Joseph LeDoux

SYNAPTIC SELF

How Our Brains Become Who We Are

Joseph LeDoux is a neuroscientist best known for his research on emotion, which he described for a general audience in his 1998 book The Emotional Brain. The present book is partly an update but, as its title indicates, it focuses more particularly on the role of synapses. These are the small spaces between neurons which determine how conduction of nerve impulses occurs within the brain and therefore, ultimately, how the brain itself functions. The scope of the book is also wider and more ambitious than that of the earlier work.

LeDoux certainly could not be accused of insulting his reader's intelligence. His new book goes into quite a lot of detail about synapses and how they work at a molecular level, and it is necessary to keep alert and concentrate to take it all in. The reward for doing this is to be treated to an up-to-date view of an important area of brain research, and therefore to an insight into current thinking about what makes us who we are.

Scientific writing about emotion encounters a linguistic problem at the outset, because the ordinary usage does not easily translate into a scientific context. In common parlance, "emotions" are more or less synonymous with "feelings": fear, anger, sexual desire and so on could be described as emotions. But these are essentially human experiences, and we rely largely on people's verbal descriptions of their feelings to know if they are having them. Much of the experimental work in neurology, however, is carried out on animals, and we cannot know what, if anything, they feel. LeDoux discusses this important subject on pp. 204 and following, and reaches this conclusion.

So my approach has been to discuss emotions (feelings) only with respect to humans, and to restrict myself to the notion of emotional processing when I talk about creatures other than humans. In this way, I avoid the construction of a theory that can never be proven, but at the expense of having one that may be incomplete. I'm comfortable with this approach, though others prefer the alternative.

There is a widespread notion in popular writings on the brain that emotion, however defined, depends on an area called the limbic system, and even scientific texts quite often echo this. LeDoux has an excellent section on the subject, in which he points out the deficiencies in the limbic system theory, which he regards as "an off-the-shelf explanation of how the brain works, one grounded in tradition rather than in facts. Deference to the concept is inhibiting creative thought about how mental life is mediated by the brain." In fact, even the existence of the limbic system as an entity is open to question, and there is little agreement about which parts of the brain are supposed to form part of it.

In any case, the most important component of the alleged limbic system, the hippocampus, is responsible for memory rather than emotion, and much of the book is about memory. I was particularly interested in the discussion of long-term potentiation, which many believe is an important part of the memory mechanism. This is a synaptic phenomenon, and LeDoux finds that the synaptic basis of memory seems to be common to a wide range of species across the spectrum of evolution. He is also good on the different types of memory (explicit, implicit, working) and on how these come together to constitute the self.

Another area where popular thinking about the brain needs to be updated relates to the notion of "reward centres". Research on rats in the early 1950s led to the claim that there are "pleasure centres" in the brain and that stimulating these could be the basis of addiction. LeDoux teases out the neurological basis for such claims, and shows that the whole subject is, as might be expected, vastly more complicated than the initial reports implied. This is just one of the many places in which he looks beneath the obvious to reveal some of the underlying complexity.

What happens in the brain when things go wrong? There is a widespread notion that much mental disorder is due to "chemical imbalance" in the brain. LeDoux refers to this idea, disparagingly, as the "soup model". It is synaptic changes that really matter, he insists; we need a circuit point of view to understand psychiatric disorders. He discusses schizophrenia, depression, and anxiety disorders in this context. Some will no doubt find this to be an excessively mechanistic approach, but perhaps they will be reassured by his view that our thoughts can actually modify the way our synapses work.

The downward mobility of thought provides a powerful means by which parallel plasticity in neural systems is coordinated… With thoughts empowered in this way, we can begin to see how the way we think about ourselves can have powerful influences on the way we are, and who we become. One's self-image is self-perpetuating.

11July 2004