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Emotions and the brain
Sub-Topics
Fear, Anxiety and Anguish

Linked
Help Linked Module: The neural bases of emotion
Original modules
Experiment Module: Identifying the Brain Structures Involved in Conditioned Fear Identifying the Brain Structures Involved in Conditioned Fear
History Module: The Quest for the “Emotional Brain”   The Quest for the “Emotional Brain”

How Different Parts of the Brain Co-operate

We now know that the brain comprises several different kinds of memory. The hippocampus and the cortex make explicit, conscious memories possible. For its part, the amygdala enables one of the forms of implicit memory: emotional memories associated with fear.

Various aspects of an especially emotional situation such as a car accident will therefore be processed both by the hippocampus and by the amygdala, working in parallel. Thanks to the hippocampus, you will remember whom you were with, what you did, and the fact that it was a particularly painful situation. However, it is because of the amygdala that when you remember the event, your palms will sweat, your heart will race, and your muscles will tense.


Suppose you are walking down the street when an unsavoury-looking character suddenly assaults you. A few days later, someone starts running toward you, and your heart begins to pound. The person runs past you without touching you, and you start to calm down. It turns out they were just running to catch a bus.

A few weeks after that, you pass by the place where you actually were attacked, and you feel sick. This time no one is running toward you. The conditioned stimulus is not present, but the situation reveals a common phenomenon, in which certain elements of the context have also been conditioned by the traumatic event. This phenomenon implies the involvement of the hippocampus.



THE AMYGDALA AND ITS ALLIES
THE TWO PATHWAYS OF FEAR

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.

 

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.


       

Linked
Link :  The emotional brain Link :  Book : THE EMOTIONAL BRAIN by Joseph E. LeDoux Link :  Fear and the Amygdala Link :  Lighting up the preconscious
Link :  Amygdala Link : Neurobiology advances emotion research Link : PET images reveal brain's response to hunger for air Link : The anatomy of anxiety
Researcher
Research :  EMOTION, MEMORY, AND THE BRAIN: What the Lab Does and Why We Do It
Original modules
Experiment Module: Identifying the Brain Structures Involved in Conditioned Fear Identifying the Brain Structures Involved in Conditioned Fear
History Module: The Quest for the “Emotional Brain”   The Quest for the “Emotional Brain”

The “wiring” of the body’s natural alarm system demonstrates the usefulness, from an evolutionary standpoint, of the automatic reactions evoked by fear. For example, a small rodent that sees a predator will freeze in place automatically. This automatic reaction is invaluable, because it happens fast, with no need for voluntary control on the rodent’s part. The rodent’s immobility, combined with its natural camouflage, will generally let it escape the predator’s notice and flee as soon as its back is turned. As mammals evolved, those rodents that were less “fearful” and did not freeze in place attracted predators’ attention more quickly and thus, though very courageous, did not leave many descendants.

If researchers condition a rat to fear a certain sound, and then surgically remove the rat’s auditory cortex, the rat will no longer be able to distinguish that sound. A human with equivalent damage would be considered deaf. Yet the rat, once recovered from its operation and to all appearances deaf, still shows fear reactions when the sound is made in its presence. The rat thus still seems to register the sound in its thalamus and amygdala, which suffices to trigger the fear reaction.
THE TWO PATHWAYS OF FEAR
THE AMYGDALA AND ITS ALLIES

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 context.

In the conditioned-fear protocol, an animal can be taught to be afraid when it hears one particular sound, but not when it hears another sound that is only slightly different. But if you destroy this animal’s auditory cortex, then it will be just as afraid of the sound that is slightly different!

The reason has been discovered in electrophysiological experiments in which the activity of neurons in the thalamus and the cortex was recorded. The cortical neurons responded to only a very narrow range of frequencies, while the thalamic neurons were activated in response to a very broad range.

Consequently, when two similar sounds are used to condition a fear, and the cortex is removed to eliminate the possibility of fine discrimination, the fear response that is controlled by the thalamus will be displayed for both auditory stimuli without distinction.

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