Education, brain plasticity and neuronal recycling

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When it comes to learning, it's pointless to pit innate against acquired, environment against heredity. As early as 1949, Canadian psychologist Donald Hebb stated:

" Two factors determine intellectual growth: innate potential, which is absolutely essential, and a stimulating environment, which is just as essential. There's no need to ask which is more important. We could assume that intelligence grows up to the limit set by heredity or by the environment - the minimum of both. In a perfect environment, it's the innate structure that sets the pace; but starting from a heredity of genius, it's the environment that dominates. "

Contemporary neuroscience confirms that there is no contradiction in asserting, simultaneously, the genetic origin of the main circuits of the human brain, and their capacity to modify themselves under the effect of learning rules, themselves governed by innate cellular and molecular mechanisms. Education takes advantage of the innate plasticity of certain brain circuits, which is maximal in children. Even a large pre- or post-natal lesion, affecting almost an entire cerebral hemisphere, when it occurs at an early age, may have only limited consequences on the development of language, visuo-spatial skills, motor skills and even visual maps. Without radically disrupting the organization of brain circuits, plasticity can redirect cerebral functions towards cortical circuits close to or symmetrical with those usually involved. A similar lesion in adulthood leads to far more dramatic consequences: hemiplegia, blind visual field, aphasia, etc.

But what exactly do we mean by brain plasticity? Over the last twenty years, numerous experiments have shown that learning is based on the reinforcement or, on the contrary, the elimination of synapses. These points of contact between neurons, which define the networks by which information is encoded and relayed, constitute the memory traces of our experiences and modify the behavior of our neurons. Recent imaging methods demonstrate that these synapses are constantly being remodeled. Neuronal activity, or the lack of it, selectively modulates synapse stability. Dendritic spines, the structures that host excitatory synapses on cortical pyramidal neurons, appear or disappear on relatively rapid timescales, from seconds to days. Axon branching is also reorganized, albeit less rapidly. Finally, axon myelination can also be altered by use.

These changes take place throughout life, but with particular intensity in childhood. A "critical period" or "sensitive period" is a window of time during which neuronal circuits display a particular capacity to adapt to the inputs they receive from their environment. The existence of critical periods is attested in the visual system: in the absence of binocular vision in the first months or years of life, visual inputs from one eye cease to be used efficiently (amblyopia). There is also a hierarchy of critical periods in other areas, most notably language. The ability to learn a second language declines steadily with age, particularly at puberty. The ability to discriminate the phonemes of the mother tongue is only present if the child has interacted with a speaker of that language before the age of 12 months. Deaf children need to be fitted with a cochlear implant as soon as possible (before the age of 3 and a half, or even 18 months according to some data), otherwise their spoken language skills decline sharply.

The opening and closing of critical periods is closely linked to the maturation of inhibitory circuits based on the neurotransmitter GABA. PV cells (expressing parvalbumin) are large inhibitory neurons, also known as basketcells, found in the cortex and hippocampus. Their maturation plays an essential role in the closure of plasticity by modulating the excitation/inhibition ratio. During cerebral maturation, these neurons become surrounded by fixed peri-neuronal networks (proteins + sugars), which limits plasticity in adulthood. However, these phenomena depend not only on age, but also on environmental and pharmacological factors. The study of plasticity in the mouse hippocampus shows that, even in adulthood, enrichment of the environment considerably increases learning capacity, while conversely, conditioned fear reduces it. These modulations of learning ability are linked to a change in the fraction of inhibitory synapses on PV cells.

Can we extrapolate these results to the human species? The example of Romanian orphanages suggests so. After spending their first months or years of life in an environment devoid of stimulation and social interaction, orphans develop irreversible abnormalities in gray matter volume, with major consequences in many areas of language and learning. However, these cognitive disorders, if treated promptly, are not irreversible. The Bucharest Early Intervention Project led by Chuck Nelson demonstrates the importance of early family placement. Children placed with families show clear advantages in physical and mental development, particularly in the areas of language and social skills. The earlier the placement, the greater the effect. In many areas, no abnormalities are detected when children are placed before 20 months.

How can we theorize these massive yet constrained effects of environmental exposure? To understand how brain circuits with a high degree of innate organization, inherited from our evolution, nevertheless manage to reorient themselves to acquire new skills, I have proposed the neuronal recycling hypothesis. This underlines the importance of anatomical constraints inherited from our evolution. Neural maps and organized brain circuits are present from infancy and bias learning. New cultural acquisitions are only possible insofar as they are compatible with these pre-existing neural architectures, which they recycle. Each cultural object must therefore find its "neural niche" in the brain: a circuit whose initial role is close enough, and whose flexibility is sufficient to be reconverted to this new use. Each circuit has intrinsic properties that make it more or less suitable for its new use. These properties constrain cultural forms and give them universal traits. We all learn to read or calculate using the same brain circuits. That's why it's essential to understand how these circuits are organized in early childhood, and how they are transformed by education, in order to optimize their functioning. Education takes advantage of the very long plasticity window of the human species, but it is not infinite and obeys precise rules.

What are the educational consequences of these discoveries? Since children's brains are plastic and equipped with powerful statistical inference algorithms, families and schools must provide them with an enriched, structured and stimulating environment from an early age. As brain plasticity is modulated, positively by environmental enrichment, and negatively by fear and negative emotions, this home and school environment must be varied, rich in positive reinforcement, and free from fear. Parents and teachers also need to know that even massive brain damage can be partially overcome, so they must not give in to discouragement. However, plasticity should not be overestimated either. Children's brains are structured from birth, giving them deep intuitions about objects, space, numbers or language, but also imposing limits on learning. All children will face similar difficulties. There are universal biological limits to brain plasticity. The existence of critical periods means that certain stimuli, particularly exposure to languages, must be provided at an early stage.