Critical periods revisited: implications for intervention with traumatized children.

Over the past decade there have been significant advances in the establishment of evidence-based treatments for traumatized children.1 Severely and multiply traumatized children, however, often have complex and difficult to treat clinical presentations. There is a perception among many clinicians, foster parents, and child protective service workers that these children are irrevocably ‘damaged’ -- especially when the trauma occurred during early developmental ‘critical periods’. The goal of this paper is to review recent advances in neuroscience that debunk the myth that early severe adverse rearing experiences lead inevitably to bad outcomes, and discuss the clinical implications of this evolving work. The belief that early deviant experience can lead to permanent changes in brain development and behavior stems partly from Hubel and Wiesel's groundbreaking experiments on monocular deprivation in cats.2 Development of central visual pathways in several mammalian species is known to be experience-dependent. Hubel and Wiesel deprived kittens of vision in one eye for different lengths of time and at different ages. They found that after suturing one lid during the first 3 months of life, there was no vision in that eye later in development after the sutures were removed, and the visual cortex did not develop normally. Effects of visual deprivation on subsequent brain development and visual processing was only evident in kittens, not in adult cats, which led to the conclusion that vision development in kittens has a ‘critical period’, and if the eyes are not exposed to the required stimuli during that period, vision would be lost and associated brain structures altered permanently. This finding has been extrapolated to traumatized children, generating the notion that severe adverse early child rearing experiences can lead to permanent alterations in brain development in regions associated with stress reactivity and emotion processing, and cause chronic and treatment-resistant psychopathology.3 Emerging findings, however are refining concepts of ‘critical periods’ and challenging previous understandings of the impact of early experience on brain development and behavior. Further studies revisiting Hubel and Wiesel's initial experiments have shown that the effects of monocular deprivation can be reversed with pharmacological interventions and environmental enrichment.4 What was previously deemed to be permanent brain damage secondary to adverse early experiences during formative periods of development has now been shown to be amenable to treatment, allowing complete function to be restored. The emerging research suggests: 1) there are sensitive periods when the brain is more susceptible to environmental inputs; 2) early experiences can have enduring effects; but 3) the effects are amenable to change. The concept of ‘critical periods’ – developmental stages when certain experiences are essential for normal development, and failure to obtain these experiences can lead to permanent alterations in brain development and behavior, has been replaced with the concept of ‘sensitive periods’ – developmental stages when certain experiences exert greater influence on brain development and behavior, but brain development and behavior continue to be malleable. So, how do early adverse rearing experiences exert relatively enduring and stable effects on brain development and behavior? How can these effects be reversed? And what are the implications for clinicians working with traumatized children? These questions are explored below.