Infants are born with far more synapses than they will need as adults.
Neural network pathways that are active while learning language, music, reading, writing, and playing sports, among other skills, are biologically reinforced by modifying synapses. Unused synapses are cannibalized to save precious metabolic energy. Psychological development literally, physically, sculpts the brain in addition to modifying synapses and thereby changes biology!
Psychology normally changes our synapses by activating internal experience-dependent plasticity mechanisms.
Our brains physically specialize as we develop psychologically. This explains why it is more difficult for older people to learn a new language.
Our neural network understanding of how psychology changes biology prepares us to understand how biology modifies psychology.
Understanding that the synapses that connect neurons contain our memories of who we are, the people that we know, the experiences we have had, and our attitudes about everyone and everything along with how we feel enables us to see that directly modifying them with legal or illegal psychoactive substances will change our psychology.
Drugs directly modify synapses pharmacologically and consequently alters our psychology.
Pharmacological psychiatry is a relatively young field. The clinical practice of selecting the right medication to make therapeutic synaptic modifications is, by and large, a trial and error business. It can take several weeks for therapeutic effects to be noticed. Therapeutic effects are often dose dependent which means that dosage may need to be systematically increased.
Neural network models enable us to see how psychology changes biology because the memory formation process that drives learning and all psychological development modifies synapses through experience-dependent plasticity neuroscience mechanisms.
This knowledge enables to understand that modifying synapses pharmacologically will also change our psychology. The causal role of synapses in learning and memory make them the missing link in psychobiology and biopsychology.
Psychology will organize around the synapse when it becomes a mature natural science just as biology organized around the cell when it became a mature natural science.