Connectome is the name for the map of the quadrillion connections between all the neurons in a human brain, unique to each individual. This sub-microscopically detailed pattern represents all of a person’s knowledge, memories, and experiences.

Consciousness is the state of sentient awareness during wakefulness which shapes an individual’s sense of self and control. From a neurological perspective, Eagleman shows how consciousness steps in when the unexpected happens, settles internal conflicts, and maintains the system’s long-term orientation, leaving routine biological operations to run without its involvement.

Eusociality is the instinct for humans to bond together into cooperative groups, irrespective of kinship. It provides a survival advantage that allows for the building of tribes and nations, but also creates the potential for conflict between the ingroup and those perceived as outsiders. Eagleman explores the role of eusociality on the brain’s development and physical response to social stimuli.

Borrowing from technological phraseology, Eagleman uses the term “hardwired” to describe behaviors and instincts that are genetically preprogrammed, as is common in many animal species. This prearranged brain wiring allows for rapid development but trades off with the flexibility to adapt to new environments.

The internal model is the brain’s constantly running, predictive model of the world, generated before it receives sensory information. Instead of building a sense of reality from scratch in each moment, the brain compares incoming data from the senses to this model, primarily processing the difference between expectation and reality. Eagleman uses this to explain why we can have rich visual experiences even with our eyes closed, such as in dreams.

“Livewired” is the author’s term to describe the evolution of the human brain as unfinished at birth, “allow[ing] itself to be shaped by the details of life experience” (6). This contrasts with the “hardwired” brains of other animals, and explains why humans can adapt, learn throughout their lives, and thrive across radically different environments.

The pain matrix is a network of several brain areas that activate in concert during the emotional experience of pain. Eagleman also discusses how this same network activates when observing someone else in pain; registering another person’s pain as physically meaningful, which is how empathy has a physical basis in the brain’s architecture.

Plasticity is the brain’s ability to reconfigure its own circuitry in response to experience and to retain those changes. Although this property is particularly associated with childhood, adult brains also demonstrate plasticity through learning new skills. Eagleman uses the example of London cabbies memorizing the city map, which results in the physical enlargement of those brain regions used for this skill.

Procedural memory is a long-term memory enabling an automatized skill that has been physically “hardwired” into the brain’s circuitry through practice. Once a skill, like riding a bicycle or cup stacking, becomes procedural, it can be performed rapidly and efficiently, without conscious effort.

Proprioception is the sensory feedback from receptors in the muscles, tendons, and joints that informs the brain about the position of one’s own limbs. This capacity, which we rarely appreciate, allows for the automatic and seamless coordination of movement. Eagleman explores proprioception as a means to distinguish between the conscious and unconscious mind, especially to emphasize the numerous hidden actions of the unconscious.

Sensory substitution is a neuroscience technique for feeding sensory information into the brain through an unusual channel, such as converting a video feed into vibrations on the tongue. In such cases, the brain can learn to read the unfamiliar signal and extract usable information from it, regardless of which sense originally carried the data. Eagleman discusses this technique as a basis to hypothesize on further, future interfaces between technology and the biological brain.

“Soft paternalism” describes an approach used in public policy based on the idea that small “nudges” in the environment can gently guide the unconscious brain toward better decisions about health, wealth, and happiness. This approach can change behavior without requiring compliance monitoring or penalties. Eagleman explores the effects of soft paternalism as a means to shape human behavior without punishments such as incarceration.

Synesthesia is a perceptual condition in which senses are blended, causing experiences such as seeing colors while hearing music or “tasting” words. The author presents this phenomenon as an example of how brains can be wired differently between individuals, leading to measurably different subjective realities.

Syndrome E is a diagnostic framework proposed by neurosurgeon Itzhak Fried to describe the psychological state that allows perpetrators of genocide to commit repetitive acts of violence. Those in this state show markedly reduced emotional reactivity and diminished empathy toward victims, a combination Fried argues produces moral disengagement and makes sustained atrocity possible. Eagleman explores the idea of Syndrome E in his discussion of personal choice, physical determinism, and culpability.

A famous ethical thought experiment which gained popularity in the early 1900s, the trolley dilemma poses scenarios where, for instance, a runaway trolley can either be left to kill five people or diverted to kill one, asking philosophical questions around intervention and the respective value of life. Eagleman uses this experiment to reveal the brain’s competing systems for decision-making: Individual’s different responses demonstrate how decision-making can be variously dominated by either rational, logical networks (treating an ethical dilemma like a math problem) or emotional networks (reacting to the gut feeling of a personal violation).