In addition to fostering the growth of synapses between neurons, an individual’s experiences also determine which of the existing synapses will survive the process of selective elimination that begins around three. Again, the principle of use-dependence applies. Experiences that utilize the connections in particular regions of the brain ensure that those connections will survive, while connections that are not utilized will be lost. Evidence for this phenomenon also comes from well-known animal studies. Kittens deprived of visual stimulation in one eye for a short time early in life lost permanently the ability to see out of that eye. In this study, and similar ones, neuroscientists concluded that the region of the brain responsible for visual perception was never encouraged to grow and maintain connections to that eye because the eye was not used (Hubel and Wiesel, 1971). For the same reason, children less than 18 months of age whose cataracts are untreated can have a dramatic and permanent loss of visual activity in the untreated eye (Boothe, Dobson and Teller, 1985), and the duration of the time in which the eye is not used is directly related to potential visual acuity once the cataract is removed and vision is restored (Mitchell and Timney, 1984).
The fact that synapses grow and form connections on a use dependent basis is indicative of the fact that the human brain is fundamentally an adaptive organ, whose physical organization is shaped by the environment. In this sense, learning is the process by which the brain responds adaptively to the environment in which a child is raised. Therefore, learning includes much more than the verbal and cognitive skills that are the focus of the K through 12 classroom education.
It is also clear from available studies that extreme deprivation, such as the rats and kittens described above were subject to, can have serious consequences for brain development. However, these studies should not be interpreted to suggest that if parents provide their children with unusual types or excessive amounts of stimulation, that their children’s brains will develop more quickly or have greater intellectual capacity. At present, there is no empirical data to suggest that providing extra stimulation above what is normally expected by the developing brain has a beneficial effect in terms of brain growth or synaptic connections. This lack of evidence should not be interpreted as a lack of effect, but reflects the fact that, at present, non-invasive techniques have demonstrated that different kinds of input have different kinds of effects on the developing human brain. However, there are a number of studies by developmental psychologists that suggests that providing infants and young children with specific enriching experiences can boost their cognitive and behavioral functioning (Infant Health and Development Project, 1990 Ramey et al., 1992).
Our exploding knowledge about the brain has also revealed that the brain is not a large, undifferentiated computer with a single control processing unit. Rather, a better metaphor would be a collection of interactive and specialized processors (Cyander and Frost, 1999). The functions of each of these processing units depend on developmentally timed molecular and cellular events that must be programmed in the correct sequence to optimize the functioning of the system.