How the brain is built

From birth to age 5, a child’s brain develops more than at any other time in life. And early brain development has a lasting impact on a child’s ability to learn and succeed in school and life. The quality of a child’s experiences in the first few years of life – positive or negative – helps shape how their brain develops. An excellent, open access review on the common mechanisms used to development the framework for both seeing and hearing.  It's obviously "heavy on neuroscience," but most readers interested in how the brain is built will learn a lot.

You will learn that the brain is not...repeat, NOT!...a "blank slate," from which individual experience then carves the specialized circuits that make humans a uniquely intelligent creature.  Instead, an inherited network of developmental genes ensures that hundreds or perhaps even thousands of specialized neurons are directed to specialized brain regions (e.g., sent to the visual cortex).  Because of where they are directed plus their unique properties, these neurons form synapses.  That is, they connect with one another to form the circuits (i.e. "wiring diagrams") that give us vision and hearing.

That fundamental, gene-directed circuitry constitutes a framework, which thereafter is modified via two types of plasticity: organizational and activational plasticity.  Organizational plasticity ensures massive movement of circuits; connections are broken and new ones made elsewhere.  This type of plasticity is experienced by the congenitally blind.  For these people, something (many possibilities exist) has gone wrong so that the "genetically intended" framework is replaced by something quite different.

The other type of plasticity, activational plasticity, is also under genetic guidance.  Many genes controlling neuronal function in brain regions controlling sensory functions have evolved to "listen" for certain environmental signals (e.g., not enough growth factors, called neurotrophins).  If they receive the signal, the fundamental framework is slightly altered.  If they receive a different signal (or scarcity of neurotrophins), the framework is slightly altered.  All of this happens because of individual experience.

This review studies only vision and hearing, two sensory regions, but the mechanisms are the same for brain regions that give us thinking, planning, and thoughts of the future: the nonsensory neocortex.

The neocortex's circuitry is framed by inherited developmental genes, and those environmental signals that result in plasticity (probably; work is ongoing) work through the brain's specialized immune system (microglia) to carve circuits that give us things like universal grammar, ease of language learning, theory of mind, intuitive supernatural agency, and various instinctive knowledge of physical laws.  Those are not learned, but rather are under genetic control in the same manner as the circuits discussed above.

Finally, so many development genes are involved in the above mechanisms it is inevitable that we all differ in our "suite" of such genes (our genotypes vary).  In fact, our DNA has "hotspots" that have evolved to mutate more often so that a greater variety of "brain building genes" results.  Natural selection requires such variety in order to become more and more "fit" (a specialized, evolutionary term with a specific meaning; don't think "fit" as in suited for the gym).  Thus, we all inherit some slight (or, in some cases like schizophrenia and autism, significant) differences in the framework that our individual experiences have to work with.  As you can imagine, if the framework is different, then the same signal received by two people will result in a slightly (or greatly) modified post-experience brain circuit.  Thus, we all behave differently...in some part...because we inherited different genes.