Environment and Development of the Nervous System

A relatively small number of species have evolved specialized brain systems that incorporate information from the environment during development to refine and better adapt the individual's brain structure. Remodeling of neural circuitry in response to experience has evolved in diverse animal species, from the honeybee to mammals, and adaptive systems can be found in numerous parts of the brain. The capacity to use experience to shape brain structure extends in mammals from the prenatal period through old age. This chapter briefly discusses the genetic contributions to the developing brain, some parts of which become specialized for learning and memory. These are not “blank slates” but rather dynamic and interactive neural structures that organize and guide plasticity. The developing nervous system helps provide and organize experience by stabilizing the visuomotor platform, rough and tumble play, exploration, and other behaviors. Affiliative behaviors between parent and child also optimize experience so that it is safe and its affective tone is stimulating but comfortable. There is thus a complex ongoing transaction during development among: brain structure, the young organism's own often playful or curious contributions to its own experience, and parental contributions. The chapter discusses two basic categories of neural plasticity: experience-expectant development involving the readiness of the brain to receive specific types of information from the environment, and experience-dependent development involving the brain's adaptation to information that is unique to an individual. The former category has a species-typical developmental period of sensitivity, as shown, for example, in studies of visual deprivation. Such processes typically overproduce synaptic connections followed by their dramatic pruning back, with synapse overproduction and pruning in prefrontal cortex seeming to occur relatively late in humans. Some information necessary for survival is unique to the individual, idiosyncratic in its timing, such that that information storage depends on the nature of the experience. A neural signal equivalent to fear or reward potentiates storage, which is not developmentally programmed but follows a lifelong pattern. Studies involving the manipulation of acrobatic training, environmental complexity, and higher education illustrate experience-dependent processes. Clinical implications for when the neural substrate is defective or experience is inadequate or incoherent, as well as strategies for helping the injured organism get back on a normative developmental trajectory, are discussed. Keywords: brain evolution; critical period; experience-dependent; human development; neural plasticity; synaptic pruning; synaptogenesis