How does your brain form its most significant memories? Studies of fear in rats have helped us learn much here. Although people and rats fear different things, the manner in which the rat and human brain and body respond to danger is similar. Because fear is at the core of many human pathologies, from panic attacks to posttraumatic stress disorder, breakthroughs in understanding the brain's fear system may lead to new ways to treat these disorders.
The core of the brain's fear system is found in a region called the "amygdala". This region receives information from all the senses and in turn controls the various networks that inspire the speeding heart, sweaty palms, wrenching stomach, muscle tension and hormonal floods that characterize being afraid.
A rat's amygdala responds to natural dangers (rats fear cats without having to learn to do so) and learns about new dangers (sounds, sights and smells that occur in anticipation of cats and other threats). It is through studies of the way the brain learns about stimuli, such as the sounds that precede danger, that our systems for learning about fear, and memory as a whole, have been elucidated through rat studies.
There is also evidence that amygdala of reptiles and birds have similar functions. 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 in this same basic 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 likely to keep an organism alive in dangerous situations. The solution was so effective that it has not changed much over the ages.
Obviously, this is not the whole story. Once the fear system detects and starts responding to danger, a brain such as the human brain, with its enormous capacity for thinking, reasoning, and musing, begins to assess what is going on and tries to determine what to do. This is when feelings of fear arise. But in order to be consciously fearful you have to have a sufficiently complex kind of brain, one 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.
So, in evolutionary terms, the fear system of the brain is very old. It is likely that it was designed before the brain was capable of experiencing what we humans refer to as "fear" in our own lives. If true, then the best way to understand how the fear system works is not to chase the elusive brain mechanisms of feelings of fear, but to study the underlying neural systems that evolved as behavioral solutions to problems of survival. In order to understand feelings, we need to step back from their superficial expression in our conscious experiences and dig deeper into how the brain works when we have these experiences.
A fundamental discovery has been that the brain has multiple memory systems, each devoted to different kinds of memory functions. For memories of fear arousing experiences, two systems are particularly important. Take this example: if you return to the scene of a recent accident, you are likely to have a physical reaction that reflects activation of memories stored in the amygdala. At the same time, you will be reminded of the accident, will remember where you were going, who you were with, and other details. These are explicit (conscious) memories mediated by another system, the "hippocampus".
In contrast, memories mediated by the amygdala are unconscious. These 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 neuro-scientists been able to figure out that these are different kinds of memory, rather than one memory with multiple forms of expression.
Many of the most common psychiatric disorders that afflict humans are emotional disorders; many of these are related to the brain's fear system. According to America's Public Health Service, about 50% of mental problems reported in the US (other than those related to substance abuse) are accounted for by anxiety disorders, including phobias, panic attacks, post-traumatic stress disorder, obsessive compulsive disorder, and generalized anxiety.
Research into the brain mechanisms of fear helps us to understand why these emotional conditions are so hard to control. Neuro-anatomists have shown that the pathways that connect 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, for 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. As we learn more, we may begin to figure out how to better treat - and even prevent - these conditions.