ABSTRACT
Human brain development is highly affected by the environments with which the child interacts during the first three years of life. The growth in size and weight of the brain is caused by increases in the size of the neurons, greater dendritic connections, initial myelination of axons, and increases in glial cell size and number. The brain growth spurt that began in infancy continues at a high pace throughout the toddler years and seems to correspond in time to the appearance of representational thinking, such as pretense and language. By age 2, the number of synapses in the toddler brain reaches adult level, and by age 3 the brain has about 1,000 trillion synapses, which is twice the density of synapses present in the adult brain. Environmental enrichment may increase neuronal complexity, improve brain function, and facilitate recovery from brain injury, and these influences on specific abilities are more pronounced during critical or sensitive periods.
