Building a Brain

The human brain is an incredibly complex structure, often considered the most complex object in the universe. Its development differs from other organs; it doesn't merely increase in size but undergoes significant changes throughout childhood and adolescence. These formative years are crucial for "imprinting," where experiences shape future behaviors, akin to how baby chicks imprint on a mother hen. The speaker's early experiences led to an interest in neuroscience and medicine.

The speaker grew up in Greenwich, Connecticut, surrounded by influential figures. His parents, immigrants from England, chose Greenwich for its proximity to Manhattan. A pivotal moment was dissecting a fetal pig in ninth-grade biology. While some classmates were repulsed, the speaker found it fascinating, marking a divergence between those inclined towards science and other fields.

The injected latex highlighted the pigs' veins and arteries. The speaker's visual-spatial ability, crucial in neurology and neuroscience, allows for mentally mapping the brain's three-dimensional structure and connections. This skill is invaluable for neurologists in diagnosing the location of strokes or brain injuries based on neurological symptoms. Neurologists and neuroscientists often seek patterns. The speaker's interest in neuroscience predates advanced imaging like CT scans and MRIs, requiring doctors to visualize the brain three-dimensionally.

The teenage brain is a puzzle. This book aims to explain the teen brain and its development. At birth, the brain is only about 40% of its adult size, with internal wiring still undergoing significant changes. The adolescent brain presents a paradox: an abundance of gray matter (neurons) but a deficit of white matter (connective wiring). It's like a new Ferrari, powerful but lacking direction. This neurological immaturity contrasts with societal expectations based on physical appearance, leading to mixed messages.

Nature builds the brain from back to front, starting with structures that manage the interaction with the environment and regulate sensory processes like vision, hearing, balance, touch, and spatial awareness. These include the cerebellum (balance and coordination), thalamus (sensory relay), and hypothalamus (body function regulation, including hunger, thirst, sex, and aggression).

The brain, light gray, has a consistency between overcooked pasta and Jell-O, weighing about three pounds and the size of two fists. Gray matter houses neurons responsible for thought, perception, motion, and bodily control. Neurons connect through white matter. Magnetic resonance imaging (MRI) distinguishes between gray and white matter. The brain's surface has valleys (sulci) and ridges (gyri). Figure 1 displays an MRI scan. The brain has two hemispheres. The cortex, the outermost layer, consists of gray matter, with white matter beneath. Neurons connect locally, but to reach other brain regions, hemispheres, or the spinal cord, they extend processes through the white matter. Myelin, a fatty insulator, coats these processes, giving white matter its color.

Brain size and weight aren't directly proportional to intellect. Whale and elephant brains are larger than human brains. The ratio of brain weight to body weight is higher in dwarf monkeys than in humans. Brain complexity matters. Albert Einstein's brain was slightly underweight but had more connections per gram. The size of the human brain is limited by the skull.

As a neurologist, monitoring children's head size growth is essential. Skull size is a crude measurement that can vary. The skull limits brain size. Eight cranial bones protect the brain. At birth, these bones are loosely connected, allowing compression during birth. The "soft spot" closes during the first year as bones fuse. Head size increases most from birth to seven years, with the largest increase in the first year due to brain development.

Evolution has maximized brain matter within the fixed skull size. Homo erectus had smaller brains (800-900 cubic centimeters) compared to modern Homo sapiens (approximately 1,500 cubic centimeters). The skull and female pelvis adapted to accommodate larger brains. The brain's design appears updated on the fly, resulting in cramped conditions and folded structures (gyri and sulci). Humans have the most complex brain folding. Simpler mammals have fewer folds. The smoother the surface, the simpler the brain.

While the brain appears symmetrical, internal differences exist. The right brain controls the left body side and vice versa. Visual and spatial perception is more prominent on the right side. The body is mapped on the brain's surface as the "homunculus." The face, lips, tongue, and fingertips get more space due to functional importance.

Wilder Penfield, a neuroscientist, created the cortical map (homunculus) by stimulating brain areas during surgery to remove epileptic seizure sources. The brain area for a body part depends on its functional complexity. The hands, fingers, lips, and mouth have ten times more area than the back. Brain regions for the same body part are close together.

The speaker's undergraduate thesis explored the brain areas devoted to body parts and whether overstimulation of a limb increases the corresponding brain area. This was an early experiment in brain plasticity. Research since the 1970s supports imprinting. David Hubel and Torsten Wiesel's work showed that kittens raised with a patched eye couldn't see out of that eye later, and the brain area for the patched eye was taken over by the open eye's connections. Kittens raised in environments with vertical lines only responded to vertical lines as adults. Stimuli during brain development alter its function. The speaker's experiment showed similar effects for touch.

The family's cat, Jill, was massaged on her paws and started using them more than other cats. She often used her left paw to eat, suggesting they stimulated her left paw more. If scientists could look into her brain, they would have seen that she had more brain space given over to her paws, and especially her left paw, than the average cat. This reallocation of brain space based on experience during life happens in people, too. This period is called the critical period, when “nurture,