Section 1, Article 3 - The corpus callosum is a long thin band of nerves that links the two brain hemispheres together. This area of the brain allows the left and right hemispheres to communicate across approximately 800 million individual fibers that develop during this age period. If development does not occur normally in the corpus callosum serious disorders can result (). This period of growth is marked with the refining of cognitive processes and designation of responsibility for which part of the brain is in charge of a given function. This division is known as .
Lateralization of the two brain hemispheres results in one side being dominant over the other for certain activities (). For example the right hemisphere controls the left side of the body and the left hemisphere controls the right side of the body. is evident by the end of the preschool years with left handed children developing thicker corpus callosum. Research indicates that the thickened area of the lefthanders brain is due to the need for more adjustments as they interact with an environment that predominantly caters to right-hand individuals. Interestingly, language is more lateralized in boys than in girls, whereas, in girls language seems to be more evenly divide. This distinction may be why girls at this age tend to pick up on language skills more easily than their male counterparts (; ). Both preservation and lateralization would not be possible without the above mentioned myelination of the nerve fibers.
Emotional development in the early childhood years is impacted by growth in the limbic system. The right hemisphere is more associated with emotional expression and works in conjunction with the amygdala, hippocampus, and hypothalamus which together make up the . The amygdala is a small area that resisters emotions, particularly the feelings of anxiety and fear (). The hippocampus is the area of the brain that is the major processor of memory and emotions, particularly related to location. The hypothalamus is a structure of the brain that interacts with the amygdala and hippocampus and signals other areas of the brain by creating hormones such as cortisol.