Theories of depression are like other theories that span the biopsychological domain in that either they are of the "mindless brain" variety and focus solely on putative biochemical or brain-circuit mechanisms, or they are of the "brainless mind" variety and focus solely on fashionable psychological models that might be impossible to implement in the brain. Western culture does, of course, have a long philosophical tradition that has encouraged this mind-brain dualism, so one should not be harder on theories of depression than on theories in other areas. Many of the chapters in this book have fallen, not surprisingly, into one or other of these categories, but it is hoped that at least some of the chapters have begun to hint at what these integrative mind-brain theories might look at in the area of depression and that they have begun to "mind the gap".
However, before a consideration of these theories, perhaps we should first point to the means by which advances can be made in these integrated mind-brain theories. That is, what methods in themselves span this false divide, in order to provide progress in our knowledge base about depression? One of these methods was discussed in the last section through the use of genetic epidemiology, by means of which some approximations to the genetic and non-genetic contributions can begin to be sketched. Such methods can help to identify candidate genes, and from there an exploration of their biological mechanisms can begin. However, there are also many other methods that span this divide and by means of which empirical progress can be made. One such is that of neuroimaging, which is very much in its early days in relation to depression (see Chapters 2 and 11).
1 have to confess to having been one of the sceptics initially about imaging approaches such as functional magnetic resonance imaging (f-MRI), because they seemed to be simply the "new phrenology" (though maybe, in fact, we have been too harsh in our long dismissal of phrenology!). Anyway, methods such as f-MRI offer the possibility of a method that immediately integrates the biological and the psychological; carefully designed "subtraction methodologies" can lead to important advances in understanding the underlying substrate and the brain circuits involved in those particular psychological processes. However, although the "subtraction methodology" can provide a very elegant approach, it does have its own pitfalls that are not yet understood. One of the key concerns is the subtraction methodology itself; it amounts to the fundamental question, "does 3 - 2 = 2 - 1?" In fact, the original exponent of the subtraction methodology approach in the nineteenth century, the Danish physiologist F.C. Donders, used the subtraction approach to study reaction times to simple and complex stimuli, the argument being that additional processes were involved, for example, if there were greater numbers of choices between stimuli. Although the method was both elegant and influential, later approaches in cognitive psychology demonstrated that Donders' principles broke down under certain important conditions (e.g., Neisser, 1967). To give a concrete example for f-MRI, let us say that a subtraction study is set up that shows neutral faces and faces showing a specific emotion such as disgust to groups of depressed and control subjects. The argument then is that any remaining differences reflect the unique status of the processing of disgust in depression. But the question, "does 3 - 2 =
2 - 1?", then applies; for example, if the control group were anxious patients rather than controls, would the same findings occur? Or if only female faces were used? Or if the faces were all familiar to the individual? Or if the stimuli were short video clips rather than photographs? So even an example as apparently straightforward as the processing of facial stimuli proves to be extremely complex once it is argued that the "equivalent" subtraction has to be achieved in several different ways before some confidence can be gained in the conclusions.
A further point that can be made briefly concerns the types of neuropsychological tests that can be used in studies such as f-MRI, psychological priming, genetic epidemiology, and so on. The tradition in cognitive psychology, and therefore in the development of neu-ropsychological and related tests, has been to use emotionally neutral stimuli; thus, the question, "What is the capital of Albania?", is an emotionally neutral knowledge question for most people, in contrast to "When did your father stop beating you?", which is emotionally valenced for everyone. Between these two extremes, we need to develop emotionally valenced tasks that are the equivalent of the emotionally neutral tasks currently in use, especially when these can be linked to f-MRI type studies. For example, in relation to depression (and other emotional disorders), there are key questions of the role of the frontal cortex in the executive control of emotion and emotion processing that have only begun to be asked. However, in order to study emotional disorders, executive function tasks need to be modified so that they have emotionally valenced versions (cf. Power et al., 2000).
Further developments will also be made in our understanding of the evolutionary role of depression (see Chapter 6), and in our understanding of the animal models that may offer the possibility of testing human models, such as in the work of LeDoux. The area of anxiety research has clearly benefited from the animal work by LeDoux (e.g., 1996), which has offered considerable support for multilevel approaches and should provide an impetus for such developments in the area of depression. Classic experimental psychology methods, such as those based on priming and subliminal activation methods of non-conscious processes, should also continue to provide further data for richer theorising about depression (see Chapters 3 and 4), but they may now be of greatest use when they are tied into f-MRI and other electrophysiological methods, so that bridges continue to be made across the mind-brain gap. They will also be of most value when they explore emotionally valenced stimuli rather than the emotionally neutral stimuli that have been used traditionally in experimental psychology. Fast-acting and other non-conscious brain mechanisms have evolved to process "emotional meaning" or the "emotional significance" of events, as noted above in the comments on developing emotionally significant neuropsychological tests.
Before we finish this section, some comments must also be made specifically about theoretical developments in the bipolar disorders. The study of bipolar disorders has suddenly opened up to psychological and social approaches, having long been ignored and left to the medicobiological approach. Even here, there have been few significant advances in theory, and the approach has focused primarily on pharmacotherapy (see Chapters 11 and 7). The promise of the psychological and the social approaches has been raised both by Wright and Lam (Chapter 12) and by Swartz et al. (Chapter 15). Although the primary focus of the last two chapters was on treatment (see also next section), the fact that the cognitive behavioural therapy (CBT) and interpersonal psychotherapy (IPT) models have an impact on the course of bipolar disorders is in itself of theoretical importance. Wright and Lam have highlighted both the role of specific types of dysfunctional beliefs that may be specific to bipolar disorders and have considered an integration of these CBT-based concepts into a conditioning-based model of approach-avoidance. In contrast, Swartz et al. (Chapter 15) have extended proposals made earlier by Goodwin and Jamison (1990) that individuals with bipolar disorders may be especially sensitive to disruption of psychosocial rhythms, a problem that many significant life events (such as job loss, the birth of a child, or retirement) and even more minor events (such as vacations, weekends, and jet-lag) possess.
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