It helps the brain recognize potential threats and helps prepare the body for fight-or-flight reactions by increasing heart and breathing rate. The amygdala is also responsible for learning on the basis of reward or punishment.
Due to its close proximity to the hippocampus, the amygdala is involved in the modulation of memory consolidation , particularly emotionally-laden memories. In fact, experiments have shown that administering stress hormones to individuals immediately after they learn something enhances their retention when they are tested two weeks later.
The figure shows the location of the amygdala from the underside ventral view of the human brain, with the front of the brain at the top of the image. The hippocampus is found deep in the temporal lobe, and is shaped like a seahorse. It consists of two horns curving back from the amygdala. Psychologists and neuroscientists dispute the precise role of the hippocampus, but generally agree that it plays an essential role in the formation of new memories about past experiences. Some researchers consider the hippocampus to be responsible for general declarative memory memories that can be explicitly verbalized, such as memory of facts and episodic memory.
Damage to the hippocampus usually results in profound difficulties in forming new memories anterograde amnesia , and may also affect access to memories formed prior to the damage retrograde amnesia. Although the retrograde effect normally extends some years prior to the brain damage, in some cases older memories remain intact; this leads to the idea that over time the hippocampus becomes less important in the storage of memory.
Both the thalamus and hypothalamus are associated with changes in emotional reactivity. The hypothalamus is a small part of the brain located just below the thalamus on both sides of the third ventricle. Lesions of the hypothalamus interfere with several unconscious functions such as respiration and metabolism and some so-called motivated behaviors like sexuality , combativeness, and hunger.
The lateral parts of the hypothalamus seem to be involved with pleasure and rage, while the medial part is linked to aversion, displeasure, and a tendency for uncontrollable and loud laughter. In this example the sensory experience is auditory rather than visual as in the emotion of faces. Much of what we know about the amygdala and its role in emotional learning and memory comes from fear conditioning, mostly but not exclusively conducted with animals.
This is an example of classical conditioning or Pavlovian conditioning. After a few such pairings the dog would salivate to the sound of the bell. The crucial aspect of classical conditioning is that it is a pairing between two stimuli.
No response is required to get the reward. In fear conditioning, an organism hears a noise or sees a visual stimulus. A few seconds, later it receives a mild shock. The reactions involve freezing, elevated blood pressure and heart rate, and it gets twitchy—startles easily.
Pathways from the thalamus to the amygdala are particularly important in emotional learning. Output pathways from the central nucleus of the amygdala make extensive connections with the brain stem for emotional responses and extensive connections with cortical areas through the nucleus basalis.
Cholinergic projections from the nucleus basalis to the cortex are thought to arouse the cortex. The following diagram provides additional information on outputs controlled by the amygdala during fear conditioning. Some pathways of fear conditioning have been discovered and this is a hot research topic in neuroscience.
If the auditory cortex pathway is lesioned, for example, basic fear conditioning is unaltered, but discrimination is altered. In the discrimination procedure one sound is paired with shock and another sound is not paired with shock.
The animals had to rely solely on the thalamus and amygdala for learning and they could not learn the discrimination; apparently the two stimuli were indistinguishable. So, the cortex is not needed for simple fear conditioning; instead it allows us to recognize an object by sight or sound— to interpret the environment. Thus, pathways from the sensory thalamus provide only a crude perception of the world, but because they involve only one neural link they are fast pathways.
Why might FAST be important? We need a quick reaction to potential danger. The thalamus—amygdala pathway provides us with this and may also prepare the amygdala to receive more highly processed information from the cortex. On the other hand, pathways from the cortex offer detailed and accurate representations of the environment. Because these pathways have multiple neural links they are slow by comparison.
If for example we see a slender curled shape behind a tree its much better to jump back and later recognize its a garden hose than to fail to quickly jump back if it were a snake. There is plenty of time later to reflect that it was foolish to be startled in our own secure garden where there are no snakes.
Fear producing visual stimuli is quickly processed by the thalamus and this information is passed to the amygdala red producing a quick response green to danger. The thalamus also passes the information to the cortex so that more careful and slower judgments can be made about the real potential danger. The amygdala is involved in pleasureful emotional learning as well as fearful emotional learning. Consider instrumental learning. Unlike classical conditioning where two stimuli are paired, in instrumental conditioning responses are followed by reward and stimulus-response associations are learned.
There are thus three events: a stimulus, a response, and a reward. It has become clear that all three pairwise combinations are learned in instrumental conditioning.
Where the amygdala comes in is that lesions of the basolateral nuclei of the amygdala disrupt the association the stimulus and rewarding attributes of the food. This amygdala memory system serves as an example of memory systems generally.
But the nucleus accumbens does not work in isolation. It maintains close relations with other centres involved in the mechanisms of pleasure, and in particular, with the ventral tegmental area VTA.
The locus coeruleus , an alarm centre of the brain and packed with norepinephrine, is another brain structure that plays an important role in drug addiction. When stimulated by a lack of the drug in question, the locus coeruleus drives the addict to do anything necessary to obtain a fix. Three structures in the limbic system also play an active part in the pleasure circuit and, consequently, in drug dependency.
The first is the amygdala , which imparts agreeable or disagreeable affective colorations to perceptions. The second is the hippocampus , the foundation of memory, which preserves the agreeable memories associated with taking the drug and, by association, all of the details of the environment in which it is taken.
Sometime in the future, these details may reawaken the desire to take the drug and perhaps contribute to recidivism in the patient. The third structure, the most anterior portion of the insular cortex , or insula, is regarded as part of the limbic system and is thought to possibly play a role in the active pleasure-seeking associated both with food and with psychoactive substances.
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