Abstract
The field of neuroscience has, after a long period of looking the other way, again embraced emotion as an important research area. Much of the progress has come from studies of fear, and especially fear conditioning. This work has pinpointed the amygdala as an important component of the system involved in the acquisition, storage, and expression of fear memory and has elucidated in detail how stimuli enter, travel through, and exit the amygdala. Some progress has also been made in understanding the cellular and molecular mechanisms that underlie fear conditioning, and recent studies have also shown that the findings from experimental animals apply to the human brain. It is important to remember why this work on emotion succeeded where past efforts failed. It focused on a psychologically well-defined aspect of emotion, avoided vague and poorly defined concepts such as “affect,” “hedonic tone,” or “emotional feelings,” and used a simple and straightforward experimental approach. With so much research being done in this area today, it is important that the mistakes of the past not be made again. It is also time to expand from this foundation into broader aspects of mind and behavior
Keyword(s): amygdala, conditioning, fear, learning, limbic system, memory
INTRODUCTION
After decades of neglect, neuroscience has again embraced emotion as a research topic. This new wave of interest raises the question of why emotion was overlooked for so long. It is instructive to consider this question before examining what has been learned about emotional circuits, as some of the factors that led brain researchers to turn away from this topic may again hamper progress unless they can be grappled with.
Why Did Interest in Emotion Wane?
During the first half of the twentieth century, brain researchers were immensely interested in the brain mechanisms of emotional behavior. Some of the early pioneers in neuroscience worked in this area, including Sherrington, Cannon, Papez, and Hebb. Responses that occur when we defend against danger, interact with sexual partners, fight with an enemy, or have a tasty bite to eat promote the survival of individuals and their species. Emotional responses are thus inherently interesting and important. So what happened? Why did research on the brain mechanisms of emotion come to a halt after midcentury? For one thing, emotion research was a victim of the cognitive revolution. The emergence of cognitive science shifted the interest of those concerned with the relation between psychological functions and neural mechanisms toward processes (perception and memory, for example) that were readily thought of in terms of computer-like operations. From the start, cognitive scientists claimed that their
field was not about emotion and other such topics (see Neisser 1967, Gardner 1987). The cognitive approach came to be the dominant approach in psychology and brain science, and research interest in emotion dwindled. Another factor that hindered work on emotions in neuroscience was that the problem of how the brain makes emotions seemed to have been solved in the early 1950s by the limbic system concept (MacLean 1949, 1952). This appealing and convincing theory was the culmination of research on the brain mechanisms of emotion by many researchers, extending back to the late nineteenth century (see LeDoux 1987, 1991). Studies of how the brain mediates cognitive processes seemingly had a long way to go to catch up with the deep understanding that had been achieved about emotions, and researchers flocked to the new and exciting topic of cognition and the brain to begin filling the gap. Cognitive questions also seemed more tractable than emotional ones, due in part to the dark cloud of subjectivity that hung over the topic of emotion. Although subjective experience and its relation to neural mechanisms is a potential difficulty for any area of psychology, cognitive scientists figured out how to study mental processes without having to solve the mind-body problem. They showed, for example, that it is possible to study how the brain processes (computes and represents) external stimuli without first resolving how the conscious perceptual experiences come about. In fact, it is widely recognized that most cognitive processes occur unconsciously, with only the end products reaching awareness, and then only sometimes (see Kihlstrom 1987). Emotion researchers, though, did not make this conceptual leap. They remained focused on subjective emotional experience. In spite of the fact that most research on emotions and the brain was, and still is, conducted with experimental animals, creatures in which subjective states are difficult if not impossible to prove, theoretical discussions of emotions and
the brain typically reverted back to the age-old question of feelings. This approach puts the mind-body problem right smack in the middle of the path of progress. The main lesson to be learned from this brief excursion into history is that emotion researchers need to figure out how to escape from the shackles of subjectivity if emotion research is to thrive. It is ironic that cognitive science, which led to the neglect of emotion research, may also be able to help in its resurrection by providing a strategy that allows the study of emotion independent of subjective emotional experiences. It is possible, for example, to ask how the brain processes emotional information (i.e. detects and responds to danger) without necessarily first solving the question of where conscious feelings come from. Contrary to popular belief, conscious feelings are not required to produce emotional responses, which, like cognitive processes, involve unconscious processing mechanisms (seeO¨ hman 1992, LeDoux 1996). If we want to understand feelings, it is likely going to be necessary to figure out how the more basic systems work. Failure to come to terms theoretically with the importance of processing systems
that operate essentially unconsciously has been a major impediment to progress in understanding the neural basis of emotion. To overcome this, brain researchers need to be more savvy about the nature of emotions, rather than simply relying on common sense beliefs about emotions as subjective feeling states. Research on emotion can also help cognitive science. A pure cognitive approach, one that omits consideration of emotions, motivations, and the like, paints an artificial, highly unrealistic view of real minds. Minds are not either cognitive or emotional, they are both, and more. Inclusion of work on emotion within the cognitive framework can help rescue this field from its sterile approach to the mind as an information-processing device that lacks goals, strivings, desires, fears, and hopes. Once a processing approach to emotion is taken, emotion and cognition can be studied similarly: as unconscious processes that can, but do not necessarily, lead to conscious experiences. This would open the door for the integration of emotion and cognition, and such integration should be a major goal for the immediate future.
CONCLUSION
Research on the emotional brain has progressed significantly in recent years, largely as a result of a highly focused approach centered on the study of fear mechanisms, and especially the mechanisms underlying fear conditioning. This work has mapped out pathways involved in fear learning in both experimental animals and humans, and it has begun to shed light on interactions between emotional and cognitive processes in the brain. Although the focus on fear conditioning has its limits, it has proven valuable as a research strategy and provides a foundation upon which to build a broader understanding of mind and brain. At the same time, there is a disturbing rush to embrace the amygdala as the new center of the emotional brain. It seems unlikely that the amygdala is the answer to how all emotions work, and it may not even explain how all aspects of fear work. There is some evidence that the amygdala participates in postitive emotional behaviors, but that role is still poorly understood. If an amygdala theory of emotion is on the horizon, let it get there by data rather than by faith. Neuroscience meetings these days have numerous papers on the role of the brain in emotion, affect, hedonic tone, and the like. Unless these vague concepts can be operationalized, as was done in the work on fear, they are likely to impede, if not recede, the progress. The future of emotion research can be bright if we keep in mind the way that emotion became respectable again: by focusing on a psychologically well-defined aspect of emotion, by using an experimental approach that simplified the problem in such a way as to make it tractable, by circumventing vague and poorly defined aspects of emotion, and by removing subjective experience as a roadblock to experimentation. This is not to suggest that the hard problems should not be worked on but instead that they should be worked on in a way that advances the field.