A child’s brain is changed by experiences.
The structure and function of a child’s brain is not only influenced by its genetic inheritance, but also by experience. While genes program certain types of nerve cell connections, experience also programs and reprograms nerve cell connections. Depending on the type of function, the relative influence of genetic versus experiential influences can differ. For example, brain centers that control breathing and heart rate are relatively hardwired at birth, whereas higher cortical functions that have to do with learning and memory are sculpted and modified by experiences. This newer understanding of brain behavior relationships yields a picture of the brain as a plastic and self organized organ in which the development and maintenance of nerve connections is based on experiential demands and not strictly predetermined (Gottlieb, Wahlsten and Lickliter, 1998).
Both the growth and elimination of synapses in the brain depends on an individual’s experiences. Experiences that stimulate activity in particular regions of the brain facilitate the growth of connections in those regions, so that synapses can be said to form in a “use-dependent” manner. Even before birth, the ongoing spontaneous firing of neurons in the immature brain is thought to stimulate the formation of synapses. For example, it has been hypothesized that neurons that happen to be triggered at the same time will connect to each other, so that cells that “fire together, wire together”(Peen and Shatz, 1999). It has been suggested that the incidence of spontaneous neural activity supports a proportion of synapse formation, and is genetically controlled in order to ensure that an adequate number of connections are formed during gestation and very early in life, before much external stimuli is available. The connections that form from these spontaneous synaptic firings are not random, but they are relatively disorganized.
Nonetheless, even as these rudimentary connections are being made before birth, the brain is capable of learning. In one experiment done in Belfast, Northern Ireland, infants of mothers who had routinely watched a BBC soap opera during their pregnancy were found to respond specifically to theme music of that show a few days after birth (Hepper, 1996). Following birth, the brain continues to create spontaneous neural activity, but, increasingly over time, synapse formation is supported by the external stimuli the infant receives from the surroundings, such as the taste of warm milk, the feeling of a mother’s caress, or the sound of a father’s voice. The brain’s response to these external stimuli is known as “sensory-driven” neural activity. Synaptic firing leads neurons to form connections to other cells that have also been activated by sensory stimuli and experiences in their new world. Because neurons that are activated by a particular type of stimulus most likely have a role to play in receiving, processing and responding to it, and because cells that are activated in synchrony become connected, sensory-driven neural activity drives the circuitry in a young child’s brain toward increasing organization.
As is the case in much of recent neurobiology research, experiments with other animals have revealed a great deal about basic mechanisms. For example, the amount or type of stimulation provided by the environment has been found to have a measurable impact on the physical development of the rat brain. One now-classic study found that rats raised in enriched environments had 30% greater synaptic density in their cerebral cortexes than did rats confined to non-enriched environments (Black, Isaacs and Anderson, 1990. See also Diamond, 1990). The rats in the enriched environment had access to challenging situations such as mazes, as well as a variety of visual stimuli. The researchers concluded that the observed difference in synaptic density between the two groups of rats was a reflection of the difference in richness of experience, and that a more natural, stimulating environment allowed a rat’s brain to develop in a normal way.