Daniel Siegel writes in The Developing Mind,
For the infant and young child, attachment relationships are the major environmental factors that shape the development of the brain during its period of maximal growth. Attachment establishes an interpersonal relationship that helps the immature brain use the mature functions of the parent’s brain to organize its own processes.
To begin to grasp the matter, all we need to do is picture a child who was never smiled at, never spoken to in a warm and loving way, never touched gently, never played with. Then we can ask ourselves: What sort of person do we envision such a child becoming?
Infants require more than the physical presence and attention of the parent. Just as the visual circuits need light waves for their development, the emotional centers of the infant brain, in particular the all-important OFC, require healthy emotional input from the parenting adults. Infants read, react to, and are developmentally influenced by the psychological states of the parents. They are affected by body language: tension in the arms that hold them, tone of voice, joyful or despondent facial expressions, and, yes, the size of the pupils. In a very real sense, the parent’s brain programs the infant’s, and this is why stressed parents will often rear children whose stress apparatus also runs in high gear, no matter how much they love their child and no matter that they strive to do their best.
The electrical activity of the infant’s brain is exquisitely sensitive to that of the nurturing adult. A study at the University of Washington in Seattle compared the brain-wave patterns of two groups of six-month-old infants: one group whose mothers were suffering postpartum depression and one group whose mothers were in normal good spirits. Electroencephalograms (EEG) showed consistent, marked differences between the two groups: the babies of the depressed mothers had EEG patterns characteristic of depression even during interactions with their mothers that were meant to elicit a joyful response. Significantly, these effects were noted only in the frontal areas of the brain, where the centers for the self-regulation of emotion are located. How does this pertain to brain development? Repeatedly firing nerve patterns become wired into the brain and will form part of a person’s habitual responses to the world. In the words of the great Canadian neuroscientist Donald Hebb, “cells that fire together, wire together.” The infants of stressed or depressed parents are likely to encode negative emotional patterns in their brains.
The long-term effect of parental mood on the biology of the child’s brain is illustrated by several studies showing that concentrations of the stress hormone cortisol are elevated in the children of clinically depressed mothers. At age three, the highest cortisol levels were found in those children whose mothers had been depressed during the child’s first year of life, rather than later. Thus we see that the brain is “experience-dependent.” Good experiences lead to healthy brain development, while the absence of good experiences or the presence of bad ones distorts development in essential brain structures. Dr. Rhawn Joseph, a scientist at the Brain Research Laboratory in San Jose, California, explains it this way:
{An} abnormal or impoverished rearing environment can decrease a thousand fold the number of synapses per axon {the long extension from the cell body that conducts electrical impulses toward another neuron}, retard growth and eliminate billions if not trillions of synapses per brain, and result in the preservation of abnormal interconnections which are normally discarded over the course of development.
Since the brain governs mood, emotional self-control, and social behavior, we can expect that the neurological consequences of adverse experiences will lead to deficits in the personal and social lives of people who suffer them in childhood, including, Dr. Joseph continues, “a reduced ability to anticipate consequences or to inhibit irrelevant or inappropriate, self-destructive behaviors.”
Dr. Gabor Maté – In the Realm of Hungry Ghosts: Close Encounters with Addiction p.348
The Sea Is Not Full 