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How the Brain Builds Identity

Posted: 16/02/2012 09:15

Susan Greenfield, Professor of Synaptic Pharmacology at Oxford, explains how character is formed by a scientific process.

How do 'memory', 'mind' and 'consciousness' map on to circuits and networks of brain cells? And if these terms can indeed be expressed in neuroscientific terms, then where and how will the additional idea of 'identity' fit in?

It is difficult to make this kind of excursion without first considering some basic facts about the physical brain itself. A full exposition of neuroanatomy, physiology and neurochemistry is beyond the scope of this essay, but essentially the brain is made up of some 100 billion specialised cells, 'neurons', and 10 times as many support cells, 'glia'. Neurons are special because they can generate an electric signal lasting about a thousandth of a second, an 'action potential'. When this happens, cells are said to be 'active'. If and when a neuron is active, the electric signals, the action potentials, zoom down to the connection that the cell makes with another neuron, which is actually a small gap known as a 'synapse'.

When the electrical signal reaches the synapse, it triggers the release of a chemical messenger, a 'transmitter': the transmitter crosses the narrow gap and locks into a kind of molecular handshake with a special molecule, a 'receptor', on the target cell. This handshake of a particular transmitter and a particular receptor is an exact fit in each case, and has been likened to a key in a lock because different transmitters each have their own specialised and specific receptors. When the interlocking takes place, it triggers a new electrical signal in the target cell, so the chain continues of electrical, chemical processes within an ever wider circuit of synapses.

These networks in turn build into anatomically defined brain regions, discernible and identifiable to the naked eye. In turn, each brain region may participate in various different mental functions, while any one mental function will be divided up among many brain regions: vision, for example, involves the interaction and participation of as many as 30 different regions.

There is great interest in how mental functions, particularly the more sophisticated human ones, relate to this interconnection of brain regions. It turns out that there is no special brain region unique to our species; rather, brains in different species conform to the same basic theme in being arranged in evolutionary layers around a primitive hub at the top of the spine. The differences between species lie in the relative size of any one brain region and its dominance and interaction with the holistic functioning of the rest of the brain.

We can understand more about what makes us so special, both as humans and as individuals, by comparing our brains to those of our nearest relatives, the chimpanzees. If we are considering that most human trait of all, our individual identity, then one area in particular needs special mention: the 'prefrontal cortex', which, as its name suggests, sits at the front of the brain, behind the forehead.

While we do not possess a qualitatively unique brain area that makes us human, the prefrontal cortex is the key brain region that shows a huge quantitative difference between our species and the rest of the animal kingdom, occupying 33% of the adult human brain but only 17 per cent in chimpanzees.

Moreover, we can see that human individual development, 'ontogeny', does indeed reflect 'phylogeny', evolution, in that the human prefrontal cortex only becomes fully matured and functional in the late teenage years and early twenties. However, even when we are adults, the prefrontal cortex can be temporarily put out of action by one particular chemical messenger, the transmitter dopamine. When you are highly excited or aroused, feeling rewarded, and indeed if you were addicted to drugs, this same single transmitter would somehow be playing a key part in these different subjective experiences. In all these cases, dopamine is playing a pivotal role by being released like a fountain from the primitive region at the top of the spine (brainstem)
outwards and upwards throughout the brain, where it then changes the responsiveness of neurons in many different areas, including the prefrontal cortex.

When dopamine reaches the prefrontal cortex, it inhibits the activity of the neurons there, and so recapitulates in some ways the immature brain state of the child; this area of the brain is only fully active in late teenage years. Just as children are highly emotional, excitable, so adults in this condition also are more reactive to the outside world and to sensations rather than inner 'cognitive' thought processes.

Schizophrenics resemble small children in many ways, not least in their hyper-reactivity to the outside world. So it should come as no surprise that schizophrenia can be characterised by an underactive prefrontal cortex and, among many other changes, by excessive levels of functional dopamine. This highly emotional state mediated by dopamine appears to be the final common conduit of all psychotropic drugs, regardless of their primary site and mode of action.

Small wonder that this dopamine system has also been linked to processes in the brain linked to feelings of reward. When we talk of having a 'sensational' time, 'letting ourselves go', and indeed 'blowing the mind', we have surely contrived situations - typically wine, women and song, or the more recent equivalent of drugs, sex and rock'n'roll - where the senses are being aximally stimulated, dopamine is released, and the prefrontal cortex is disabled. In addition, the prefrontal cortex has more inputs to all other cortical areas than any other region of cortex and therefore a key role in operational brain cohesion.

So if this key area is underactive, there could be a profound effect on holistic brain operations, again contributing to the mindset where sensory trumps cognitive - and where, I'm about to suggest, individual identity is less emphasised.

We can think of the normal human condition as comprising two basic modes. In the first, there is strong prefrontal cortex activation, where thinking dominates and there are normal levels of dopamine: in this scenario we are mindful of consequences, and have a past, a present and a future. In the other mode, in contrast, we can revert to the world of the small child, where one is completely in the here-and-now present, reacting to the external sensory input of an atomised moment that is independent of preceding events, of future consequences, and hence of 'meaning' - a state once described by the musician John Cage as 'no purpose, just sound'.

But can we actually speak of the 'mind' in down-to-earth neuroscience terms? In the past, the brain and mind were regarded as distinct. The mind was the province of the philosopher and thus had a monopoly on exotic and insubstantial thoughts and feelings disembodied from the squalor of biology. In contrast the physical brain excited little interest in and of itself; after all, it has no moving parts, has an unspectacular appearance and yields the secrets as to how it works with obstinate reluctance, as any neuroscience researcher will tell you. How easy, therefore, to accede to the traditional distinction of minds versus brain, of mental versus physical.

Over the last few decades, however, we have started to realise that this kind of polarisation is as unhelpful as it is unrealistic. The more we learn about the astonishing ability of the brain to adapt, 'plasticity', the more we realise that the mind, one's own individual take on the world, can actually be the personalisation of the physical brain through tangible, physical mechanisms.

Despite the first use of the term plasticity in 1894, by the pioneering Spanish neuroanatomist Santiago Ramon y Cajal, an empirical demonstration of brain adaptability that was experience-dependent was not shown until the 1940s, when lab rats experienced a stimulating 'novel' environment. Further studies on such 'enriched' animals have shown improvements in memory and reduced levels of anxiety, relative to their standard-caged counterparts. More recently, neuroscientists have observed that such cognitive discrepancies are likely to be the result of corresponding anatomical differences.

For example, the brains of enriched animals are heavier, with a thicker outer layer (cortex), and have neurons with increased size and protein content and more branches ('dendrites'); they also have increased blood supply, supporting an increased metabolism. The paradigm of an environmental enrichment has proved a useful tool in testing key theories of development, as
well as investigating mechanisms of learning and memory, with potentially important implications for human development.

The wonderful thing about being born a human being, as opposed to say a goldfish, is that although we are born with pretty much all the neurons we will ever have, it is the growth and connections between the brain cells that accounts for the growth of the brain after birth. This post-natal development of the human brain means that, unlike the goldfish, we have the potential for experiences to leave their mark on those brain cell connections. Let's be honest, goldfish do not have great personalities. Indeed if a goldfish died it could be replaced by a successor purchased that day from a pet shop without the child, or indeed anyone who saw the goldfish, being any the wiser. But as the brain of an animal becomes more complex, so we shift from the narrow stereotype that is dictated by genes, i.e. instinct, through to learning and adaption to the environment.

We human beings don't run particularly fast, we don't see particularly well, and we are not particularly strong compared to other species in the animal kingdom. But more than any other we are fantastic at adapting, and we occupy more ecological niches than any other species on the planet. This is because of the basic property of the plasticity of the brain, especially and exceptionally in the brains of humans, enabling us to adapt to every input, every moment of our existence.

Perhaps the most well-known example of the astonishing plasticity of the human brain is a study made in 2000 of London taxi drivers. London taxi drivers are renowned throughout the world for their detailed knowledge of the streets, configurations and one-way systems of central London. Most astonishing is that they have learnt all this information by heart and can navigate the streets of the big city, taking passengers from A to B without recourse to a manual. On average it takes a candidate driver two years to learn this massive amount of material, so that they can pass an ominous oral exam, tellingly called 'The Knowledge'. Needless to say, London taxi drivers have placed a huge burden on their memories, specifically on a form known as 'working memory', where rules and facts have to be constantly in mind in determining ongoing actions.

Eleanor Macguire and her colleagues were fascinated to discover what might have changed in the brains of London taxi drivers as a result of the very unusual experience of constantly stretching their working memory. Amazingly they found in brain scans that a particular area of the brain related to working memory (the 'hippocampus') was actually bigger in the drivers than in other people of the same age. Moreover, it was not the case that having a big hippocampus had predisposed those individuals to become London taxi drivers, as the difference in hippocampal size was larger in proportion to how long the taxi drivers in question had been plying their trade. This study was a clear example of brain plasticity, of the 'use it or lose it' principle: the brain, like the muscles of the body, grows stronger and larger with exercise. But you do not need to wait for years to see an effect.

In a particularly intriguing experiment conducted by Pascual Leone and his research group in 1995, three groups of adult human volunteers, none of whom could play the piano, volunteered for a five-day experiment. The control group, those exposed to the environment but not to the all-important factor under observation, merely stared at a piano. A second group learnt five-finger piano exercises, and even over five days showed an astonishing change in their brain scans. However, there was a third group that was more remarkable still; in this group the subjects were required merely to imagine they were playing the piano, and their brain scans showed almost identical changes to those undergoing physical practice.

We must abandon the notion that 'mental' phenomena do not have a physical basis in the brain - they do, even though the question of how it all happens remains subject to much speculation.

This is an edited extract from Susan Greenfield's essay for Notting Hill Editions, You and Me - the Neuroscience of Identity


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