video might be appropriate sensory stimulation for a four-year-old child, they are likely inappropriate for an infant or a toddler. A more appropriate pattern and intensity of sensory stimulation for a one-year-old would be socially interacting with a nurturing and interesting caregiver. The implications of experience instructing fine-tuning of brain development are critical for custodial issues. If early life experience does not activate the attachment system, it is likely that the development of future attachment formation will be compromised. This seems to have occurred in some orphans adopted from China and Romania. Or, if early life attachment coexists with fear, then the activity of these systems could be overly coordinated.6 Of course, exploring these issues in the human brain is extremely difficult, but animal research in both rodents and primates certainly supports this view, as discussed below.
Importantly, we also know that no brain area functions in isolation and that brain changes induced by early life experiences are ubiquitous throughout the brain. Thus, information about brain development for a given brain area needs to be interpreted within the context of other neural changes because brain activity is a coordinated process of functional connectivity between areas. Moreover, the contribution of learning and interventions, which can dramatically alter brain activity, needs to be considered as we relate neuroscience to behavior and policy.7