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Experience-Dependent, Experience-Expectant Processes

Humans have evolved the unique capacity to store enormous amounts of information from birth to old age. The human brain is already large and complex at birth and is programmed to incorporate specific information (e.g., vision, language, attachment) on a developmental schedule. The brain continues to grow considerably for decades while incorporating additional information as needed (e.g., details of the environment, school, culture, personal experience). The neural mechanisms to store expected information during development are similar to those used on demand in adulthood but the differences have important clinical implications.

Evolutionary Background

Learning is an example of convergent evolution, where different species have independently evolved homologous information storage systems. For example, honeybees store spatial information about food location to communicate with other bees. Octopi are capable of learning to solve puzzles by observation and retain information for long periods. When learning their complex songs, young birds grow new neurons and remodel synaptic connections, and adult birds can be adept at puzzles and recall. These species evolved neural plasticity separately and apparently use neural mechanisms quite different from those shared by mammals.

Mammals make extensive use of two types of neural mechanisms to incorporate experience; the first uses early experience to refine brain development and the second allows individuals to incorporate information useful for survival.

The developing brain can reliably expect information being available when needed to form maps of visual space or to assemble motor programs for reaching or speech. This use of experience that can be reliably expected during development, such as with a kitten’s eyelids opening or a baby’s first stepping or grasping, has been termed experience expectant. Other types of information can be essential for individual survival, but the timing and quality are very idiosyncratic. This is neural plasticity on demand—whether information is incorporated depends on its relevance, so it is termed experience dependent. The mature brain is assembled from long, flexible sequences of genetically programmed and experience-expectant components. At almost all points of development, however, experience-dependent learning can occur when needed.

Experience-Expectant Processes

Human brain development has successive waves of growth and pruning back in a developmental sequence organized to incorporate experience. Different regions have peaks in brain cell formation, extension of dendritic and axonal branching, synapse formation, and myelination, followed by programmed pruning back of these same structures. This sculpting of final brain architecture through waves of overproduction and pruning back also follow a regional schedule, starting with sensory cortex, followed by association cortex and lastly prefrontal cortex. Each area uses experience to refine its connections and store information on a developmental schedule, improving and shaping the information to be used by the next region. Thus, visual cortex uses the earliest visual experience to finalize its structure, whereas information in later development helps refine cortical regions for language, social cognition, and executive function.

These events result in a linked developmental cascade, where reliable experiences build on each other and are incorporated in sequence to assemble brain structure. The components of the cascade are often termed critical periods because they rely on the quality and timing of experience typical for that species. For example, the infant’s early visual experience helps organize the infant’s understanding of the proximal and distal visual environments, and this is integrated with motor information for walking and grasping.

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