Early on, we pass through a maze of critical periods, windows of opportunity when both brain and immune system are particularly sensive to specific types of information. Once a critical period has passed, a pattern of behavioral or immunological response becomes instilled. Like accents, we develop pre-dispositions that last for life. The brain has chosen what ensembles it will wear for life.
After the age of three synaptic connections and even entire neural pathways are progressively culled. Our subconscious minds are continually choosing which neural connections are unnecessary and eliminating them, mostly during REM sleep. After the age of three we begin losing synaptic connections, and fast.
Counterintuitively, as we soak up more and more information the number of synapstic connections in our brains declines tenfold, from a thousand trillion at the age of three to approximately 100 trillion in adulthood. By the age of three we’ve already decided which cards we’ll play and have sloughed off the rest.
In animals, we call this process imprinting. Through the ages of 12 to 30 groups of neurons and their pathways make their connections stronger, with certain groups becoming more widely connected than others. The strength of neurons that grew and survived during critical periods – and their connections – continues to grow. As we specialize, we become more adept at social skills and professional thinking. If we make it through adolescence, odds are we’ve learned some degree of emotional control and risk avoidance. The inherited tags on our epigenome can directly influence this process.
Until 1992, the coming together process of genes/DNA (nature) and experience (nurture) seemed like tidy way of looking at things. Biological development, genetic diseases, and adult psychology were believed to be determined by the interplay of our genes, mothering, and early learning. Nature/Nurture explained recurrent health problems, the way we process information, natural selection as theorized by Darwin, and the subconscious drives as described by Freud. According to this nature/nurture view “the die is cast” during childhood.