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Charlie Teo: What Happens In Your Brain When You Run

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Thinking about the brain

Think for a few moments about a very special machine, your brain - an organ of just 1.2 kg, containing one hundred billion nerve cells, none of which alone has any idea who or what you are. In fact, the very idea that a cell can have an idea seems silly. A single cell, after all, is far too simple an entity. However, conscious awareness of one’s self-comes from just that: nerve cells communicating with one another by a hundred trillion interconnections. When you think about it this is a deeply puzzling fact of life. It may not be entirely unreasonable therefore to suppose that such a machine must be endowed with miraculous properties. But while the world is full of mystery, science has no place for miracles and the 21st century’s most challenging scientific problem is nothing short of explaining how the brain works in purely material terms.

Thinking about your brain is itself something of a conundrum because you can only think about your brain with your brain. You’ll appreciate the curious circularity of this riddle if you consider the consequence of concluding, as you might, that your brain is the most exquisitely complex and extraordinary machine in the known universe. Clearly, this is and may be nothing more than, the opinion of your brain about itself: the brain’s way of thinking about the brain. So it seems we are caught in the logical paradox of a self-referencing, and in this case also a self-obsessed, system. Perhaps the only reliable conclusion from this thought experiment is that the brain is about as conceited as it is possible to be!

Notwithstanding the brain’s well-developed personal vanity, we must grant that it provides you with some very distinctive abilities. It operates in the background of your every action, sensation, and thought. It allows you to reflect vividly on the past, to make informed judgments about the present, and to plan rational courses of action into the future. It endows you with the seemingly effortless ability to form pictures in your mind, to perceive music in noise, to dream, to dance, to fall in love, cry, and laugh. Perhaps most remarkable of all, however, is the brain’s ability to generate conscious awareness, which convinces you that you are free to choose what you will do next.

We have no idea how consciousness arises from a physical machine and in trying to understand how the brain does that we may well be up against the most awkward of scientific challenges. That is not to say that the problem cannot in principle be solved, just that the brain is a finite machine and presumably has a finite capacity for understanding. But what are the limits of its intellectual capacity and, at that limit, might we still be asking unanswerable questions about the brain? Neuroscientists accept that they are faced with an awesome challenge. The accelerating pace of discovery in neuroscience, however, shows that we are a long way from any theoretical upper limit on our capacity for understanding that might exist. So rather than despairing of the limitations of the human intellect, we should be optimistic in our striving for a complete physical understanding of the brain and of its most puzzling of properties – consciousness and the sensation of free will.

Although we have barely started these texts we have already made a fundamental conceptual error in the way we have referred to ‘the brain’. The brain is not an independent agent, residing in splendid and lofty superiority in our skulls. Rather it is part of an extended system reaching out to permeate, influence, and be influenced by, every corner and extremity of your body. As the spinal cord, your brain extends the length of the backbone, periodically sprouting nerves that convey information to and from every part of you. Practically nothing is out of its reach. Every breath you take, every beat of your heart, your every emotion, every movement, including involuntary ones such as the bristling of the hairs on the back of your neck and the movement of food through your guts – all of these are controlled directly or indirectly by the action of the nervous system, of which the brain is the ultimate part.

From this perspective the brain is not simply a centre for issuing instructions, it is itself bombarded by a constant barrage of information flowing in from our bodies and the outside world. Specialized cells called sensory receptor neurons feed information via sensory nerves into the nervous system, providing the brain with real-time data on both the internal state of the body and about the outside world. Furthermore, information flowing into and out of the brain is carried not only by nerve cells. About 20 per cent of the volume of the brain is occupied by blood vessels, which supply the oxygen and glucose for the brain’s exceptionally high energy demand. The blood supply provides an alternative communication channel between the brain and the body and between the body and brain. Endocrine glands throughout the body release hormones into the blood stream. These hormones inform the brain about the state of bodily functions, whilst the brain deposits hormonal instructions into its blood supply for distribution globally to the rest of the body.

So when we say the brain does x or y, the word ‘brain’ is a shorthand for all of the interdependent interactive processes of a complex dynamical system consisting of the brain, the body, and the outside world. The human brain is a highly evolved and stupendously complex ‘machine’ that is often compared to the most complex of man-made machines, digital computers. But brains and computers differ fundamentally. The brain is an evolved biological entity made from materials such as small organic molecules, proteins, lipids, and carbohydrates, a few trace elements, and quite a lot of salty water. A modern computer is built with electronic components and switches made from silicon, metal, and plastic. Does it matter what a machine is made of? For computers the answer is no – computer operations are ‘medium independent’. That is to say, any computation can in principle be performed in any medium, using components made from any suitable material. Thus cogs and levers, hydraulics or optical devices for that matter could replace the electronics of a modern computer, without affecting (except in terms of speed and convenience) the machine’s ability to compete.

It seems extraordinarily unlikely either that the brain is simply performing computational algorithms or that thinking could equally well be achieved with cogs and levers as with nerve cells. So perhaps we cannot expect computers to perform like brains unless we find a way to build them in a biological medium.

From marks to meaning

To gain an insight into questions about the brain that must be answered, and to set the stage for later chapters, I will now briefly examine the activity of the brain in the context of a familiar act of everyday life. Let us consider the behaviour in which you are currently engaged – namely, reading these words. What exactly is your brain is doing right now? What kind of behaviour is reading and what must the brain do in order to achieve it?

Obviously, the brain must first learn how to read and equally obviously reading is a means of learning and engages our imagination. Reading also demands concentration and attention. Therefore as you read these words your brain must direct your attention away from the many potential distractions that are constantly in the background, all around you. You need not worry however because, without bothering your conscious awareness, your brain is keeping a watchful ‘eye’ on external events. It can at any moment redirect your attention away from this page and towards something more important. Your attention can also be distracted by events internal to the brain, the various thoughts that constantly pass through it and compete for your consciousness.

Reading, when reduced to the rather prosaic level of motor actions, depends on the brain’s ability to orchestrate a series of eye movements. Now, as you read these words, your brain is commanding your eyes to make small but very rapid (about 500° per second) left-to-right movements called saccades (right-to-left or up-and-down for some other written languages). You are not consciously aware of it, but these rapid movements are frequently interrupted by brief periods when the eyes are fixed in position.

Watch someone reading and you will see exactly what I mean. You’ll notice that the eyes do not sweep smoothly along the line of text, rather they dart from one fixation to another. It is only during the fixations, when the eyes dwell for about a fifth of a second, that the brain is able to examine the text in detail. Reading is not possible during the darting saccadic movements because the eyes are moving too quickly across the page. You are not aware of the blur and confusion during a saccade because fortunately there is a brain mechanism that suppresses vision and protects you from visual overload.

Reading is only possible between saccades, not only because the eyes are then stationary but also because gaze is centred on the retina’s fovea. The fovea is the only part of the retina specialized for high acuity vision (see Chapter 5), but it scrutinizes a very small area of our visual world. As a literal rule of thumb, foveal vision is restricted approximately to the area of your vision covered by your thumbnail held at arm’s length. It is a small window of clear vision within which you are able to decipher just 7 or 8 letters of normal print size at a time. The task for the brain is to generate a precise series of motor commands to the eye muscles which ensure that at the end of each saccade your high acuity vision is fixed on that part of the text you need to see most clearly next. As your eyes approach the end of a line, the brain generates a carriage return. Of course, the return saccade must be to the left, of the correct magnitude and associated with a slight downward shift in gaze in order to bring the first word on the next line onto the fovea.

I have considered only the simple case of the brain directing eye movements alone as if nothing else affects gaze direction. But of course, the relative positions of the eye and page are affected continuously by head, body, and book motion. Thus the brain must continually monitor and anticipate factors affecting the future position of your eyes relative to the text. The fact that you can effortlessly read on a moving train while eating a sandwich is evidence that your brain can solve this problem quite easily.

Importantly, it is done automatically and on an unconscious level without you having to think through every step. If you had to consciously think about the mechanical process of reading, you would be illiterate!  Our lack of conscious awareness of underlying brain processes can also be illustrated by reflecting on the subjective experience that the comprehension of written material represents. While reading we are not conscious of the fragmented nature of comprehension imposed by underlying move-stop-move-stop activity of the eyes I’ve just described or by the fact that only 7 or 8 letters can be deciphered at each stop. On the contrary, our strong subjective impression is that comprehension of the text flows uninterrupted and moreover that we can read several words or even whole sentences ‘at a glance’. That this is not the case can be illustrated by reading a sentence containing a word that has more than one meaning and pronunciation. For example, the word tear has two very different meanings and pronunciations in English – tear the noun of crying and tear the verb of ripping apart. Clearly, such word ambiguity complicates the brain’s task of providing you with an uninterrupted comprehension. If for instance, the word tear occurred at the beginning of a sentence its meaning might remain ambiguous until the subject of the sentence appears later. Because you cannot read the whole sentence at a glance your brain may be left with no option but to choose one of the alternative meanings (or sounds, if you are reading aloud) of a word and hope for the best.

While we cannot read whole sentences at a glance, the brain does recognize each word as a whole. What is quite surprising however is that the order of the letters is not particularly important (good news for poor spellers). That is why you will be able to read the following passage without consciously having to decode it.

We will now consider how and in what form textual information at the gaze point enters the brain. Light-sensitive cells called photoreceptors capture light focused as two slightly different images on the left and right retinae. The photoreceptors undertake a fundamental and remarkable transformation of energy, a transformation that must occur for all of our senses. This process is known as sensory transduction and always involves converting the energy in the sensory stimulus, in this case, light energy, into an electrical signal. This is because the nervous system cannot use light or sound or touch or smell directly as a currency of information transmission. In the brain electricity is the critically important currency of information flow.

The brain interprets or decodes electrical signals according to their address and destination. We see an electrical signal coming from the eyes, hear electrical signals from the ears, and feel the electrical signals coming from touch sensitive cells in the skin. You can demonstrate the importance of signal origin by pressing very gently with your little finger into the corner of your closed right eye, next to your nose. The touch pressure will locally distort the retina and produce an electrical signal that will be transmitted to the brain.

Your brain will ‘see’ a small spot of light in the visual field caused by touch. Notice that the light appears to be coming from the peripheral visual field somewhere off to the right; a moment’s thought should tell you why this is so. The photoreceptor cells of the retina are not connected directly to the brain. They communicate with a network of retinal neurons through a mechanism that couples the fluctuating electrical signal in the photoreceptor to the release of a variety of chemicals known as neurotransmitters. In their turn, neurotransmitters convey signals from one neuron to another by generating or suppressing electrical signals in neighbouring neurons that are specifically sensitive to particular neurotransmitters. This transformation of an electrical into a chemical signal occurs mostly at specialized sites called chemical synapses. Electrical signals can also pass directly between neurons at sites known as electrical synapses. Thus, through a combination of direct electrical transmission between neurons and the release of chemical messengers, information about the visual image captured by the eyes is processed in the retina before being conveyed by the optic nerve to the brain.

There are about one million output neurons in the retina, known as retinal ganglion cells, and each one extends a long, slender fibre or axon in the optic nerve. Axons are specialized for the high-speed (up to 120m/second), long-distance, and faithful transmission of brief electrical impulses. Impulses travelling along the axons of the retinal ganglion cells in the optic nerve reach the first neurons in the brain about 35 thousandths of a second after the capture of photons by the photoreceptors. In the brain, the axons of the retinal ganglion cells terminate and form synapses with a variety of other neurons which in turn interconnect with many others, a process which results finally in the conscious awareness of a vivid picture in your mind of what your eyes are looking at.

Somehow this astonishing electrochemical process that involves no conscious effort whatsoever produces meaningful pictures in your mind – close your eyes and the picture goes away, open them and it appears to you apparently instantaneously and effortlessly. Truly amazing!

Reading does not come naturally; it is a difficult skill that must be acquired painfully. Once learnt however it is rarely, if ever, forgotten – thankfully. So we do not have to worry about forgetting how to read because the skill is robustly established in our long-term memory banks. Although the enabling skill of reading is retained in permanent memory, an entirely different type of memory is required during the active process of reading itself. While reading, we must retain a short-term ‘working memory’ for what has just been read. Some of the information acquired while reading may be committed to long-term memory but much is remembered for just long enough to enable you to understand the text. Memories must somehow be represented physically in the brain. Brain chemistry and structure is altered by experience and the stability of these physicochemical changes presumably corresponds to the retention duration of memory. So what exactly is a memory? What kind of physical trace is left in the brain after we have learnt some new skill or fact? What is forgetting and why are some memories quickly forgotten and others never? These are questions to which I shall return later.

Finally, we must consider one of the most elusive of problems. While accepting that everything that the brain does depends on lawful processes occurring within and between the brain’s cells, how can we explain how ‘meanings’ arise in our minds while reading words? How do marks on paper become images in the mind, how do they make you think? How can any of this be explained completely by the responses of individual brain cells and interactions between them? Consider for example what happens when I recognize the word banana. I instantly call up an image of a yellow, curved object about 20 cm in length, 4 cm in girth, that is edible and incidentally delicious. We might propose that there is a single neuron in my brain that responds when I read ‘banana’ and triggers all of the remembered associated thoughts. Maybe this is the same neuron that responds when I see a real banana.

According to this logic, different neuron fire when I look at an apple and another recognizes my grandmother. While it is true that neurons can respond rather specifically to particular stimuli, most neuroscientists believe that there can be no one-to-one correspondence between the response of an individual neuron and a perception. Surely a separate neuron cannot detect and represent every object and percept? After all, in order to know that that object is a banana, information about shape, size, texture, and colour must somehow be bound together with stored knowledge about fruit, my appetite, and so on. These processes are associated with different networks of neurons in different parts of the brain and there is no known way they could all converge on a single neuron which when activated could trigger ‘aha, a banana for lunch’. Another way to think about the relationship between the activities of neurons and a perception is to consider how assemblies of nerve cells in different parts of the brain cooperate with one another in parallel. Having said that, we are far from understanding how objects, meanings, and perceptions are encoded in the brain by the activities of neurons. This is one of the most intriguing of problems in neuroscience. While the notion that there is a separate nerve cell in the brain for each object, meaning, and perception (parodied by the term ‘Grandmother cell’) has been roundly rejected, there is a lasting appeal in this simple idea. Indeed provocative research published at the time of writing this sheds a fresh perspective on the way objects are represented in the brain. It suggests that the idea there may be a neuron in your brain that only recognizes your grandmother deserves some serious reconsideration.

Starting with a historical perspective on the development of modern brain science I go on to describe electrical and chemical signalling mechanisms that underlie all mental functions, how the nervous systems evolved, how the brain responds to sensations and perceptions, the formation of memories and what can be done when the brain is damaged. The potential for interfacing the brain with computers is discussed, as is the contribution of neuroscience to developments in robotics and artificial nervous systems. 

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