A complete picture of psychiatric illness must confront another even subtler dilemma faced by many other sciences (65
)—how to integrate distinct perspectives on the same underlying process. For our field, the most prominent perspectival dilemma is that of how brain- versus mind-based approaches will interrelate in explanations of psychiatric disorders.
My approach to this question begins with the work of Marr (66
), who proposed that a biologically complex information-processing system like the mammalian visual system can be realized (or understood) from three complementary perspectives. These three perspectives are as follows, with Marr's original description of them in quotes and my rephrasing in italics (66
, p. 25):
- Computational theory: “What is the goal of the computation … and what is the logic of the strategy by which it can be carried out?” What is the task this mind/brain system is designed to accomplish?
- Representation and algorithm: “How can this computational theory be implemented? In particular, what is the representation for the input and the output and what is the algorithm for the transformation?” What functional processes are required to accomplish the task?
- Hardware implementation: “How can the representation and algorithm be realized physically?” How are those processes actually implemented in brain “wetware”?
The key feature of Marr's approach—which provides a hierarchy of explanation—is that the biology (“hardware implementation” in his terminology) is understood in the context of functional explanations that articulate the goals of the system. Note that Marr's implementation perspective focuses on the biological means by which a mechanism is executed while the representational and computational perspectives examine content. (To reemphasize, Marr is proposing different perspectives on the same psychobiological process—here the mammalian visual system. This is in contrast to the first part of this essay, which examined different parts of a single broad mechanism.)
To try to build models of brain function from the bottom up, Marr suggests, is hopeless. If you took such an approach and began at the level of individual molecular processes as they occur within neurons and tried to work up from there to higher functions such as perception or motor behavior, let alone mood or “reality testing,” you would not be able to see the forest for the trees. Rather, you also need a top-down perspective, beginning with the task that the neural machinery was designed to execute.
Marr's levels were designed for neural systems that process information. However, at a deeper level, his approach implements the old physiological distinction between understanding structure (hardware/brain) and function (processes/algorithms). These two complementary approaches have often been used iteratively in the scientific approach toward understanding complex biological systems and have, for example, been central to the development of cell biology (62
Marr's three perspectives could be used unaltered for guiding our approach toward understanding the mechanisms underlying certain psychiatric symptoms, such as auditory hallucinations or biased threat perception. Although many psychiatric problems are not currently amenable to an information-processing perspective, Marr's underlying logic is nonetheless valid for psychiatry across the board. Obtaining a complete explanation of psychiatric disorders will require detailed understanding from a biological perspective. But this will not emerge from the bottom up—wherein biology would replace psychology—as predicted by the hard reductionists. Rather, it will happen by supplementing such strategies with top-down approaches, which allow biological explanations to be pursued and understood in the context of prior models articulated using psychological constructs.
Let me come at this key concept—that biological explanations need to be understood top-down in a context defined by mental constructs—from three additional, interrelated vantage points. First, both cognitive and evolutionary psychology advocate a “reverse-engineering” approach to brain/mind functioning (67
). The brain contains, this view suggests, many different neural subsystems, all of which evolved to accomplish distinct tasks. Such tasks range from the relatively simple—such as maintaining an appropriate respiratory rate—to the extremely complex, such as the perception of meaning in human speech.
Imagine coming upon an old machine full of gears, sprockets, springs, and levers. What would be required to “explain” the workings of this machine? The concept of reverse-engineering provides a simple answer. To “explain” it, you must understand what its purpose is. Once you know the purpose, you can, with patience and ingenuity, “reverse-engineer” what the components of the machine are doing. So understanding the function of a machine forms the framework for an explanation of how it works. While the human brain is not a machine designed by a human, it is, functionally, a machine designed by evolution. So once we decide to try to understand what the brain is doing, we find that we cannot proceed without considering higher levels of analysis such as cognition, emotion, and perception.
How can we conceptualize what different parts of our brains are supposed to do? We can do that only in the language of function, which means using psychological constructs. Neurochemical terms will not work. So a reverse-engineering approach to understanding how brains make minds also suggests that biological explanations for the disturbed brain/mind systems that underlie psychiatric disorders have to be placed within the context of psychological processes.
The second perspective on this problem contrasts two different ways in which a more abstract (or higher-order) theory can relate to a more basic (or lower-order) theory. One possible form of that relationship is replacement, another term for hard-core reduction. Consider the physical concepts of temperature and mean kinetic energy. Once you know the mean kinetic energy of a gas, you learn nothing more by knowing its temperature. The higher-order construct (temperature) becomes redundant and is replaced by the more basic construct (mean kinetic energy). The second possible form of this relationship is implementation. Here, the more basic theory provides the mechanistic details of how the functions proposed by the more abstract theory actually get accomplished. In this case, the higher- and lower-order theories work together to provide a complete explanation. The lower-order theory does not replace the higher-order theory.
Which of these two relationships best describes how psychology and biology will interrelate in the explanation of psychiatric disorders? I argue that implementation is the more accurate depiction and fits well within the mechanistic framework here advocated. Following Gold and Stoljar (69
), we can gain some confidence about the correct answer to this question by examining recent neurobiological research. For this purpose, they examine the work of Kandel on learning in Aplysia
) and conclude that his research program is best understood as an example of implementation, not replacement—that is, his work is best seen as “the fleshing out of a psychological story in neurobiological terms” (69
, p. 822). Along the same lines, Hatfield (70
) and Bechtel (5
) have shown, respectively, that for three areas of visual perception (binocular single vision, stereopsis, and color vision) and for memory, neuroscientists have progressed by figuring out how the brain has implemented processes first worked out by psychologists.
Our third perspective on the relationship between biological explanations and mental constructs can be best illustrated with a hypothetical story: A research team shows definitively that gene X is associated with schizophrenia. The results are widely replicated. This research team then shows that gene X produces protein Y. A large definitive study shows that protein Y is abnormal in schizophrenia. This too is replicated, at which point they call a press conference to declare that they have “solved the riddle” of schizophrenia. Amid the triumphalist rhetoric, a young psychiatric resident raises her hand and asks, “But how does this abnormality in protein Y lead to the characteristic symptoms of schizophrenia—delusions, hallucinations, thought disorder, and negative symptoms?” The lead scientist, a bit stunned by the questions, replies, “We have no idea.”
A more comprehensive explanation of a psychiatric disorder must include an understanding of the production of the key symptoms and signs underlying that disorder. Parts of these explanations will have to be framed in psychological terms. Genes and molecules will surely prove to be critical causes of schizophrenia and thus will explain important things about the illness. But alone, they cannot explain it completely.
To say this in another way, psychology frames questions about how biological processes implement psychological functions. Moreover, as we understand the brain processes, we need to “back-translate” the biology into an understanding—in psychological terms—of the key psychopathological constructs under investigation (e.g., sad mood, drug craving, hallucinations, and compulsions). Merely showing a strong odds ratio between a particular genetic or molecular variant and illness is not enough. As Hatfield writes, “Researchers seek to understand the microactivities of neurons by asking how they contribute to one or another more global brain function, psychologically described” (70
, p. 257).
Thus, eschewing hard reduction or hard emergence, we have the most to hope from a perspective in which biological explanations will sit within and implement “wetware” functions that are articulated in the language of psychology. This will not be a one-step procedure, as psychologists will not always “get it right.” An iterative relationship between psychology and biology—where initial psychological constructs are better defined and subdivided by initial biological findings, which in turn help clarify the biology—will be needed to reach a more complete understanding. In short, biological and psychological perspectives will coevolve.
These arguments do not imply that useful insights cannot come from the hard reductive or emergent perspectives. Indeed, effective therapies can be developed from basic biological research (such as associated genes) without having any idea of how the gene variant produces symptoms. Furthermore, important approaches to treatment, such as cognitive-behavioral therapies, have emerged from psychological constructs that contained no biology. However, these perspectives will leave us with only part of the picture.