" The human brain is the most complex entity in the universe. It has between fifty and one hundred billion nerve cells, or neurons, each branched to form thousands of possible connections with other nerve cells. It has been estimated that laid end to end, the nerve cables of a single human brain would extend into a line several hundred thousand miles long. The total number of connections, or synapses, is in the trillions. The parallel and simultaneous activity of innumerable brain circuits, and networks of circuits, produces millions of firing patterns each and every second of our lives. The brain has well been described as “a supersystcm of systems.”
Even though fully half of the roughly hundred thousand genes in the human organism are dedicated to the central nervous system, the genetic code simply cannot carry enough information to predetermine the infinite number of potential brain circuits. For this reason alone, biological heredity could not by itself account for the densely intertwined psychology and neurophysiology of attention deficit disorder.
Experience in the world determines the fine wiring of the brain. As the neurologist and neuroscientist Antonio Damasio puts it, “Much of each brain’s circuitry, at any given moment in adult life, is individual and unique, truly reflective of that particular organism’s history and circumstances.” This is no less true of children and infants. Not even in the brains of genetically identical twins will the same patterns be found in the shape of nerve cells or the numbers and configuration of their synapses with other neurons.
The microcircuitry of the brain is formatted by influences during the first few years of life, a period when the human brain undergoes astonishingly rapid growth. Five-sixths of the branching of nerve cells in the brain occurs after birth. At times in the first year of life, new synapses are being established at a rate of three billion a second. In large part, each infant’s individual experiences in the early years determine which brain structures will develop and how well, and which nerve centers will be connected with which other nerve centers, and establish the networks controlling behavior.
The intricately programmed interactions between heredity and environment that make for the development of the human brain are determined by a “fantastic, almost surrealistically complex choreography,” in the apt phrase of Dr. J. S. Grotstein of the department of psychiatry at UCLA. Attention deficit disorder results from the miswiring of brain circuits, in susceptible infants, during this crucial period of growth. "
― Gabor Maté , Scattered Minds: The Origins and Healing of Attention Deficit Disorder
