56: Experience and the Refinement of Synaptic Connections

Introduction

• Development of Human Mental Function Is Influenced by Early Experience

  • Early Experience Has Lifelong Effects on Social Behaviors

  • Development of Visual Perception Requires Visual Experience

• Development of Binocular Circuits in the Visual Cortex Depends on Postnatal Activity

  • Visual Experience Affects the Structure and Function of the Visual Cortex

  • Patterns of Electrical Activity Organize Binocular Circuits in the Visual Cortex

• Reorganization of Visual Circuits During a Critical Period Involves Alterations in Synaptic Connections

  • Reorganization Depends on a Change in the Balance of Excitatory and Inhibitory Inputs

  • Postsynaptic Structures Are Rearranged During the Critical Period

  • Thalamic Inputs Are Also Remodeled

  • Synaptic Stabilization Contributes to Closing the Critical Period

• Segregation of Retinal Inputs in the Lateral Geniculate Nucleus Is Driven by Spontaneous Neural Activity In Utero

• Activity-Dependent Refinement of Connections Is a General Feature of Circuits in the Central Nervous System

  • Many Aspects of Visual System Development Are Activity-Dependent

  • Auditory Maps Are Refined During a Critical Period

  • Distinct Regions of the Brain Have Different Critical Periods of Development

• Critical Periods Can Be Reopened in Adulthood

• An Overall View

The human nervous system is functional at birth: Newborn babies can see, hear, breathe, and suckle. However, the capabilities of human infants are quite rudimentary compared to those of other species. Wildebeest calves can stand and run within minutes of birth, and many birds can fly shortly after they hatch from their eggs. In contrast, a human baby cannot lift its head until it is 2 months old, cannot bring food to its mouth until it is 6 months old, and cannot survive without parental care for a decade.

What accounts for the delayed maturation of our motor, perceptual, and cognitive abilities? One main factor is that the embryonic connectivity of the nervous system, discussed in Chapters 52 through 55, is only a "rough draft" of the neural circuits that exist in our adult selves. After birth, embryonic circuits are refined by sensory stimulation—our experiences. This two-part sequence—genetically determined connectivity followed by experience-dependent reorganization— is a common feature of mammalian neural development, but in humans the second phase is especially prolonged.

At first glance this delay in human neural development might seem dysfunctional. Although it does exact a toll, it also provides an advantage. Because our mental abilities are shaped largely by experience, we gain the ability to custom fit our nervous systems to our individual bodies and unique environments. It could be argued that it is not the large size of the human brain but rather its experience-dependent maturation that makes our mental capabilities superior to those of other species.

The plasticity of the nervous system in response to experience endures throughout life. Nevertheless, periods of heightened susceptibility to modification, known as sensitive periods, occur at particular times in development. In some cases the adverse effects of deprivation or atypical experience early in life cannot easily be reversed by providing appropriate experience ...