Therefore, not only does experience play an essential role in brain development, but the timing of certain experiences is important as well. In the first few years of life, individual regions of a child’s brain go through periods of time which neuroscientists call “critical periods.” During these periods the brain appears to be relatively more plastic, and therefore both amenable and vulnerable to the influence of experience. As the child ages, existing synapses in a particular region of the brain are thought to stabilize, suggesting that it may become more difficult to create new connections in that region. Therefore, neuroscientists suggest that the critical period of each developing region of the brain represents a window of opportunity during which specific experiences and stimuli are required in order to promote the usedependent synaptic growth described above (Cyander and Frost, 1999). Most of these studies documenting the existence and role of critical periods have focused on sensory processing (i.e., vision, hearing) in animals and on certain animal imprinting behaviors. In addition, there is growing evidence for critical and sensitive periods for other higher critical functions as well (Cyander and Frost, 1999: Cicchetti and Cannon, 1999).
However, it is important to emphasize that there is no single critical period of brain development in early childhood, or at any point in an individual’s life. Instead there are multiple critical periods. Different regions of the brain, each corresponding to a particular set of abilities or behaviors, become connected to the other regions at different times in a hierarchical fashion (Chugani, 1998). When a child is born, the brainstem, the “lowest” region of the brain responsible for basic functions such as heart rate and body temperature regulation, is immediately wired and stabilized because of its essential role in ensuring survival. Subsequently, other regions of the brain associated with child’s developing emotional and sensory motor capacities begin to develop a web of selectively maintained synapses, driven by the interplay of genetic and environmental influences. Because the various regions develop, organize and become fully functional at different times, specific kinds of experiences facilitate development in each region during that region’s critical period (Perry, 1997).
The development of language is one example of how critical periods influence brain development. The capacity to hear and produce all of the sounds, or phonemes, used in any human language is latent at birth.One study found that while both one-month-old American and Japanese infants were able to distinguish between the English sounds of L and R. the Japanese infants could not make that distinction a mere five months later (Kuhl, 1997). Because the potential capacity to distinguish and produce specific phonemes that are not present in a child’s native language gradually decreases as the child ages, adult native Japanese speakers find it extremely difficult to distinguish between these unfamiliar sounds. However, it has also been shown that adult Japanese speakers can learn to distinguish between L and R if they participate in a program specifically designed to teach them the difference - in effect, reprogramming their neural connections through additional experience (McClelland, 1999). Researchers have also identified a window for acquiring the sounds of a new language without an accent between ages 2 and 14 (Flege, J.E. et al., 1995).