4.1.2 The brain, hemispheric specialisation and the endocrine system

Explained

1. Hindbrain and midbrain: the old brain.

• These parts are shared with lower animals and keep us alive without thought. Damage to the medulla can stop breathing; this is why a blow to the base of the skull is dangerous.
• The cerebellum stores skills that become automatic (cycling, typing). Alcohol impairs it early, causing the unsteady walk and slurred movements that police roadside tests look for.
• The RAS filters incoming stimuli and keeps the cortex awake; a sleeping parent wakes to the baby's cry but not to traffic. Damage to it can cause coma.

2. Forebrain structures below the cortex.

• Thalamus: sorts sensory messages (vision, hearing, taste, touch) and sends them to the right area of the cortex. Smell goes directly to the olfactory areas.
• Hypothalamus: small (about the size of an almond; the pituitary below it is pea-sized) but powerful. It keeps homeostasis: it starts eating, drinking and shivering or sweating as needed, and it is a centre for emotional and sexual behaviour and aggression. It links the nervous and endocrine systems by controlling the pituitary.
• Limbic system (a ring of structures around the brainstem; Broca named the "limbic lobe" in 1878, Papez described an emotion circuit in 1937, and MacLean named the "limbic system" in 1952): includes the hippocampus, amygdala, cingulate gyrus, and is linked with the hypothalamus.
  • Hippocampus damage (the famous patient H.M., 1953) leaves old memories but prevents new long-term ones (anterograde amnesia).
  • Amygdala damage makes animals tame and fearless; stimulation can trigger rage or fear.

3. The cerebral cortex and its lobes.

• About 2–4 mm thick, but folded into ridges (gyri) and grooves (sulci) so that a large area fits inside the skull. The cerebrum (cortex plus the white matter under it) makes up about 80% of the brain's weight.
• Landmarks: the central sulcus (fissure of Rolando) separates the frontal and parietal lobes; the lateral sulcus (Sylvian fissure) sets off the temporal lobe.

• The motor and sensory strips are laid out like an upside-down map of the body (the "homunculus"); hands, lips and tongue get the largest areas because they need the finest control and sensitivity.
• Association areas (most of the cortex, especially the prefrontal) combine information for thinking, planning and language.
• Phineas Gage (1848): an iron rod damaged his frontal lobe; he survived but became impulsive and irresponsible, showing the frontal lobe's role in personality and self-control.
• Aphasias: damage to Broca's area → slow, effortful speech with good comprehension (expressive aphasia). Damage to Wernicke's area → fluent but meaningless speech with poor comprehension (receptive aphasia).

4. Hemispheric specialisation.

• Lateralisation = the differing roles of the two sides of the brain in a task. It is relative, not absolute: both hemispheres take part in almost every task, sharing information through the corpus callosum (about 200 million fibres).
• Language is in the left hemisphere in about 95% of right-handers and in most (roughly 70%) of left-handers.
• Vision crosses too: what we see to the left of the fixation point (left visual field) reaches the right hemisphere, and vice versa.

5. The split-brain experiments.

• Cutting the corpus callosum (commissurotomy / callosotomy) stopped severe epileptic seizures spreading from one hemisphere to the other. Patients seemed normal in daily life.
• In the laboratory, a word or picture was flashed to one visual field. Shown "spoon" in the right field (left hemisphere), the patient could say "spoon". Shown it in the left field (right hemisphere), he said he saw nothing, yet his left hand could pick out the spoon by touch.
• Conclusion: the left hemisphere controls speech; the right understands simple words and objects but cannot name them; each hemisphere can work with its own knowledge when disconnected.

6. Hemisphericity.

• Hemisphericity = an individual's tendency to rely more on one hemisphere's mode of processing information: left (verbal, logical, analytical), right (non-verbal, intuitive, holistic) or integrated (both equally). It is described as a person's style of learning and thinking.
• It is measured by tests such as Torrance's "Your Style of Learning and Thinking" (SOLAT), used in Indian educational research (Indian adaptation by D. Venkataraman).
• Caution: "left-brained" and "right-brained" people are a popular simplification; in normal people the two hemispheres always work together.

7. The endocrine system and behaviour.

• Hormones act more slowly than nerve impulses but their effects last longer and spread through the whole body. The nervous system is like a phone call; the endocrine system is like a broadcast.
• The hypothalamus → pituitary → target gland chain controls most glands; hormones in the blood feed back to switch the chain off (negative feedback).
• Pituitary. Anterior lobe: growth hormone (too much in childhood → gigantism; in adults → acromegaly; too little → dwarfism), TSH (thyroid), ACTH (adrenal cortex), FSH/LH (gonads), prolactin (milk production). Posterior lobe releases two hormones made in the hypothalamus: ADH / vasopressin (water balance) and oxytocin.
• Oxytocin: uterine contractions in childbirth and milk let-down in breastfeeding; also bonding, trust, social recognition between mother and child and between partners (the "love" or "bonding" hormone).
• Thyroid (thyroxine): sets the body's metabolic rate. Too little (hypothyroidism): sluggish, tired, slow thinking, weight gain, depression; in infancy it causes cretinism (stunted growth and intellectual disability). Too much (hyperthyroidism): restless, anxious, irritable, sleepless, losing weight. Iodine deficiency → goitre.
• Adrenal glands (above the kidneys). The medulla releases adrenaline and noradrenaline in emergencies: faster heart, more blood sugar, alertness; it backs up the sympathetic nervous system. The cortex releases cortisol, the main long-term stress hormone (raises blood sugar, reduces inflammation; prolonged high levels weaken immunity).
• Pancreas: insulin lowers blood sugar, glucagon raises it. Too little insulin → diabetes; too much (low blood sugar) → anxiety, irritability, confusion, fainting.
• Pineal gland: releases melatonin in darkness; it makes us sleepy and keeps the body's daily (circadian) rhythm. Jet lag and night shifts upset it.
• Gonads: testosterone (testes) → male secondary sex characteristics, sex drive, linked to aggression and dominance; estrogen and progesterone (ovaries) → female sex characteristics, menstrual cycle, pregnancy. The adrenal cortex also makes small amounts of sex hormones in both sexes.
• Parathyroid: keeps blood calcium normal; too little parathormone → muscle twitching (tetany), irritability.
• Thymus: helps the immune system mature; large in childhood, shrinks after puberty.