How Your Brain Works: Inside the most complicated object in the known universe (New Scientist Instant Expert) Read online

  How Your Brain Works

  Inside the Most Complicated Object in the Known Universe

  NEW SCIENTIST

  www.hodder.co.uk

  Contents

  Series introduction

  Contributors

  Introduction

  1 Welcome to your brain

  2 Memory

  3 Intelligence

  4 Emotions

  5 Sensation and perception

  6 Consciousness

  7 Ages and sexes of the brain

  8 Sleep

  9 Technology to improve your brain

  10 Make the most of it

  Fifty ideas

  Picture credits

  Series introduction

  New Scientist’s Instant Expert books shine light on the subjects that we all wish we knew more about: topics that challenge, engage enquiring minds and open up a deeper understanding of the world around us. Instant Expert books are definitive and accessible entry points for curious readers who want to know how things work and why. Look out for the other titles in the series:

  Scheduled for publication in spring 2017:

  The End of Money

  The Quantum World

  Where the Universe Came From

  Scheduled for publication in autumn 2017:

  How Evolution Explains Everything About Life

  Machines That Think

  Why The Universe Exists

  Your Conscious Mind

  Contributors

  Editor-in-chief: Alison George is Instant Expert editor for New Scientist.

  Editor: Caroline Williams is a UK-based science journalist and editor. She is author of Override (Scribe, 2017).

  Articles in this book are based on talks at the 2016 New Scientist masterclass “How your brain works” and articles previously published in New Scientist.

  Academic contributors

  Daniel Bor is an author and cognitive neuroscientist at the Sackler Centre for Consciousness Science at the University of Sussex, UK.

  Derk-Jan Dijk is professor of sleep and physiology at the University of Surrey in Guildford, UK and director of the Surrey sleep research centre.

  Jonathan K. Foster is professor in clinical neuropsychology and behavioural neuroscience, affiliated to Curtin University in Perth, Australia, the neurosciences unit of the health department of Western Australia and the University of Western Australia.

  Linda Gottfredson is professor emeritus of education at the University of Delaware in Newark and focuses on the social implications of intelligence.

  Andrew Jackson is at the Institute of Neuroscience, Newcastle University, and is working on a neural prosthesis to restore hand movement after spinal injury, and on a brain implant to control epilepsy.

  George Mather is professor of vision science at the University of Lincoln, UK. He specialises in the perception of movement and of visual art.

  Michael O'Shea is professor of neuroscience and co-director of the centre for computational neuroscience and robotics at the University of Sussex, UK.

  Tiffany Watt Smith is a research fellow at the centre for the history of the emotions at Queen Mary University of London.

  Raphaëlle Winsky-Sommerer researches circadian rhythms and sleep at the University of Surrey in Guildford, UK.

  Thanks also to the following writers:

  Sally Adee, Anil Ananthaswamy, Colin Barras, Andy Coghlan, Catherine de Lange, Linda Geddes, Alison George, Jessica Griggs, Anna Gosline, Jessica Hamzelou, Bob Holmes, Courtney Humphries, Christian Jarret, Graham Lawton, Jessica Marshall, Alison Motluk, Helen Phillips, Michael Reilly, David Robson, Laura Spinney, Kayt Sukel, Helen Thomson, Sonia van Gilder Cooke, Kirsten Weir, Caroline Williams, Clare Wilson, Emma Young.

  Introduction

  If you are reading this, you are the proud owner of one of the most complex objects in the known universe: the human brain.

  You wouldn’t know this from looking at it: at first glance it’s a 1.4 kilogram pinkish wrinkled blob with roughly the consistency of tofu. It looks so uninspiring, in fact, that until 2,500 years ago it was thought to do nothing more complex than cool the blood.

  Now, of course, we know that the brain is a rich tangle of 86 billion neurons which, through a complex ballet of electrical and chemical activity, allows us to experience the world, feel, taste and remember. Over the course of human history it is what has enabled our species to build civilisations, create great art and fly to the moon.

  The question of how it manages these feats has kept great minds busy for centuries. In recent decades, though, neuroscientists have had the distinct advantage of being able to use modern brain-imaging techniques to observe in real time as patterns of electrical activity and blood flow hint at what is going on inside.

  As these techniques continue to reveal the brain’s workings, neuroscience is forging into new territory, trying to piece together the entire wiring diagram of the human brain. It is, without doubt, the most exciting time in the history of brain science.

  As we enter this exciting period of discovery, this New Scientist Instant Expert guide tells you everything you need to know about the human brain. Gathering together the thoughts of leading neuroscientists, and the very best of New Scientist magazine, it will bring you up to date with what the best brains in science know. If you have ever wondered how the brain senses, remembers, how it becomes conscious and what it is doing while we sleep, then read on.

  Caroline Williams, Editor

  1

  Welcome to your brain

  The brain is the most confusing, complicated and arguably the ugliest organ in our body – yet, in essence, it is simply a collection of nerve cells gathered together in one place to simplify the wiring. A brain can be just a handful of cells, as found in some simple invertebrates, or billions, as in humans. It allows animals to adapt their behaviour to changes in the environment on a much quicker timescale than evolution. Thanks to advances in the field of neuroscience, we now have an exquisite understanding of the brain’s underlying architecture. But how did our human brains evolve, and what makes them different from those of other animals? And what are the philosophical implications of being “just a brain”? Here is a whistlestop tour of your grey matter.

  The birth of neuroscience

  The birth of neuroscience began with Hippocrates some 2,500 years ago. While his contemporaries, including Aristotle, believed that the mind was to be found in the heart, Hippocrates argued that the brain is the seat of thought, sensation, emotion and cognition.

  It was a monumental step, but a deeper understanding of the brain took a long time to follow, with many early theories ignoring the solid brain tissue in favour of fluid-filled cavities, or ventricles. The 2nd-century physician Galen – perhaps the most notable proponent of this idea – believed the human brain to have three ventricles, with each one responsible for a different mental faculty: imagination, reason and memory. According to his theory, the brain controlled our body’s activities by pumping fluid from the ventricles through nerves to other organs.

  Such was Galen’s authority that the idea cast a long shadow over our understanding of the brain, and fluid theories of the brain dominated until well into the 17th century. Even such luminaries as French philosopher René Descartes compared the brain to a hydraulically powered machine. Yet the idea had a major flaw: a fluid could not move quickly enough to explain the speed of our reactions.

  A more enlightened approach came when a new generation of anatomists began depicting the structure of the brain with increasing accuracy. Prominent among them was the 17th-century English doctor Thomas Willis, who argued that the key to how the brain worked lay in the solid cerebral tissues, not the ventricles. Then, 100 years later, Italian scientists Luigi Galvani and Alessandro Volta showed that an external source of electricity could activate nerves and muscle. This was a crucial development, since it finally suggested why we respond so rapidly to events. But it was not until the 19th century that German physiologist Emil du Bois-Reymond confirmed that nerves and muscles themselves generate electrical impulses.

  This paved the way for the modern era of neuroscience, beginning with the work of the Spanish anatomist Santiago Ramón y Cajal at the dawn of the 20th century. His spectacular observations identified neurons as the building blocks of the brain. He found them to have a diversity of forms that is not found in the cells of other organs. Most surprisingly, he noted that insect neurons matched and sometimes exceeded the complexity of human brain cells. This suggested that our abilities depend on the way neurons are connected, not on any special features of the cells themselves. Cajal’s “connectionist” view opened the door to a new way of thinking about information processing in the brain, which still dominates today.

  Wired to think

  While investigating the anatomy of neurons in the 19th century, Santiago Ramón y Cajal proposed that signals flow through neurons in one direction. The cell body and its branched projections, known as dendrites, gather incoming information from other cells. Processed information is then transmitted along the neuron’s long nerve fibre, called the axon, to the synapse, where the message is passed to the next neuron (see diagram, below).

  It took until the 19
40s and 50s for neuroscientists to get to grips with the finer details of this electrical signalling. We now know that the messages are transmitted as brief pulses called action potentials. They carry a small voltage – just 0.1 volts – and last only a few thousandths of a second, but they can travel great distances during that time, reaching speeds of 120 metres per second (m/s).

  The nerve impulse’s journey comes to an end when it hits a synapse, triggering the release of molecules called neurotransmitters, which carry the signal across the gap between neurons. Once they reach the other side, these molecules briefly flip electrical switches on the surface of the receiving neuron. This can either excite the neuron into sending its own signal, or it can temporarily inhibit its activity, making it less likely to fire in response to other incoming signals. Each is important for directing the flow of information that ultimately makes up our thoughts and feelings.

  The complexity of the resulting network is staggering. We have around 86 billion neurons in our brains, each with around 1,000 synapses. If you started to count them at one per second you would still be counting 30 million years from now.

  FIGURE 1.1 Structure of a neuron

  Unlike the electronic components of a computer, our networks of neurons are flexible thanks to a special class of neurotransmitter. These neuromodulators act a bit like a volume control, altering the amount of other neurotransmitters released at the synapse and the degree to which neurons respond to incoming signals. Some of these changes help to fine-tune brain activity in response to immediate events, while others rewire the brain in the long term, which is thought to explain how memories are stored.

  Many neuromodulators act on just a few neurons, but some can penetrate through large swathes of brain tissue creating sweeping changes. Nitric oxide, for example, is so small (it’s the 10th smallest molecule in the known universe, in fact) that it can easily spread away from the neuron at its source. It alters receptive neurons by changing the amount of neurotransmitter released with each nerve impulse, kicking off the changes that are necessary for memory formation in the hippocampus.

  Through the actions of a multitude of chemical transmitters and modulators, the brain is constantly changing, allowing us to learn, change and adapt to the world around us.

  How did our brains get so complicated?

  14 million years ago a small ape lived in Africa. It was a very smart ape but the brains of most of its descendants – orang-utans, gorillas and chimpanzees – do not appear to have changed greatly compared with the branch of its family that led to modern humans. What made us different?

  We can only speculate as to why their brains began to grow bigger around 2.5 million years ago, but it is possible that serendipity played a part.

  In other primates, the “bite” muscle exerts a strong force across the whole of the skull, constraining its growth. In our forebears, this muscle was weakened by a single mutation, perhaps opening the way for the skull to expand. This mutation occurred around the same time as the first hominids with weaker jaws and bigger skulls and brains appeared.

  The development of tools to kill and butcher animals around 2 million years ago would have been essential for the expansion of the human brain, since meat is such a rich source of nutrients. A richer diet, in turn, would have opened the door to further brain growth.

  Primatologist Richard Wrangham at Harvard University thinks that fire played a similar role by allowing us to get more nutrients from our food. Eating cooked food led to the shrinking of our guts, he suggests. Since gut tissue is expensive to grow and maintain, this loss would have freed up precious resources, again favouring further brain growth.

  Our big brains might also have a lot to do with our complex social lives. If modern primates are anything to go by, our ancestors likely lived in groups. Mastering the social niceties of group living requires a lot of brain power. Robin Dunbar at the University of Oxford thinks this might explain the enormous expansion of the frontal regions of the primate neocortex, particularly in the apes. Dunbar has shown there is a strong relationship between the size of primate groups, the frequency of their interactions with one another and the size of the brain regions that deal with them.

  Overall, it looks as if a virtuous cycle involving our diet, culture, technology, social relationships and genes led to the modern human brain coming into existence in Africa by about 200,000 years ago.

  So where do we go from here? The answer is that we are still evolving. According to one recent study, the visual cortex has grown larger in people who migrated from Africa to northern latitudes, perhaps to help make up for the dimmer light up there.

  Interestingly, we may have reached a point at which there is no advantage in our brains getting any bigger. There may have come a time in our recent evolutionary past when the advantages of bigger brains started to be outweighed by the dangers of giving birth to children with big heads. Or it might have been that our brains got too hungry to feed. Our brains already burn 20 per cent of our food intake and it could simply be that we can’t afford to allocate any more energy to the job of thinking.

  What’s more, our brains might even be shrinking. In the past 10,000 years or so the average size of the human brain compared with our body has shrunk by 3 or 4 per cent. Some people wonder if it means we are getting stupider (see Chapter 3 for more on this). Others are more hopeful, suggesting that perhaps the brain’s wiring is more efficient than it used to be.

  Mapping the mind

  The brain may be a tangle of neurons but it is anything but disorganised. As each brain develops before birth it organises itself into a characteristic shape that, details aside, looks much the same in all of us. There is more than one way to carve up something as complicated as this and different regions have a dizzying number of names and descriptions. At its simplest, though, the brain can be divided into three parts, each of which deals with a particular kind of processing.

  Hindbrain

  As its name suggests, the hindbrain is located at the base of the skull, just above the neck. Comparisons of different organisms suggest it was the first brain structure to have evolved, with its precursor emerging in the earliest vertebrates. In humans it is made up of three structures: the medulla oblongata, pons and cerebellum

  The medulla oblongata is responsible for many of the automatic behaviours that keep us alive, such as breathing, regulating our heartbeat and swallowing. Significantly, its axons cross from one side of the brain to the other as they descend to the spinal cord, which explains why each side of the brain controls the opposite side of the body.

  A little further up is the pons, which also controls vital functions such as breathing, heart rate, blood pressure and sleep. It also plays an important role in the control of facial expressions and in receiving information about the movements and orientation of the body in space.

  The most prominent part of the hindbrain is the cerebellum, which has a very distinctive rippled surface with deep fissures. It is richly supplied with sensory information about the position and movements of the body and can encode and memorise the information needed to carry out complex fine-motor skills and movements. Recent research has also linked it with fine-tuning of our emotional and cognitive skills.

  Midbrain

  The midbrain plays a role in many of our physical actions. One of its central structures is the substantia nigra, so-called because it is a rich source of the neurotransmitter dopamine, which turns black in post-mortem tissue. Because dopamine is essential for the control of movement, the substantia nigra is said to “oil the wheels of motion”. Dopamine is also the “reward” neurotransmitter and is necessary for many forms of learning, compulsive behaviour and addiction.

  Other regions of the midbrain are concerned with hearing, visual information processing, the control of eye movements and the regulation of mood.

  FIGURE 1.2 Identifying the major parts of the brain (and below)

  Forebrain

  Many of our uniquely human capabilities arise in the forebrain, which expanded rapidly during the evolution of our mammalian ancestors. It includes the thalamus, a relay station that directs sensory information to the cerebral cortex – the outer, wrinkly area of the brain – for higher processing; the hypothalamus, which releases hormones into the bloodstream for distribution to the rest of the body; the amygdala, which deals with emotion; and the hippocampus, which plays a major role in the formation of memories.