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The Primacy of Infancy: Implications for Brain and Cognitive Development

We generally assume that brain development relates to cognitive development. That concept came to my attention from Nobel Prize Laureate Eric Kandel during my introductory neuroscience course at Columbia University in 1978 when he stated, “behavior depends on the formation of appropriate interconnections among neurons in the brain.” That astute statement foreshadowed the enormity of primate and human neuroscience research that continued to the turn of the century. In the period of 1990 to 2000, known as the Decade of the Brain, launched by the Presidential decree of George H.W. Bush, there was enhanced public awareness of the benefits of brain research. This occurred through a variety of activities including publications and programs aimed at introducing politicians, educators, private and public funding agencies and the general public to cutting-edge research and encouraged public dialog on its ethical, philosophical, and humanistic implications. One revelation that attracted attention was the process of cortical synaptogenesis that guided brain connectivity and neural circuitry. An exuberant burst of synaptic formation that starts prenatally and continues shortly after birth, is followed by programmed synaptic elimination throughout motor, sensory, associative and visual lobe cortices in a process that continues throughout early infancy, early childhood and into adolescence before reaching the adult state of normal synaptic density.
The field of developmental neuroscience entered a new era offering the promise of discovering meaningful links between specific brain and cognitive developments. With a relatively complete wiring diagram for several important regions of the brain, investigators related the sequence of cortical synaptogenesis to the timing of sensorimotor development, laying the foundations for important cognitive capabilities that sequentially emerged in infancy. This well-defined period of exuberant synaptogenesis that marked the emergence of each specific new cognitive capacity, while not leading to an instantaneous change in all skills, coincided with refinements in the morphology of the portion of the brain serving that new function, and advancing it toward the next neurocognitive milestone in a march toward maturity. For example, the first vocalizations, early fixated gaze, and simple motor actions were all achieved by the infant at the height of synaptogenesis.

However, more complex sensorimotor toddler achievements such as symbolization, as originally described by Piaget, moving beyond action systems to cognitively evoke an object, event, or person, coincided instead with the refining or “pruning” of synapses. These processes occurred concurrently throughout all cortical regions of the brain, tapering by age 3 in primates, and age 7 years in humans.  Add to this, the observation that the morphology and functionality of certain specialized brain functions such as visual circuitry could be experimentally altered during these critical periods of synaptic formation. And it was not long before societal concerns mounted for infants at greatest risk either due to prenatal or postnatal illness, inherent poverty or deprivations in the home. In what became the famous motto “use it or lose it,” attention focused on synaptogenesis as something to be modified. Such that by the beginning of the twentieth century, our nation saw a heightened awareness of public health policies directed at the newborn’s basic needs for safety, nourishment, nurturing and if needed, early educational intervention, expanding childcare programs for working parents, and enforcing mandatory unpaid family and maternity leave. The importance of the early childhood years, in particular the first 3, became synonymous for the lay public and policy makers, with acquiring the neurocognitive and developmental attributes of self-confidence, trust, intellectual inquisitiveness, physical and mental health.       

In the years that have ensued, there has been a wealth of new information on the functional connectivity and neural plasticity of the human brain and its implications for learning, memory, and mental health, that take us beyond the myths, determinism and primacy of the first three years of life.