60 parts drawn from 3 sides. For each one: what it does, an everyday example, and what happens when it is damaged. The appendix follows the old circuits that still fire in everyday life.
Side view
The left hemisphere, front to the left. Dashed parts lie hidden beneath the surface.
Midline cut
Split down the middle and seen from the cut, front to the left. Dashed parts lie off the midline.
Top down
From above, front at the top.
Every part
Numbers match the drawings. Under each name: the views it appears in. Lobes and large regions such as the prefrontal cortex light up when you pick them here or tap their number.
Landmarks
Lobes
Frontal lobe
Parietal lobe
Temporal lobe
Occipital lobe
Cingulate cortex
Deep structures
Brainstem and cerebellum
Appendix
Old circuits in a modern life
Responses shaped long ago for a world of small groups, predators and scarce food, and what they do now. Each entry starts with an everyday scene, then follows what happens in the brain, step by step, and why it feels the way it does.
There is no lizard brain. The old idea of a reptile core wrapped in newer layers has been dropped by neuroscience. The amygdala and the cortex evolved together and work as 1 system. "Old circuits" here means responses shaped long ago, now meeting a very different world.
Walking past a group of people
The scene
You walk past a table of strangers outside a café. One of them looks up, then says something to the others, and they laugh.
Evolved for
For most of human history people lived in small groups. Being accepted meant food, protection and a partner; being rejected could mean death. Paying close attention to what others thought of you kept you alive.
What happens, in order
Why it feels like this
Your body is getting ready to fight or run, but the threat is social, so the energy has nowhere to go. That is the racing heart, the hot face and the urge to walk faster. It feels personal because what is at stake is your standing. In fact people notice you far less than it feels: in the "spotlight effect" studies, students made to wear an embarrassing T-shirt guessed that about twice as many people had noticed it as really had.
Evidence
A review of 208 laboratory studies found the biggest cortisol rises when people were judged by others in a situation they could not control (Dickerson and Kemeny 2004). The amygdala reacts to threatening faces even when they are flashed too fast to see (Morris, Öhman and Dolan 1998). The spotlight effect: Gilovich, Medvec and Savitsky 2000. Why we evolved this way is a widely held account rather than proven fact.Widely held account
A sudden bang
The scene
A door slams behind you in a quiet office.
Evolved for
Protecting the body from a blow or an attack before there is time to think.
What happens, in order
Why it feels like this
You jump before you know why, then feel a jolt of adrenaline and often a moment of embarrassment. The reflex is built to be fast, not accurate.
Evidence
The startle to a sudden sound runs through a short circuit in the lower brainstem, and fear makes it stronger (Koch 1999).
A phone notification
The scene
Your phone buzzes on the desk while you are trying to finish a piece of work.
Evolved for
Learning which places and actions pay off, in a world where food and company turned up unpredictably.
What happens, in order
Why it feels like this
The pull is wanting, not liking. You feel driven to check even though most messages are dull. Rewards that come at random keep the urge strongest, the same pattern that keeps people at a slot machine.
Evidence
Dopamine cells signal the difference between the reward you expected and the one you got (Schultz, Dayan and Montague 1997). Dopamine drives wanting more than pleasure itself (Berridge and Robinson 1998). Applying this to phones is a widely held account.Widely held account
Craving sugar and fat
The scene
You have just finished a big dinner, and someone brings out a chocolate cake.
Evolved for
Surviving long stretches when food was scarce, by prizing sweet, fatty, high-energy food whenever it appeared.
What happens, in order
Why it feels like this
Sweet and fatty foods feel rewarding because for most of human history they were rare. Wanting can outrun hunger, which is why there is always room for dessert.
Evidence
Taste is processed in the insula and value in the orbitofrontal cortex (StatPearls neuroanatomy series). The scarcity explanation is a widely held evolutionary account.Widely held account
Being left out
The scene
You see photos of a party your friends held, and you were not invited.
Evolved for
Staying in the group. A painful alarm at the first signs of being excluded pushed people to repair their relationships before they were cast out.
What happens, in order
Why it feels like this
We call rejection "hurt" because it can feel like physical pain. Part of the brain's system for the unpleasantness of pain may be shared between the 2.
Evidence
In a ball-tossing computer game, being left out raised activity in the anterior cingulate, and that activity tracked how upset people felt (Eisenberger, Lieberman and Williams 2003). In 1 study, 3 weeks of paracetamol reduced daily reports of hurt feelings (DeWall et al. 2010). Whether the anterior cingulate is really specific to pain is disputed (Wager et al. 2016).Disputed
Disgust at spoiled food
The scene
You open the milk and it smells sour.
Evolved for
Avoiding infection before it happens, by pulling back from rot, waste and signs of illness.
What happens, in order
Why it feels like this
Disgust makes you pull back, wrinkle your nose and lose your appetite. It errs on the side of caution, so it also fires at things that are perfectly safe.
Evidence
Smelling something disgusting and watching someone else's disgusted face activate the same part of the insula (Wicker et al. 2003). The behavioural immune system account is reviewed by Schaller 2011.Widely held account
Seeing faces in things
The scene
The front of a car, with its headlights and grille, looks like it is smiling at you.
Evolved for
Spotting faces, friend or foe, as fast as possible. Mistaking a rock for a face cost little; missing a real face could cost a lot.
What happens, in order
Why it feels like this
Once you have seen the face, it is almost impossible to unsee. The face detector is set to fire easily.
Evidence
The fusiform face area was described by Kanwisher, McDermott and Chun 1997. When people thought they saw faces in pure visual noise, the right fusiform face area responded specifically (Liu et al. 2014). The cost-of-mistakes explanation is a widely held account.Widely held account
Jumping at a snake-shaped stick
The scene
On a woodland walk you leap sideways from a curved stick on the path.
Evolved for
Primates have lived alongside venomous snakes for tens of millions of years. Spotting one fast paid off.
What happens, in order
Why it feels like this
You react first and realise afterwards. The fast route trades accuracy for speed, so false alarms are the price of never missing a real snake.
Evidence
A fast route from the thalamus to the amygdala, alongside a slower one through the cortex, is described by LeDoux 2000. In monkeys, cells in the thalamus respond fastest and most strongly to pictures of snakes (Van Le et al. 2013). People learn to fear snakes and spiders more readily than other objects (Öhman and Mineka 2001).
Comparing yourself online
The scene
You scroll past a former classmate's promotion and holiday photos on the same evening.
Evolved for
Keeping track of your rank in a group small enough that you knew everyone, so you knew whom to defer to and when to compete.
What happens, in order
Why it feels like this
A good result can feel bad if someone near you did better. A feed shows you the best moments of thousands of people, far more comparisons than any small group ever offered.
Evidence
When 2 people were paid in neighbouring brain scanners, the reward response in the striatum depended on what the other person got (Fliessbach et al. 2007). The figure of about 150 people in a natural human group comes from Dunbar's work on primate brain size and is debated. The mismatch with social media is a widely held account.Widely held account
Worrying at 3 am
The scene
You wake at 3 am and a small problem at work suddenly feels like a disaster. By breakfast it seems manageable again.
Evolved for
No circuit was built for night-time worry. Several systems overlap: cortisol starts rising in the early morning to get you ready to wake, and a tired brain keeps emotions in check less well.
What happens, in order
Why it feels like this
Problems feel bigger at night because there is less to distract you and less control from the front of the brain. Daylight and sleep restore both.
Evidence
After a night without sleep, the amygdala reacted about 60% more to unpleasant pictures, and its link with the front of the brain was weaker (Yoo et al. 2007). The full 3 am explanation is a widely held account.Widely held account
This page explains; it does not diagnose. If anxiety stops you doing things you want to do, a doctor or psychologist can help, and it responds well to treatment.
About this drawing
An original drawing, not traced from a scan or an atlas. The main folds and grooves are placed as in a typical adult brain; the small folds are drawn for texture and vary from person to person. Structures that sit off the cut or under the surface are drawn dashed where they would project.
Sources: the StatPearls neuroanatomy series (NCBI Bookshelf); Dickerson and Kemeny 2004, Psychol Bull; Morris, Öhman and Dolan 1998, Nature; Gilovich, Medvec and Savitsky 2000, J Pers Soc Psychol; Koch 1999, Prog Neurobiol; Schultz, Dayan and Montague 1997, Science; Berridge and Robinson 1998, Brain Res Rev; Eisenberger, Lieberman and Williams 2003, Science; DeWall et al. 2010, Psychol Sci; Wager et al. 2016, PNAS; Wicker et al. 2003, Neuron; Schaller 2011, Phil Trans R Soc B; Kanwisher, McDermott and Chun 1997, J Neurosci; Liu et al. 2014, Cortex; LeDoux 2000, Annu Rev Neurosci; Van Le et al. 2013, PNAS; Öhman and Mineka 2001, Psychol Rev; Fliessbach et al. 2007, Science; Dunbar 1992, J Hum Evol; Yoo et al. 2007, Curr Biol; Ulrich-Lai and Herman 2009, Nat Rev Neurosci. Damage cases: Damasio et al. 1994, Science (Phineas Gage); Dronkers et al. 2007, Brain (Broca's patients); Scoville and Milner 1957 (Henry Molaison); Feinstein et al. 2011, Curr Biol (patient SM); Naqvi et al. 2007, Science (insula and smoking); Gazzaniga 2005, Nat Rev Neurosci (split brain); Bisiach and Luzzatti 1978, Cortex (neglect); Herculano-Houzel 2009, Front Hum Neurosci (neuron counts).