Inside your head is an object capable of feats of computation, creativity and understanding unrivalled in the known universe – and all using the power of a 20-watt light bulb.
We have made huge strides in understanding the human brain. In recent years, we have discovered that brain cells can regenerate and pinned down what happens when you start talking before you know what you want to say. Yet, the more we learn, the more we realise how much we still don’t know. In the following pages, we explore the biggest questions about the brain to reveal the mechanisms and mysteries of this phenomenal blob of grey goo.
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What makes our brain special?
The human brain, we love to tell ourselves, is exceptional. Other animals might use tools or solve mazes, but can they invent computers or write sonnets?
Yet even with our extraordinary mental prowess, it isn’t easy to explain what makes the human brain so special. At around 1.5 kilograms, our brains are about a third the weight of an elephant’s and a fifth that of a sperm whale.
If body size is taken into account, however, our brains are unusually large: between seven and eight times what would be expected for a mammal our size. But this crude measure isn’t enough to explain our intelligence. The brain-to-body-size ratio of a capuchin monkey is higher than that of a gorilla, yet gorillas are considered smarter.
Clearly size isn’t everything. A more important metric might be the number of neurons – the brain’s processing units. Humans have about 86 billion, according to Suzana Herculano-Houzel of Vanderbilt University in Tennessee, who has pioneered methods to count them.
In fact, primate brains have more neurons than other mammal brains of comparable size. Humans, having the largest brain of any primate, also have the most neurons of any primate, and probably any animal. This was possibly enabled by the invention of cooking, which releases more calories from food to fuel this energetically expensive organ.
It isn’t only the number of neurons that matters, but also where they are. Our remarkable abilities probably stem from having more neurons in our cerebral cortex – the brain’s wrinkly outermost layer – than other animals, says Herculano-Houzel. This structure allows us to develop more complex behaviours rather than simply responding to stimuli. “If you have a cortex, you are no longer a slave to what happens around you. You have the flexibility to choose to do things otherwise,” says Herculano-Houzel.
What’s more, her team recently discovered that across warm-blooded animals, the number of neurons in the cortex . Herculano-Houzel thinks this is also a factor in our cognitive superiority: humans take years to reach maturity. “We take a really long time to put that brain together,” she says. “All the while, you are assimilating information from the world.”
Neurons aren’t the whole story: cells called astrocytes also play an important role in intelligence (see “What makes a brain”). But the sheer computing power afforded by our 16 billion cortical neurons is likely to be the critical factor behind our cognitive dominance.
Sam Wong
What is consciousness?
Think of the conscious mind as a furnace. If you are deeply asleep, the flame of consciousness has died down to a low but persistent level. In REM sleep, when you dream, the flame is jumping and burning brightly but erratically. In a coma, it is a glowing ember.
Consciousness, in other words, exists in a range of states. One explanation for this is that full consciousness erupts when many parts of the brain broadcast information to a network of neurons known as the global workspace. When this broadcast doesn’t take place, sensations remain subconscious. When the broadcast is incomplete, you get different levels of consciousness, such as when you are dreaming or have had a blow to the head.
By studying these states, we should one day be able to pinpoint the brain mechanisms that give rise to consciousness. So why then, has explaining consciousness – how a kilogram or so of nerve cells conjures up the swirl of thoughts and emotions that make up our mental experience – been dubbed “the hard problem”? One reason is that philosophers have focused on explaining how we become aware of experiences. Their term “qualia” describes the properties of experiences we have, such as the redness of a strawberry or the perception of the taste of wine. Trying to find explanations for qualia has caused no end of confusion among neuroscientists.
“If our brain is a smartphone, consciousness is the screen”
One solution to the problem is to ignore it. “‘Qualia’ is a term of art, introduced by philosophers who want to make the questions about the nature of consciousness answerable only by spooky, non-biological accounts,” says Patricia Churchland at the University of California, San Diego. After all, we don’t normally talk about our qualia, we talk about things such as being tired, needing to eat or even being in love – feelings that have a straightforward, non-spooky biological origin.
Most people aren’t aware of their qualia until they are prompted by philosophers, says Daniel Dennett of Tufts University, Massachusetts. Dennett not only thinks that there is no hard problem, he believes consciousness itself is a kind of illusion. “It takes some substantial coaxing and cajoling to get people to ‘notice’ their qualia,” he says, “and when they think they do notice them, they are falling for another illusion.”
The illusion, according to Dennett, is this: each of us believes that we have privileged access to some remarkable properties of our own mental states, with which we are intimately acquainted and which we perceive as experiences. But the brain presents to our consciousness only what matters to us, and does so in a way we can understand. For example, it is why we see things as being a particular colour. The real world isn’t like that, but our visual systems effectively colour-code the world for us, to simplify it.
Illusions of the mind
Dennett argues that much as colour is an illusion created by the brain, so too is consciousness. “Consciousness is a user illusion designed by evolution to make life easier for the brain that must guide a body through a perilous life,” he says.
Smartphone designers call the screen of a phone a user illusion. The screen is the interface to the computer underneath, but the icons on it – such as the envelope that depicts a message – are symbolic, and don’t bear any relation to the actual hardware and software of the phone’s messaging system. If our brain is a smartphone, consciousness is the screen, our interface to the brain. However, as Churchland says, the metaphor isn’t exact. When we feel dizzy or pinpoint where a sound is coming from, for example, it is the result of physical processes in the brain. Perhaps consciousness is more like a smartphone screen that presents different apps according to the amount of battery left or how much it has been shaken.
Consciousness, in other words, is a partial illusion, a picture knitted together by the brain as a result of all the inputs it is receiving and the completeness of the brain broadcast.
Rowan Hooper
Are smarter people’s brains different?
The short answer is yes. People vary in their intelligence, so how else could we account for this if not for differences in the structure or function of the brain? Exactly what those differences are, however, is a matter of intense investigation.
The first thing to note is that people with larger brains really do tend to have higher IQs, but there is more to this than size (see “What makes our brain special?”). To find out more, we need to zoom into the white and grey matter that makes up our brain. The latter is comprised of the main bodies of neurons, whereas white matter is made of the fibres down which they send signals. Rogier Kievit at the MRC Cognition and Brain Sciences Unit in Cambridge, UK, and his colleagues have in the frontal lobe (see diagram) connected to fluid intelligence, which is the ability to solve novel problems. They also found this was linked to the amount of white matter connections between the two halves of the prefrontal part of the brain.
It isn’t just the amount of tissue that counts. One of the most striking features of the mammalian brain is the way it has deep folds of grey matter on its surface, giving it a walnut-like appearance. These increase its surface area, bringing cells closer together and allowing them to communicate faster. And sure enough, the and working memory: smarter people have more-folded brains.
850,000
kilometres of nerve fibres
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But this still doesn’t tell us where in the brain intelligence resides. To find this out, we can turn to one of the most popular ideas about its location, something called . This proposes that the biological basis of intelligence is a network connecting different brain hotspots.
Clues about these hotspots can be found in brain-imaging studies. By examining how parts of the brain became activated during cognitive tasks, Ulrike Basten at Goethe University Frankfurt in Germany and her colleagues identified a network connecting 20 different areas in the frontal and parietal regions that were . People with more grey matter or higher neural activity in these regions were smarter.
It feels like we are getting somewhere, but that result doesn’t just mean that smarter people have physically different brains: they seem to have ones that work more efficiently too. A brain might have the right chassis for high performance, says Emiliano Santarnecchi of Harvard Medical School, “but it’s not that relevant without an on-board computer regulating how power is delivered and when to allocate resources at any given moment”.
Santarnecchi’s work suggests that intelligence can be boosted by to increase the brain’s processing efficiency. He also emphasises the importance of . Perhaps some people’s brains are inherently more plastic, more capable of learning.
This is to say nothing of genetics. Although we know that hundreds of genes contribute to intelligence, it is going to take a long time to discover the nuances of their impact. But then it was never going to be a simple matter to find the location of intelligence, the richest human trait, in the brain, the most complex known object in the universe.
RH
What happens when we think?
Think about thinking, and it doesn’t take long for your mind to go down a rabbit hole. Thoughts come naturally to us, but pinning down exactly what they are is more complicated. Once they were viewed as immaterial entities, separate from the biological matter of the brain. Now we know that our every thought – whether about a simple object or an abstract idea – is the result of electrical signals pulsing through the brain’s network of 86 billion neurons.
“For me, a thought is simply the transformation of inputs to outputs by the brain,” says Ethan Solomon at the University of Pennsylvania.
But if you ask 100 neuroscientists for a definition, you will get 100 different answers, says Avgusta Shestyuk at the University of California, Berkeley. “‘Thinking’ is an umbrella term that covers multiple different cognitive processes,” she says. Some thoughts take the form of pictures, others seem to be comprised of words, and many take place at the unconscious level, without us even noticing.
The latest neurological studies let us tune into the electrical signals that underlie thinking. They show that even a basic thought involves a surprising amount of activity, with different brain areas firing up and sending information to others, and certain “hub” regions directing the traffic.
“Mind-reading devices are no longer the stuff of science fiction”
Last year, for example, Shestyuk and her colleagues recorded the by measuring the electrical signals involved when people were asked to recall and say a word. The first areas to show activity were the visual and auditory cortices, which receive signals from the eyes and ears. Next, the brain’s command centre – the prefrontal cortex – kicked in.
The harder the memory task, the more of the prefrontal cortex that became activated, and the longer it took to respond, reflecting the time taken for the region to recruit other brain areas such as networks where memories are stored. Finally, the motor cortex revved up to generate a spoken response. Surprisingly, this happened before the prefrontal cortex had decided on a response. “That’s why we sometimes start speaking before we know what we want to say,” says Shestyuk.
So, the prefrontal cortex helps orchestrate thought processes, but the signals involved also need to be coordinated. That is the job of brainwaves: ripples of neural activity oscillating at different frequencies across the brain. Solomon’s research reveals that during a memory test, in the various brain regions involved become coordinated, and this synchronisation probably allows information to be communicated between the regions.
Our new-found ability to eavesdrop on individual thoughts means that mind-reading devices are no longer just the stuff of fiction. Earlier this year, electrodes on the brain were used to translate brainwaves into words spoken by a computer. Techniques like these could help people with locked-in syndrome – who are conscious but unable to move – to communicate. All thanks to the power of thought.
Alison George
Are you really left or right-brained?
Chances are you have thought of yourself as left or right-brained: rational and logical, or creative and free-spirited. Appealing as this concept is, it is also a complete myth.
It is easy to see how the idea was born. In the 1960s, we discovered that certain functions occur solely on one side of the brain. Most people process language in the left hemisphere, whereas our emotions are dealt with in the right. It was soon said that our left hemisphere held a monopoly over the virtues of logic, reason and language. The right side of the brain was responsible for driving our emotions, musicality and impulsiveness. From this came the popular maxim that whichever side of your brain dominated, determined your personality.
The . For instance, although your left hemisphere produces complex speech, the right allows you to understand the emotional and metaphorical content of those words – it gives you some linguistic finesse. Creative thought, on the other hand, activates a widespread network of cells that favour neither hemisphere.
Moreover, there is . Jeffrey Anderson at the University of Utah has scanned the brains of more than 1000 people while they performed various tasks and showed that none revealed a dominance for using one side of their brain over the other.
Top to bottom
Other ideas abound. The “theory of cognitive modes”, developed by Stephen Kosslyn at Harvard University, holds that our cognitive style is determined by whether we are top or bottom-brained. The top regions of our brain are involved in formulating and carrying out plans, and revising them when they go wrong. Lower regions are largely concerned with processing inputs from the senses, classifying objects and events, and giving them meaning.
Everyone uses their whole brain all of the time, says Kosslyn, but each of us to some extent relies more on top or bottom systems, and this affects our behaviour. A , for instance, will be more of a creative go-getter, but sometimes ineffective because they don’t update plans based on current circumstances. Bottom-brained types think through the details of a plan but are less likely to initiate complex schemes.
Anderson, however, posits that our personalities probably and how rich those links are. For instance, people who are open to new experiences are more likely to get goosebumps when they see a beautiful sunset. Brain scans show that they have more connections between areas that process sensory information and regions responsible for our inner voice.
We can feed this information into deep-learning machines that can make accurate predictions about personality traits based on a person’s brain scan, says Anderson. “It’s not about anyone using the left or right side of the brain more,” he says, “it’s about subtle differences in connections across the whole brain.”
Helen Thomson
Is your brain ever off?
When you rest, it sometimes feels as if your brain switches off too. It doesn’t. If you are alive, your neurons are firing. “There is a lot of processing going on even when you’re not seemingly doing anything at all,” says Deniz Vatansever, a cognitive neuroscientist at Fudan University in China.
It could hardly be any other way. Moment-to-moment readiness was a matter of life and death for our ancestors. These days, most of us don’t have to worry about leopards leaping from the undergrowth. But we still need to be alert to dangers and opportunities, and that requires a brain that is working constantly.
~1 petabyte
Estimated memory capacity of the human brain [equivalent to that of the World Wide Web]
Source:
Back in the 1990s, neuroscientists noticed that people lying quietly in brain scanners with their eyes closed showed surprising levels of brain activity. The researchers soon mapped the brain regions that are most active during rest, and this became known as the default mode network. This shows little activity when we are engaged in tasks requiring attention, but fires up when we “switch off”, allowing our minds to wander.
Some evidence suggests that the default mode network is involved in mulling over past experiences and speculating about the future. In that sense, it is vital because daydreaming is considered to be one of the abilities that sets us apart from other animals. But the default mode network does more than that. In 2017, Vatansever and his colleagues demonstrated that it underlies our ability to do certain things without paying attention, such as tying our shoelaces or driving along a familiar route – our .
Shutting down
The brain is a hive of activity during sleep, too. Once consciousness is lost, it gets to work on all manner of chores: clearing out toxic molecules, regulating hormone levels and conjuring dreams, which are thought to provide a safe environment to simulate new behaviours that could help during waking life. The sleeping brain also files away experiences for later recall.
Even when someone is in a vegetative state, unconscious and apparently unresponsive for a prolonged period, their brain continues to work at some level. When some people in this state are asked to imagine themselves playing tennis, for instance, blood flow increases in brain regions associated with motor skills, suggesting that the neurons there are firing. In one case, a in response to yes-or-no questions.
Only when you die do your neurons completely shut down. Except even then, there is a final burst of activity, as Jed Hartings at the University of Cincinnati in Ohio and his colleagues recently showed for the first time in humans. When the heart is no longer pumping blood to the brain, starving it of oxygen, neurons draw on energy reserves to before they produce one last burst of electrochemical energy. Only then is the brain switched off for good.
Daniel Cossins
Does the gut influence the mind?
We often say we make decisions on the basis of gut feelings, and this may be truer than we realise. Nausea, for instance, makes us judge certain moral violations more harshly. This is just one of many ways in which our gut influences what goes on in our head.
It is easy to forget that the gut is a sense organ, detecting incoming nutrients, toxins and pathogens, and relaying that information to our brains. It contains some 500 million neurons that coordinate the process of digestion.
The gut is also home to about 2 kilograms of bacteria: our microbiome, which influences every organ in the body, including the brain. A wealth of studies in mice show that changing the bacteria in the gut can change behaviour – in some cases, turning the animals into antisocial loners.
The microbiome may be particularly important in childhood, while the brain is still developing. Mice that lack microbes called Bifidobacteria in their gut during infancy seem worse at learning new information, for example.
Evidence from humans is accumulating, too. One imaging study found that containing various strains of live bacteria had a profound effect on people’s resting brain activity and their responses to seeing faces showing emotion. This year, a study of 1054 people in Belgium found that certain types of gut bacteria are .
“The human brain starts to shrink around the age of 40”
There are also tantalising hints that certain neurological conditions, such as , and diseases, like Alzheimer’s, may originate in the gut. In Parkinson’s, synuclein fibres, a hallmark of the disease, seem to appear first in the gut before spreading to the brain. We don’t know what triggers it, but it could be an unknown microbe or toxin. In epilepsy, may explain why the high-fat “keto” diet prevents seizures in some people.
Research on the gut-brain connection is in its infancy, but it has sparked the idea of medicines that target the microbiome to improve our mental health – dubbed psychobiotics. John Cryan at University College Cork, Ireland, believes it is an exciting prospect, but a lot more work is needed to pinpoint which bacteria are beneficial for which conditions and how to deliver them to the gut.
How bacteria actually influence the brain is also something of a mystery, but the picture is becoming clearer. The tens of trillions of bacteria in our gut are a hive of metabolic activity, producing a wealth of chemicals that we can absorb. Working out which ones get to the brain and exert effects there is a big focus of current research. Some bacteria even feed on GABA, a brain chemical implicated in depression.
It might seem odd that our brains are influenced by what is in our guts, but it isn’t so surprising when you consider that these microbes have always been with us, says Cryan. “I see them as friends with social benefits because they really affect the social brain in early life and development,” he says. “This relationship is very important and I think it’s been evolutionarily wired.”
Sam Wong
What makes a brain
A few years ago, scientists took human brain cells and injected them into mice. A year later, the cells had multiplied and the mice had got smarter, learning more effectively than mice with regular brains. Perhaps that isn’t so surprising – until you hear that these brains cells weren’t neurons.
As building blocks of the brain go, neurons hog the attention. There are some 86 billion of these stringy cells carrying electrical impulses around the brain, helping us to control our bodies and think thoughts. But there are plenty of cells in the brain that aren’t electrically active. These are known as glial cells, and they are at least as numerous as neurons. A type of glia called an astrocyte was injected into those mouse brains, suggesting – not for the first time – that they could be important in learning.
Glia used to be considered mere gap fillers. No longer. “There is a lot of evidence to say they are more than just glue,” says Anne Cooke, chief executive of the British Neuroscience Association. “They are the unsung heroes of the brain.” They come in different types. Small ones called microglia, for instance, roam the brain gobbling up foreign material to protect the neurons.
Astrocytes take care of the neurons’ environment too, controlling levels of chemical messengers known as neurotransmitters and helping to repair damage. Evidence is mounting that these cells also have a role in the development of human intelligence. We know that babies start off with many connections between their neurons and that these are gradually pruned down to create smaller numbers of stronger signalling pathways. Astrocytes, it seems, are . “While neurons are still very important, it seems that glial cells are involved in setting the gain on the system,” says Ed Lein at the Allen Institute for Brain Science in Seattle.
It isn’t all about cells. Holes play a part too. Deep inside your head there are slim chambers called ventricles that produce the fluid that bathes the cells of your brain. We make as much as 500 millilitres of this cerebrospinal fluid daily, which keeps everything in working order by providing cushioning and nutrients, and washing waste away.
There is no doubt plenty more to be discovered. Just last year, researchers identified a new type of brain cell, dubbed a rosehip neuron due to its resemblance to the shape of a rosebush fruit. It may only exist in humans.
Other secrets may well be revealed by Lein’s ongoing efforts to create a map of all the brain’s cell types, painstaking detective work that looks at the genes single cells express. He has recently studied the neocortex – the outer part of the human brain that deals with higher processes and makes up 80 per cent of its mass – and found 75 different types of cells there alone.
Joshua Howgego
What makes some brains more resistant to decline?
It is a harsh fact of life: as you get older, your cognitive abilities start to wane. But why is it that some people reach a ripe old age with little more than the odd “senior moment”, while others have far greater mental decline?
The , with cells deteriorating most quickly in the frontal lobe, the striatum and the hippocampus – areas involved in our most complex thoughts, movement and memory. How resistant you are to the effects of this decline is likely to be associated with your cognitive reserve. This is a kind of mental buffer that allows your brain to sustain more damage before you notice changes in your cognition.
Cognitive reserve isn’t just down to someone having more neurons than another person, but also to how well their neurons engage with each other across different networks in the brain. This allows the brain to compensate when age-related decline occurs or disease takes hold, and helps reroute information so that the organ can continue to work optimally. It is a bit like boosting the processing power in a computer: more things can go wrong before you start to notice it slowing down.
Our environment can also influence cognitive reserve. A high level of education offers one of the biggest boosts, whereas obesity and insulin resistance seem to reduce it. Several genes also help us resist cognitive decline. Tiny genetic variations are associated with our susceptibility to Alzheimer’s as well as with how the brain utilises energy reserves and reacts to injury and pathogens.
Exercise the mind
Brain shrinkage over time sounds bleak, but there is some good news. Although most of our brain cells are created soon after birth, we can make certain types of neuron even into our 90s. This ability might go some way in explaining why some people’s brains fare better against the ravages of old age.
There are a few other ways to boost cognitive reserve. Continuing to educate yourself throughout your life appears to provide one of the biggest benefits, but playing a musical instrument, socialising, getting the right amount of sleep and speaking more than one language also help.
Don’t put your feet up for too long though: the adage “healthy body, healthy mind” turns out to be true. “If you’re looking to maintain brain health, you need to exercise,” says Steve Harridge, director of the Centre for Human and Applied Physiological Sciences at King’s College London. Regular workouts bring about significant improvements in memory, attention, processing speed and executive functions, such as planning and multitasking.
And don’t leave it too late. Richard Henson at the University of Cambridge and his colleagues have discovered that the things we do in midlife – outside of work and education – make a unique contribution to brain health in older age. The activities retirees do in their old age, however, had . “Midlife seems to be a good time to intervene, to nudge people into taking part in more activities – physical, intellectual and social – that might bode well for them 20 or 30 years later,” says Henson.
Helen Thomson
Article amended on 18 July 2019
We corrected the labelling of the primary motor cortex
