by Bruce Hood
So this is how the brain basically works. Just like Keanu Reeves’s Neo, you have no direct connection with reality. Everything you experience is processed into patterns of neural activity that form your mental life. You are living in your own Matrix. Wilder Penfield, the famous Canadian neurosurgeon who reported how he could induce dreamlike flashbacks in his conscious patients when he directly stimulated their cortex during operations, most dramatically demonstrated this. He wrote, ‘They were electrical activations of the sequential record of consciousness, a record that had been laid down during the patient’s earlier experience.’8 He even operated on his own sister and showed that direct stimulation of the cortex triggered motor actions, sensations and thoughts. It’s these patterns of connectivity that encode all the information we process, memories we store and plans that we intend execute. Love, hate, the capital of France, the winners of the last World Cup soccer tournament, how to pitch a tent, how to divide by ten, the plot of your next novel, the taste of chocolate and the smell of oranges – every feeling, bit of knowledge and experience you have or plan to have is possible because of the cascading activation of neurons. Everything we are, can do and will do is nothing more than this. Otherwise we would need ghosts in the brain and, so far, none have been found.
How the Developing Brain Gets Organized
Of course, the human brain is considerably more organized than a chaotic jumble of overlapping circuits. Many areas have been mapped that correspond to different tasks or functions that the brain undertakes. There are brain regions that process information as it arrives from the senses. There are brain regions that plan, initiate and control movements. There are brain regions where personal memories are stored. There are regions that perform calculations. There are centres for emotion, aggression, pleasure and arousal – the fire in the belly of the machine that gets us out of bed in the morning and motivates us to act on the world.
One way to consider how the brain is organized structurally and functionally is to consider it like an onion. At the core of the onion is the brain stem that regulates the basic body functions that keep us alive, such as breathing and blood circulation. Above the brain stem is the midbrain region that controls activity levels such as wakefulness and appetite. The midbrain also governs basic motor control and sensory processing. Arising out of the midbrain is the limbic system, a network that controls emotions and drives such as aggression and sex. This has been called the ‘reptilian brain’ because it controls the sorts of functions we share with lizards and snakes.9 These functions are simply triggered by the sight of a competitor or a potential mate – like a knee-jerk reaction. Deep in the history of our species, we behaved in this automatic way but eventually we evolved higher levels of brain machinery that enabled us to control these reptilian urges. Sitting on top of everything is the cortex, a thin layer on the surface of the brain packed with neurons that support higher-order processing for interpreting the world, generating knowledge and planning actions.
Figure 3: Illustration of structural and functional hierarchy of brain systems
One of the most surprising discoveries in recent years is that the cortex is not where the majority of neurons are found. Most neurons are densely packed into a specialized region in the base at the back of the brain known as cerebellum, which controls movement.10 Only about a fifth of neurons are found in the remaining areas of the cortex that we usually associate with higher level thinking. This is surprising as one would assume that the complex mental processes involving thought would benefit from having more processors. However, the power is not in the number of neurons but the amount of connections. Like many performance issues in life, it’s not how much you have, but what you do with it and who you know. Even though the cortex has fewer neurons that one might expect, it has much greater connectivity with more extensive and longer fibres that join together different, widely distributed populations. This is the secret to the power of the human cortex – communication. By integrating information from diverse areas, the brain can generate rich, multidimensional experiences. Somehow, out of this richness comes our conscious self. Without cortical activity, you lose consciousness – you lose your self.
Not only does this multilayered model represent one of the major organizational layouts of the brain, but it also illustrates the relative developmental progression that has taken place in the brain through evolution, with the lower systems being more mature and operational than the upper systems which continue to develop into adulthood. Babies start out with functioning lower centres. With time and experience these lower regions become increasingly interconnected with the higher centres that exert influence and control so that the brain operates in a coordinated way.
You can see this coordination emerging throughout childhood. In fact, many scientists like myself believe that much of the change in early development can be attributed to not only the emergence of higher brain centres, but also the integration between these systems and their control over lower mechanisms. For example, something as simple as eye movement is controlled initially by lower brain systems below the cortex that are working from birth.11 The problem is that these lower systems are fairly dumb. Those that control eye movement have evolved simply to direct your gaze to the darkest and brightest objects in the world. So for very young infants, the brightest things usually get their attention, but the trouble is that they lack the control to look away easily. For example, below two months of age, they have ‘sticky fixation’ – when they get stuck on a particular visually compelling target.12 The trouble is that if the most visible thing always captures your gaze, then you are going to miss everything else in view. In fact, when I worked at a specialized unit for children with visual problems, we used to get young mothers coming in worried that their healthy babies were blind because they did not seem to move their eyes a lot. They seemed to be in some sort of trance, staring fixedly at the window. They wanted to know why their young baby didn’t look them straight in the eye.
The behaviour of these babies, like many of the limitations found in young infants, reflect the immaturity of their brains. During the early weeks babies have very little cortical control. Over time, cortical mechanisms start to exert increasing control over the lower mechanisms through a process called inhibition that works like a vetoing system to shut down activity. Inhibition helps to reign in the lower centres to allow more flexibility. In the case of sticky fixation, the cortical mechanisms enable the baby to look away from highly visible targets, such as the bright light streaming in through the window, and direct their gaze to less obvious things in the world.
It turns out that most human functions require some degree of inhibitory control. Here’s a cruel trick to play on an eight-month-old baby who has developed the ability to reach out for toys. Show them a desirable, colourful toy that they really want but put it in a large clear plastic container. At first they will bash their tiny little hands against the clear surface as they reach for it. Even though they will keep bashing their hands against the transparent plastic, they find it hard to stop reaching straight for the toy.13 The sight of the toy is so compelling that they cannot inhibit their reaches. In fact, inhibiting our impulsive thoughts and behaviours is one of the main changes over the course of a lifetime that contributes to the development of the self. When these regulatory systems fail, then the integrity of the self is compromised.
It is as if our brain is a complex machine made up of many subdivisions that compete for control of the body – like a complex factory under the control of a senior manager who oversees production. It is this senior manager in our head office that we all experience as the self. You may be able to find your own senior manager by a bit of introspection – the process of focusing in on your mental state. Try this out. Find a quiet spot and close your eyes. Turn your attention to your self. Try to locate where that self is. With both hands, point with your index fingers to the sides of your head where you think your inner self is currently located. When both fingers are pointing to
where you think you are having experience at this very moment in time inside your head, keep one finger pointing and with the other hand point to this same place from the front of your head so you can accurately triangulate the site of your consciousness. Now draw the imaginary lines to find the intersection where ‘X’ marks the spot.
You have just located your own ‘point zero’ – where the ‘you’ inside your head sits. Figure 4 is taken from a study to map out where people think their point zero is located.14 It reveals that when we become mindful of our inner state, for most of us, it seems like we exist inside our heads, somewhere behind our eyes. We believe that this is the place where we are listening to a running commentary of thought, experiencing the sensations that the world throws at us and somehow controlling the levers that work the action and motions of our bodies.
Take a further moment to experience your body in this quiet state. If you concentrate you can feel its inner workings. As you read these lines, can you feel the subtle movements of your tongue bobbing up and down inside your mouth? Now that your attention has been drawn towards it, can you feel the pressure of the chair you are sitting on pressing against your backside? We can be in touch with our bodies but we are more than just our bodies. We control our bodies like some skilled operator of a complex meat machine.
Figure 4: Plot of locations where individuals typically feel their ‘self’ is located (based on study by Ferrari et al., 2008. Copyright permission given)
This internal self is sometimes called the ‘homunculus’ and this little chap is a real troublemaker. The homunculus is a problem because you are left none the wiser about the location of the self. In fact, considering the homunculus reveals why the reality of the self is a problem. There can be no single individual inside your head for the simple reason that, if true, then this homunculus would require an inner self as well. You would need a ‘mini-me’ inside the ‘you’ that is inside your head. But if the ‘mini-me’ inside your head is a homunculus, then who is inside the head of mini-me and so on, and so on? This would become an infinite regression leading to no end. Like an endless series of Russian matryoshka dolls, one inside another, the homunculus simply restates the initial problem of where the self is located in the mind. This is what the philosopher Dan Dennett has called the illusion of the Cartesian Theatre after the famous French philosopher, René Descartes, who thought that each of us possess a mind that inhabits our bodies. Dennett described this like sitting in the audience inside our heads watching the world of experience unfold like a play on a stage. But who is inside the head of the person watching the play in the Cartesian Theatre? Proposing an inner self simply does not help in solving the problem of where we are inside our heads.
Are we like a factory made up of lots of autonomous little workers inside our heads carrying out all the various tasks and functions that humans can achieve? To some extent we are, in that many of the subdivisions can operate independently. But there is not a worker army of homunculi any more than there is a chief executive in charge. Rather, our minds are a multitude of different processes and decisions that are often in conflict with each other, which often can occur below our level consciousness. This is why we will need to abandon the notion of internal individuals, which is inadequate to explain the complexity of our brain, and ultimately discard the notion that there is an inner self.
Mapping the Mind Machine
If the brain is a complex machine organized into different processing subdivisions, where does this organization come from? Who sets up all the domino patterns in the first place? This question is one of the major battlegrounds in neuroscience. To what extent are we preconfigured for the world by our genes and to what extent does that configuration emerge through our interaction with the world? It’s the old ‘nature versus nurture’ issue but at the basic biological level. It all depends on what aspect of being human you are considering but even the simplest features appear to combine biology with experience.
It is quite clear that we are born with many basic neural patterns in place. Many sensory and motor areas are well specified at birth even though they have yet to reach their full adult potential.15 But babies are not just passive sponges soaking up sensation from their environment – they can also act upon the world. For example, each human newborn is equipped with a repertoire of behaviours known as reflexes that play some vital role in development. Consider the rooting reflex, triggered by gently stroking the cheek of a newborn, which makes the baby turn their head and pucker up their lips in anticipation of a tasty nipple. If a nipple (or at least something of a similar shape) is touched to the baby’s lips, this then triggers a sucking reflex. You might think that the baby has decided to feed, but the truth is that these behaviours are completely involuntary and automatic and do not require any thinking. In fact, you do not need a very sophisticated brain to execute them. Anencephalic babies, born without any cortex, can still execute sucking reflexes because these behaviours are supported by primitive neural circuitry that lies beneath the cortex. But anencephalic babies are never destined to experience what it is to be human. They do not learn. They do not get bored.16 They simply respond. They will never develop a sense of their own self. Most die within days.
In contrast to the unfortunate babies born with brain damage, healthy infants are equipped with a brain that is designed to learn about its environment and this learning starts very early. We now know that the unborn baby can learn the sound of their mother’s voice, develop a preference for the food she eats while pregnant and even remember the theme tune to the TV soap operas she watches while waiting for the big day to arrive.17 All of this proves that the brain is already functioning and storing patterns of connections that represent the outside world. This is one reason why separating the relative influence of nature from nurture is always going to be hard and contentious. When do you start measuring? From conception or from birth?
Neuroscientists argue about how much of the adult brain structure is already evident in the infant, but it is quite clear that even if much of the blueprint for brain architecture has been passed on in the genetic code we inherit, there is still considerable scope for making amendments and building extensions to the original plan. This is where the environment shapes the brain by sculpting the matrix of neuronal connectivity that generates our minds.
Plastic Brains
I once bought a ‘Grow Your Own Brain’ gimmick toy, which was basically a compressed tiny plastic foam brain that you put in water, and it eventually expands to a much greater size. It’s amusing but not really a useful teaching aid. It is true that as babies grow their brains expand, but they are not simply swelling. The human newborn baby’s brain weighs about a quarter of the weight of an adult brain but within the first year more than half of the difference in weight is made up. What may surprise you is that this weight change is not because the brain is growing more neurons. In fact, newborn babies have almost their full complement of neurons that will remain with them throughout the rest of their lives. Rather most of that weight change is due the rapid expansion of communications between the neurons.18
As you can see in Figure 5, a diagram of the cortex taken from newborns through to fifteen months old, the human brain undergoes a massive explosion in connectivity between neurons during infancy.19 For example, during peak activity, the rat pup brain is generating neuronal connections at the rate of 250,000 every second. That’s fifteen million connections every minute. We do not know how fast the process occurs in humans. If anything it may well be even faster.
Figure 5: Illustration of neurons’ increasing connectivity during development
These structural changes reflect the way that biological processes interact with the world to shape the brain to fit into its environment. Two complementary processes create this sculpting.20 First, genetic commands tell the neurons to start growing more and more connections. This creates an initial over-production of connectivity between the neurons. That’s why the diagram looks like the underground root s
ystem of weeds growing in your garden. Second, this bout of over-production is then followed by a period of pruning, where connections are lost between neurons.21 Around four out of every ten connections are lost with about 100,000 lost every second during the peak rate. This loss of connectivity is particularly interesting and at first surprising. Why would nature put in all the effort to build bridges between neurons only to knock them down almost equally as fast at a later date?
It turns out that the over-production and subsequent cull of connections may be a cunning strategy to shape the brain to its environment. A massive connectivity means that the brain is wired up for every potential pattern of activation that it may encounter from experience. But remember, only neurons that fire together, wire together. When neurons are not reciprocally activated, nature prunes their connections through inactivity. Returning to the metaphor of our extended neighbourhood, ‘If you don’t return my call, I am not going to bother contacting you later.’ Or for those of you familiar with social networking such as Facebook or Twitter, then it’s the case of ‘un-following’ followers who do not follow you back.