Barrett, Lisa Feldman. _Seven and a Half Lessons about the Brain_. Boston: Houghton Mifflin Harcourt, 2020. # Progressive Summary One of the main themes of the book is that we are not some advanced result of evolution. Other animals are not inferior to us. An example of our need to feel superior is the triune brain theory, which says that being rational is to control our emotions and instincts. But the triune theory is a myth. The main purpose of the brain is to manage and budget our body's resources such as water, salt and glucose. # Key Points The brain did not evolve for thinking. It's main function is to manage our body's energy budgets by making predictions based on past experiences and then picking a behaviour accordingly. Our cerebral cortex doesn't make us special. In fact, other mammals have a big cortex relative to the size of their brains. - There is an instructive explanation of why it's more important to look at relative proportions. If we went to a big house and found a big kitchen, should we assume the people who live there love to cook? Instead, if we went to a small house and found a big kitchen relative to its size, there's probably a better chance that the people there are gourmet chefs. ## Brain as a network The human brain is a network of 128 billion neurons. The brain network is always on. Some signals may be stronger or weaker, but the neurons are always conversing until the day we die. Each neuron passes on signals to a few thousand other neurons, and receives signals from a few thousand others, generating about five hundred trillion connections. Neurons are grouped into clusters. Most connections into the clusters are local. But some clusters are hubs, and they have long-distance connections to other parts of the brain. Hubs allow the brain to operate efficiently without depleting the body budget. ## How the network changes **Fast Changes** Neurotransmitters such as glutamate, serotonin and dopamine help to speed up or slow down signals between synapses. Some neurotransmitters (such as serotonin and dopamine) can act on other neurotransmitters, and are called neuromodulators. Neurotransmitters and neuromodulators allow the brain to take on trillions of different patterns of activity. **Slow Changes** There's a saying that "neurons that fire together, wire together." This expresses the slow changes in wiring called plasticity. Over time, the connections between neurons can grow or die off depending on what is used. *Tuning* strengthens the connections between neurons, especially ones that are important for budgeting the body's resources. Branch-like dendrites become bushier. Trunk-like axons develop a thicker myelin sheath, which makes the signals travel faster. *Pruning* means that less-used connections die off. These processes continue throughout life. Our dendrites keep sprouting new buds. If they aren't tuned within days, then they are pruned. **Degeneracy** The same function can be performed by different sets of neurons. This redundancy is called degeneracy. It reminds me of the stacking functions in permaculture. The human brain takes 25 years to complete its wiring. A human embryo creates twice as many neurons as an adult brain needs. Infant neurons are bushier than an adult ones. # Comments # Quotes > So you don’t have an inner lizard or an emotional beast-brain. There is no such thing as a limbic system dedicated to emotions. And your misnamed neocortex is not a new part; many other vertebrates grow the same neurons that, in some animals, organize into a cerebral cortex if key stages run for long enough. Anything you read or hear that proclaims the human neocortex, cere- bral cortex, or prefrontal cortex to be the root of rationality, or says that the frontal lobe regulates so-called emotional brain areas to keep irrational behavior in check, is simply outdated or woefully incomplete. The triune brain idea and its epic battle between emotion, instinct, and rationality is a modern myth. > Natural selection did not aim itself toward us – we’re just an interesting sort of animal with particular adaptations that helped us survive and reproduce in particular environments. Other animals are not inferior to humans. They are uniquely and effectively adapted to their environments. Your brain is not more evolved than a rat or lizard brain, just differently evolved. > Perhaps rationality is better defined in terms of the brain’s most important job: body budgeting – managing all the water, salt, glucose, and other bodily resources we use every day. In this view, rationality means spending or saving resources to succeed in your immediate environment. > If you’ve heard that the left side of your brain is logical and the right side is creative, that’s just a metaphor. So is the idea that your brain has a “System 1” for quick, instinctive responses and a “System 2” for slower, more thoughtful processing, concepts discussed in the book Thinking, Fast and Slow by psychologist Daniel Kahneman. (Kahneman is very clear that Systems 1 and 2 are metaphors about the mind; but they are often mistaken for brain structures.) Some scientists describe the human mind as a collection of “mental organs” for fear, empathy, jealousy, and other psychological tools that evolved for survival, but the brain itself isn’t structured like that. Your brain also does not “light up” with activity, as if some parts are on and others off. It does not “store” memories like computer files to be retrieved and opened later. These ideas are metaphors that emerged from beliefs about the brain that are now outdated.