The concept of synaptic pruning had an impact on mainstream thinking about early education soon after its discovery. The fact that millions of synapses are eliminated between early childhood and adulthood was mind boggling. But conceptually, it made sense that the brain circuitry starts with more connections than are needed, and subsequently can be sculpted based on input. Although it may seem inefficient, the early overabundance of connections enables a process for selective elimination and refinement that occurs over years, as children learn. But how does synaptic pruning relate to childhood learning? Education scholars and advocates had for centuries been alert to the positive impacts of education on child development. In 1979, education initiatives such as the Head Start program in the United States were already in place. The discovery of synaptic pruning then supplied a new and specific biological basis supporting the benefits of early childhood education.

Discoveries about synaptic pruning and brain plasticity helped to set in motion a new wave of proposals for educational interventions to, in essence, “train the brain.” The idea of brain plasticity also triggered a flurry of articles in the popular press in the 1990s, in Newsweek, Time, US News and World Report and other venues, with titles like “Brain talk” or “How a child’s brain develops.” The New York Times reported in an obituary in August 2013 that Peter’s findings had “influenced education and government policy and parents’ priorities, putting increased emphasis on the importance of early education. Today, parents of infants and toddlers encourage bilingualism or violin lessons at what they hope will be peak synaptic moments, school systems focus more on kindergarten and pre-kindergarten programs, and aging baby boomers download Sudoku apps in an effort to preserve precious neurons.”

These ideas for how to educate young children were not without controversy. Is there in fact a “critical window for learning”? The idea that emerged during this time was to “teach children young so your child has more synapses!” Why do young children have increased capacity to learn and recover language after brain injury? Why do young children learn a second language easily, and without an accent? There is substantial evidence to support a kind of critical window that correlates with pruning in primary sensory regions of the brain, the areas that regulate hearing and vision and are associated with language acquisition. The development of musical ability, like language, is also an early process, for example in the association of perfect pitch with early exposure to music.

Plasticity in the development of language is a particularly intriguing area, since having advanced language skills is a process that in the words of Peter Huttenlocher is “uniquely human.” And, accordingly, language processing in the brain is complex. “Within the human species, we find little evidence for a circumscribed language organ. The whole brain participates in language …,” said Elizabeth Bates in 1993, as quoted in Peter’s book Neural Plasticity [Reference Huttenlocher1]. However, specific regions of the brain had been implicated in language development, as early as the mid-1800s, based on studies of patients with unilateral brain injuries. In 1861, Paul Broca reported a Parisian shoemaker who had suffered a stroke previously and retained the ability to understand but was unable to speak or write due to a lesion in what became known as “Broca’s area” in the frontal lobe. In 1879, Carl Wernicke described patients that retained the ability to speak but were unable to understand language due to lesions on the left back side of the brain, in an area now referred to as Wernicke’s area. This supported the idea that regions of the brain had specialized functions in language. What is particularly interesting is what happens if these unilateral brain injuries occur earlier in life, during a period of substantial plasticity or capability for pruning and refining synapses. In contrast to adults with brain injuries in areas that mediate language and result in the inability to speak or understand language (aphasia), lesions in the same region of the brain of infants or young children often do not lead to significant language deficits. Peter’s collaborator Susan Levine said, in an interview in February 2022, “it is shocking how well kids can do after brain injury to regions that affect language.”

As Peter stated in Neural Plasticity: “There appears to be universal agreement language functions after focal injury such as stroke in infancy are remarkably different from those after stroke in the adult” [Reference Huttenlocher1]. Focal brain lesions in infants prior to the onset of language are not associated with aphasia or language deficits later in life, although there can be some delay in language development. The recovery is remarkable as compared to adult brain injuries. This is due to the significant brain plasticity in young children and represents a “window” in development when there is a high potential for recovery after injury. Later in Peter’s career he became particularly interested in studying the brains of children after injury to further dissect brain plasticity and the potential for recovery using functional MRI. This work was funded as part of a large National Institutes of Health program project grant on longitudinal language on which Janellen served as the overall principal investigator. Peter’s component of the grant was focused on using functional MRI to image the brains of children and to study the impact of brain injury in young children in collaboration with Susan Levine.

Despite clear evidence for early plasticity and the potential for learning during this time even after brain injury, the skeptics were many, including Peter himself. Are the first three years of life, when there is robust synaptic remodeling, important for long-term learning? Peter often noted that “more synapses or a density of synapses in the brain does not mean someone is smarter”; more is not necessarily better. Peter also believed that most types of learning continue throughout life and are not limited to specific early developmental windows. Bruer wrote in a critical commentary in Nature Neuroscience in 2002, “Peter Huttenlocher’s work on changes in synaptic density over the life span has been prominent in media, policy and education articles” to support early intervention [Reference Bruer2]. Bruer referenced other scientists who similarly suggested the importance of early education and its relationship to brain plasticity. But Bruer then wrote: “Unfortunately, in each case, it is the spurious, throwaway speculation, not the sound scientific result, that has captured the public’s attention.” Bruer did not think brain science should be applied to education and child development and referred to it as the “pediatrician’s error.” Peter, normally quiet in dissent, countered with a response, also published in Nature Neuroscience [Reference Huttenlocher3]. Although Peter agreed that learning continues throughout life, he argued that more science was needed to connect brain science and an understanding of how to optimize education experiences for young children. Peter wrote: “Old borders between disciplines are rapidly disappearing, in large part driven by new methods of investigation such as functional magnetic resonance imaging (fMRI). Examples of neuroscience data with practical implications are on the increase.” He went on to say, “Bruer ridicules early intervention. Yet, many recent studies show that early education programs, begun before children are two years old, carry long-lasting benefits in selected populations, including disadvantaged and prematurely born infants.”

Peter and his wife Janellen had long since wedded their passions for understanding brain plasticity and the role of the environment in child development and learning. In the above commentary, Peter noted, “The interface between developmental neuroscience and child development is an exciting area of investigation, driven by new technologies such as functional imaging. Collaboration between neuroscientists, developmental psychologists and educators will best advance this field.”