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Who are you? The answer, of course, lies in your brain. But how your brain becomes and continues to be who you are is still poorly understood. Neuroscientists have been quite successful in figuring out how pieces of the brain puzzle work (perception, movement, learning, emotion) but have not made much progress in putting the pieces together to build the kind of global picture of brain function that would be necessary to understand how one's personal identity, one's self, is represented in neural tissue.

The self has been of more interest to philosophers and psychologists than brain researchers. From Descartes through Locke and into modern

times philosophers have stressed consciousness as the defining aspect of self. Carl Rogers, a pioneering self-psychologist, followed the philosophers in defining the self as "the organised, consistent conceptual gestalt composed of perceptions of the characteristics of the 'I' or 'me"'. Many contemporary self-psychologists similarly focus on self-consciousness. They do not deny that some aspects of mental life occur unconsciously, but they tend to minimise the importance of the unconscious.

Recently, there has been a growing interest in a more partitioned view of the self. One partition is between the minimum and the narrative self. The former is an immediate consciousness of one's self; the latter a coherent self-consciousness that extends into the past and future. But these conscious partitions, which themselves may be based on different mechanisms, are, as Freud noted, only the tip of the iceberg. Terms such as the primitive, core, ecological and non-conceptual self, refer to unconscious aspects of personal identity that define who we are. The study of implicit or unconscious aspects of the self are now major themes in social psychology. In contrast to the narrative and minimal self notions, which depend on language to encode our awareness of who we are in consciousness, these implicit aspects of the self are not accessible for verbal self-reflection.

Although the self has not been a major research interest for neuroscientists, some have ventured into the territory. Michael Gazzaniga and Antonio Damasio, for example, emphasise - as I do - the importance of understanding the conscious self in the context of the unconscious workings of the brain. But unlike Damasio and Gazzaniga, whose ideas are about the organisation of the mind and experience, I have been attempting to develop a theory that links the self to the detailed understanding of the cellular basis of brain function that is emerging

in neuroscience. Before I can explain this, though, I need to discuss the relation of the self to consciousness in more detail.

Evolution of the self Consciousness, at least the kind of consciousness we have in mind when we talk about our own mental states, was very likely added to the brain in recent evolutionary history. It was layered on top of all the other processes that were already there in our animal ancestors. Although animals are not conscious in the human sense, neither are they simply objects like rocks or chairs. Non-human animals are living creatures with nervous systems that represent external events internally and that allow their bodies to interact with and change the material world. The concept of a conscious person, a conscious self, while useful as a way of evaluating issues related to being human (in assigning moral and legal responsibility, for example), is less valuable as a concept for understanding existence in the context of our animal ancestry. When the self was viewed solely in terms of linguistically constructed conscious experience, it was possible to ignore the evolutionary links of the self to processes in other animals. But the new view, emphasising non-verbal, non-conceptual, unconscious, aspects of the self, requires an evolutionary context. To the extent that many of the systems that function non-consciously in the human brain function similarly in the brains of other animals, there is likely to be considerable overlap in the non-conscious aspects of the self between species. Obviously, the more similar the brains, the greater the overlap. Having a self does not depend on having the capacity to be self-aware.

While only humans have the unique aspects of the self made possible by the human brain, other animals have the kinds of selves made possible by their brains. We often talk about the personalities of animals. One neighbour has a mean dog, another a lovely cat. We have no trouble assessing the individuality of these creatures. And the factors that go into the shaping of their personalities are similar, up to a point, to those that shape ours. Because each biological organism is unique, it will differ from other similar creatures. By defining the self this way, we frame the problem in terms of biological, and specifically brain, mechanisms.

Genes, synapses and experience The particular aspects of the self that define "you" are present in your brain alone. In order for you to remain who you are from minute to minute and year to year, your brain must somehow retain the essence of who you are over time. Memory is thus central to understanding the self in terms of neural mechanisms. While memory implies the ability to recollect the past consciously, the new view is that memory is stored by systems that function both consciously (explicitly) and unconsciously (implicitly). The self is, in essence, a complex set of implicit and explicit memories.

In the late 19th century, a debate raged over whether the brain contained individual cells that were interconnected or whether it was composed of a mesh of continuously connected elements. Proponents of the former theory, which emerged as the winner, included two great pioneers in the study of the brain - Santiago Ramon y Cajal and Charles Sherrington - as well as the young Sigmund Freud. The outcome of this debate was not only the realisation that the brain is composed of neurons, but also that connections between neurons, so-called synapses, underlie all aspects of brain function.

Biologically, memories can be understood as changes in patterns of connectivity between brain cells or neurons. Points of connection between neurons are called synapses, tiny spaces through which the brain does its business. Your memory of a particular experience involves changes in the synaptic connections among the neurons that are engaged by the stimuli that constitute the experience. To the extent that the self is a set of memories, the particular patterns of synaptic connections in an individual's brain and the information encoded by these connections are the keys to who that person is.

Genes, too, make important contributions to personality and the self by shaping the brain. All of the capacities that we have as homo sapiens, including our capacities to learn and remember, are made possible by the genetic make-up of our species. What we put in memory systems as individuals is up to experience, but the existence and basic mode of operation of these systems is due to our species' genes.

At the same time, we each have a family genetic history that is a variation on the theme of being a human, and a personal set of genes that is a variation on our family's. All these variations influence who we are.

The best-articulated view of the role of genes in shaping behavioural and mental characteristics comes from biological trait theories of personality. These propose that a person's enduring qualities are caused by their genetic background. Considerable evidence has been amassed to support the view that some traits, such as the extent to which one is extroverted or introverted, are highly influenced by one's genetic history. Nevertheless, genes usually account for at most 50 per cent of any particular personality trait. For many traits the influence is far less. Further, life's experiences, in the form of learning and memory, shape how one's genotype is expressed. The concept of phenotypic plasticity describes the fact that genes can give rise to different outcomes in different circumstances. Even the most ardent proponents of genetic determination of behaviour admit that genes and environment interact to shape trait expression.

While the fact that both nature and nurture contribute to who we are is widely acknowledged, less well understood is that, from the point of view of how the brain works, nature and nurture are not different things but different ways of doing the same thing: wiring synapses. That is, both genes and experiences have their effects on our minds and behavioural reactions by shaping the way synapses are formed. Moreover, in many ways, the genetic influence on personality can also be thought of as memory - a memory encoded across generations and species rather than by individual experience. Synapses are the key to both genetic and learned influences on who we are.

Progress in understanding the molecular biology of genes has led to a surge of interest in the role of genes in brain function, including the role of genes in personality. But it is important not to lose sight of the contribution of experience, of learning and memory. Through learning and memory processes, and the underlying synaptic changes, personality builds up in a cohesive way. Without learning and memory, personality would be an empty expression of our genetic constitution. Learning allows us to transcend our genes.

Synaptic connections are also at the core of mental disorders. These were long thought of simply as chemical imbalances. In fact, the key is not the chemicals themselves, but the circuits in which the chemicals act. For example, many drugs that are used to treat mental disorders alter the monoamine class of chemicals in the brain (serotonin, dopamine, norepinephrine, acetylcholine). These chemicals are widely distributed across the brain, but the alterations that affect a particular problem, such as schizophrenia, are now believed to be restricted to a subset of the many circuits that use the chemical in question. Systems and circuits are, in essence, integrated sets of synapses.

Treatment of mental illness, whether by drugs or psychotherapy, is a process of changing one's mental states and behaviours. Many of the drugs used to treat depression and anxiety disorders affect the same molecular cascades that have been implicated in learning and memory. This suggests that drug therapy is a way of placing the brain in a state conducive to learning.

Holding it all together If our self is encoded in the synaptic connections of systems that function consciously and unconsciously, will we know what a person is when we figure out how these systems function? Actually, no. Figuring out the synaptic mechanisms underlying each mental process is itself quite a challenge. But we need to go beyond the mere explanation of how each process works in isolation. We need to understand how the many processes interact, and how the particular interactions that take place inside each of our brains give rise to and sustain who we are. Synaptic interactions between the systems that underlie the individual processes are key in keeping the self integrated in space (across brain systems) and time (across the days of our lives).

The integration problem is amplified by the fact that so many systems in the brain are able to change as a result of experience. How is a coherent personality ever established and maintained if different systems are able to learn and store information on their own? Why don't the systems come to function independently of one another? One reason is that although the different systems have different functions (seeing and hearing, controlling movements, planning and decision-making, and so on) they experience the same world. They process information differently, but about the same life events.

Another reason is the existence of convergence zones, regions that are able to integrate the activity of other regions. Convergence zones tend to engage in so-called higher-order processing. Not only can convergence areas put information together, but they can also send commands back to the lower-order systems, allowing some high-level co-ordination across the specific systems.

Then there is the widespread nature of certain chemical systems, such as monoamines. When these systems are turned on, they release their chemicals throughout the brain. These chemicals can serve as signals that facilitate learning across widely distributed systems. Monoamine systems tend to be activated during significant events, such as emotionally charged experiences. Indeed, activation of emotion systems is one of the key ways that the self is glued together. The brain has a number of emotion systems, including networks involved in the identification of sexual partners and food sources, as well as detecting and defending against danger. When one of these systems is active, the others tend to be inhibited. For example, other things being equal, animals will hang out in areas where they feel safe. So when the time comes to search for food, their fear of certain locations, like wide open spaces or places where they've previously encountered a predator, might have to be overcome if that's where food is likely to be found. The hungrier the animal, the more it will tolerate fear and anxiety and take risks to get food. Similarly, both hunger and sexual arousal are decreased by activation of systems involved in fear and stress. But once aroused, sexual desire can override many other brain systems - people risk all sorts of adverse consequences for a fling. Not only does the arousal of an emotional state bring many of the brain's cognitive resources to bear on that state, it also shuts down other emotion systems. As a result, the learning that occurs is relevant to the current emotional situation.

The broader the range of emotions that a child expresses, the broader will be the emotional range of the self that develops. This is why childhood abuse is so devastating.

If a significant proportion of early emotional experiences are caused by activation of the fear system, then the characteristic personality that begins to build up from the parallel learning processes co-ordinated by the emotional state is one drenched in negativity and hopelessness rather than in affection and optimism. Similarly, dissociative states that exist in certain forms of mental disorder might be thought of as emotionally driven synaptic configurations that partition the mind and behaviour in abnormal, maladaptive ways.

Most of us, most of the time, are able to piece together synaptic connections that hold our selves together. Sometimes, though, thoughts, emotions and motivations come uncoupled. When this happens, the self is likely to begin disintegrating, and mental health to deteriorate. When thoughts are radically dissociated from emotions and motivations, as in schizophrenia, personality can change drastically. When emotions run wild, as in anxiety disorders or depression, you are no longer the person you once were. And when motivations are captured by drug addiction, the emotional and intellectual aspects of life suffer.

Thinking broadly about who we are Given the importance of synaptic transmission in brain function, it's practically a truism to say that the self is synaptic. What else could it be? But not everyone will be happy with this conclusion. Many people will counter that the self is psychological, social or spiritual, rather than neural, in nature. My assertion that synapses are the basis of personality does not assume that your personality is determined by synapses; it's the other way around. Synapses are simply the brain's way of receiving, storing and retrieving our personalities, as determined by all the psychological, cultural and genetic factors. So as we begin to understand ourselves in neural, especially synaptic, terms, we do not have to sacrifice the other ways of understanding existence. A neural understanding of human nature broadens rather than constricts our sense of who we are.

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