What the Lab Does and Why We Do It

How does the brain form memories of life's significant events? This is the question that motivates the research in our laboratory. More specifically, our work is focused on how traumatic memories are formed, stored, and retrieved.

A fundamental assumption in this work is that the brain has multiple memory systems, each devoted to different kinds of memory functions. For traumatic memory, two systems are particularly important. For example, if you return to the scene of an accident, you will be reminded of the accident and will remember where you were going, who you were with, and other details about the experience. These are explicit (conscious) memories mediated by the hippocampus and other aspects of the temporal lobe memory system. In addition, your blood pressure and heart rate may rise, you may begin to sweat, and your muscles may tighten up. These are implicit (unconscious) memories mediated by the amygdala and its neural connections. They are memories in the sense that they cause your body to respond in a particular way as a result of past experiences. The conscious memory of the past experience and the physiological responses elicited thus reflect the operation of two separate memory systems that operate in parallel. Only by taking these systems apart in the brain have neuroscientists been able to figure out that these are different kinds of memory, rather than one memory with multiple forms of expression. The work of our lab has been focused on the neural system underlying the formation of implicit emotional memories.

We have used classical fear conditioning as a behavioral assay for studying emotional memories. In fear conditioning, the subject receives a neutral stimulus in connection with some unpleasant event. As a result of its past association with the unpleasant event, the neutral stimulus acquires the capacity of elicit protective reactions in anticipation of danger. If you were bitten by your neighbor's dog yesterday, the sight of the beast today (and for some time to come) will certainly put you on guard, causing you, for example, to freeze dead in your tracks, or perhaps to run away, and will also lead to a host of physiological responses.

Using fear conditioning as a behavioral assay of the emotion humans experience as "fear", it has been possible, through studies of experimental animals, to map out in great detail just how the fear system of the brain works. Although much of the research has involved laboratory rats, there have also been studies of a variety of other mammals. Remarkably, the results in all these species lead to the same conclusion. Learning and responding to stimuli that warn of danger involves neural pathways that send information about the outside world to the amygdala, which determines the significance of the stimulus and triggers emotional responses, like freezing or fleeing, as well changes in the inner workings of the body's organs and glands. There is also evidence that the amygdala of reptiles and birds has similar functions. And recent studies of humans with damage to the amygdala, due to neurological disease or as a consequence of surgery to control epilepsy, show that our brains also work the same way. The implication of these findings is that early on (perhaps since dinosaurs ruled the earth, or even before) evolution hit upon a way of wiring the brain to produce responses that are likely to keep the organism alive in dangerous situations. The solution was so effective that it has not been messed with much, and works pretty much the same in rats and people, as well as many if not all other vertebrate animals. Evolution seems to have gone with an "if it ain't broke, don't fix it" rule when it comes to the fear system of the brain. The things that make rats and people afraid are very different, but the way the brain deals with danger appears to be similar. We can, as a result, learn quite a lot about how emotional situations are detected and responded to by the human brain through studies of other animals.

Obviously, this is not the whole story of an emotion, especially not in humans. Once the fear system detects and starts responding to danger, a brain like the human brain, with its enormous capacity for thinking, reasoning, and just plain musing, will begin to assess what is going on and try to figure out what to do about it. This is when the feeling of fear enters the picture. But in order to be consciously fearful you have to have a sufficiently complex kind of brain, one that can be aware of its own activities. While this is undoubtedly true of the human brain, it is not at all clear which (if any) other animals have this capacity.

The point is that the so-called fear system of the brain is very old, evolutionarily speaking, and it is very likely that it was designed before the brain was capable of experiencing what we humans refer to as "fear" in our own lives. If this is true, then the best way to understand how the fear system works is not to chase the elusive brain mechanisms of fearful feelings, but instead is to study the underlying neural systems that evolved as behavioral solutions to problems of survival. This is not to say that fear and other conscious emotions are not important, or that they should not be studied. They are important, but in order to understand them we may need to step back from their superficial expression in our own conscious experiences and dig deeper into how the brain works when we have these experiences.

Many of the most common psychiatric disorders that afflict humans are emotional disorders, and many of these are related to brain's fear system. According to the Public Health Service, about 50% of mental problems reported in the U.S. (other than those related to substance abuse) are accounted for by the anxiety disorders, including phobias, panic attacks, post-traumatic stress disorder, obsessive compulsive disorder, and generalized anxiety. Research into the brain mechanisms of fear help us understand why these emotional conditions are so hard to control. Neuroanatomists have shown that the pathways that connect the emotional processing system of fear, the amygdala, with the thinking brain, the neocortex, are not symmetrical -the connections from the cortex to the amygdala are considerably weaker than those from the amygdala to the cortex. This may explain why, once an emotion is aroused, it is so hard for us to turn it off at will. The asymmetry of these connections may also help us understand why psychotherapy is often such a difficult and prolonged process- it relies on imperfect channels of communication between brain systems involved in cognition and emotion.

Studies of the basic biology of the fear system are likely to continue to reveal important information both about where our emotions come from and what goes wrong in emotional disorders. And as we learn more, we hope to also figure out how to better treat and even prevent these conditions.