Sorting Out Memories and Emotion
An Interview with Joseph LeDoux, Ph.D.


by Brenda Patoine

April, 2007

Q: You often make the distinction between “memories of emotions” and “emotional memories.” What is the difference?

A: We remember life’s important moments especially well. Emotional experiences, whether good or bad, leave strong traces in the brain. It was once thought that there was a single memory system in the brain. Now, however, we know that memories are formed in a variety of systems that can be roughly divided into two broad categories: systems that support conscious memory (i.e., explicit memory systems), and systems that store information unconsciously (i.e., implicit memory systems). Memories about emotional situations are often stored in both kinds of systems.

Much of our understanding of the neural systems’ underlying implicit emotional memory has come from studies utilizing Pavlovian fear conditioning as a behavioral paradigm. This work has implicated the amygdala in the formation and storage of emotional memories. That these memories are implicit is illustrated by the fact that people can be conditioned to respond to stimuli that they are not conscious of. Moreover, damage to the amygdala interferes with the ability of humans to be conditioned in this way. Such people have conscious explicit memories of being conditioned (i.e., they have memories about the emotional situation), but do not have the implicit emotional memories that allow the stimulus to elicit emotional responses.

Q: How is the amygdala involved in emotional memory?

A: In the case of implicit memory the information is learned and stored in the system that processes the relevant stimulus information and produces the learned response. This differs from explicit memory, which involves a system (the medial temporal lobe system) that has no obligatory responses associated with it. The amygdala is hard-wired by genetics to respond to certain kinds of stimuli that have traditionally been dangerous to our species. When such stimuli are encountered, behavioral, autonomic nervous system, and hormonal responses are expressed that help the organism cope with the danger. In such situations, stimuli that are associated with the danger, and thus predictive of future dangers, acquire the capacity to elicit emotional responses. Thus, amygdala is involved in emotional memory because it is involved in emotional processing, and it is involved in emotional processing because of its wiring to sensory and motor systems.

Q: You’ve suggested that the long-held concept of a “limbic system” that governs emotions is misguided. What has led you to this conclusion? Is there an “emotion circuit” in the brain?

A: The limbic system was proposed as an all-purpose solution to the problem of how the brain makes emotions. Several concepts were key to the theory. First, the limbic system is involved in emotion and not cognition. Second, the hippocampus is the centerpiece of the limbic system, and hence the emotional brain, because it integrates the internal and external environment. Third, the limbic system exists anatomically. Each of these ideas has been called into question. Soon after the limbic system theory was proposed, it was shown that the hippocampus, the system’s purported centerpiece, was involved in cognitive memory. As it turns out, the hippocampus does not play a key role in emotion. This weakened the first two concepts about the limbic system. Regarding the third point, it has been very difficult to come up with criteria that allow one to say which brain areas belong to the limbic system and which do not.

More than any other region, the amygdala has been consistently implicated in emotion. Because the amygdala was part of the limbic system, evidence implicating the amygdala in emotion has been viewed as vindication of the limbic system concept. This constitutes faulty reasoning.

One thing we need to guard against is replacing the limbic system concept with the amygdala concept of emotion. Much of what we know about the amygdala has come from studies of fear, and moreover, of a limited kind of simple fear. We need much more research on emotions and their neural basis before we can assign a particular brain region or circuit as the “emotion system.” My preference is to talk about the “limbic forebrain”—regions related to the medial cortex—without referring to these as a functional system. Functional systems have to be discovered through research. Broad theories like the limbic system are probably detrimental because they imply that we know the answers.

The limbic system was brilliant in its time, especially as a psychological theory about differences between emotional and cognitive evolution. The brain part was not quite right. But Paul MacLean, who originated the concept, should be praised rather than criticized. We know a lot more now than was available at the time, and he did a heck of a job with what was available.

Q: Beyond “academics,” why should the public care about research on emotional memories? What are the clinical implications of this kind of work?

A: There is both an upside and a downside to the fact that emotional states make memories stronger. The upside is that we remember our emotional experiences to a greater extent than non-emotional ones. The downside is that we remember our emotional experiences to a greater extent than non-emotional ones. By understanding how emotional memories, and memories about emotions, are formed and stored, we hope to be in a better position to help relieve suffering in people who live with traumatic memories that intrude upon daily life. Recent studies in rats suggest that it is possible to weaken emotional memories by giving certain drugs during the retrieval of the memory. This has implications not only for traumatic memory but also for the implicit memories that sustain addiction.

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Q: What gets you most excited about your research? What are the next steps?

A: The most exciting thing about my research is the young people I work with. I am constantly being challenged and amazed by their creativity and insights. My next steps are really steps we take together. It’s hard to say where we are going. I don’t run the lab with a master plan. I kind of let today let us know what to do tomorrow.