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.