| THE AMYGDALA AND ITS ALLIES |
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The amygdala is a brain structure that is essential for decoding emotions, and in particular stimuli that are threatening to the organism. As a result of evolution, many of our body’s alarm circuits are grouped together in the amygdala.
Consequently, many sensory inputs converge in the amygdala to inform it of potential dangers in its environment. This sensory information comes to the amygdala either directly from the sensory thalamus or from the various sensory cortexes.
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But there are several other regions of the brain that project their axons to the amygdala; examples include the hypothalamus, the septum and the reticular formation of the brainstem.
The amygdala also receives numerous connections from the hippocampus. Since the hippocampus is involved in storing and retrieving explicit memories, its connections to the amygdala may be the origin of strong emotions triggered by particular memories.
The hippocampus also specializes in processing sets of stimuli (as opposed to individual stimuli)–in other words, the context of a situation. Hence it is because of the hippocampus and its close connections with the amygdala that the entire context associated with a traumatic event can provoke anxiety. |  |
Major connections to the the amygdala also come from the medial prefrontal cortex. These connections appear to be involved in the process of extinction, whereby a stimulus that triggers a conditioned fear gradually loses this effect. This happens if that stimulus is repeatedly presented to the subject without the unconditional nociceptive stimulus that was initially associated with it to produce the conditioned fear.
The prefrontal cortex also seems to be involved in the final phase of confronting a danger, where, after the initial automatic, emotional reaction, we are forced to react and choose the course of action that can best get us out of danger. In people whose frontal cortex is damaged (people with “frontal syndrome"), planning the slightest task is very difficult, if not impossible.
Thus, the ability that our superior mental structures give us to voluntarily plan an emotional response suited to the situation is a wonderful complement to our system of rapid, automatic responses. The connections from the prefrontal cortex to the amygdala also enable us to exercise a certain conscious control over our anxiety. However, at the same time, this faculty can create anxiety by allowing us to imagine the failure of a given scenario or even the presence of dangers that do not actually exist.
| THE TWO PATHWAYS OF FEAR |
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Information from an external stimulus reaches the amygdala in two different ways: by a short, fast, but imprecise route, directly from the thalamus; and by a long, slow, but precise route, by way of the cortex.
It is the short, more direct route that lets us start preparing for a potential danger before we even know exactly what it is. In some situations, these precious fractions of a second can mean the difference between life and death.
 | Here is an example. Suppose you are walking through a forest when you suddenly see a long, narrow shape coiled up at your feet. This snake-like shape very quickly, via the short route, sets in motion the physiological reactions of fear that are so useful for mobilizing you to face the danger. But this same visual stimulus, after passing through the thalamus, will also be relayed to your cortex. A few fractions of a second later, the cortex, thanks to its discriminatory faculty, will realize that the shape you thought was a snake was really just a discarded piece of garden hose. Your heart will then stop racing, and you will just have had a moment’s scare. |
But if your cortex had confirmed that the shape really was a snake, you probably would not have just been startled. You would probably have taken off with all the alacrity that the physiological changes triggered by your amygdala allowed.
Thus, the fast route from the thalamus to the amygdala does not take any chances. It alerts you to anything that seems to represent a danger. The cortex then makes appropriate adjustments, suppressing any reactions that turn out to be inappropriate. Thus, we see, from an evolutionary perspective, how these two complementary pathways may have become established. From the standpoint of survival, the consequences of mistaking a garden hose for a snake are less severe than those of mistaking a snake for a garden hose.
But the cortex is not the only part of the brain that puts in its two cents by specifying the nature of the object. The hippocampus can also come into play by giving you information about contexto.
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