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The amygdala (; plural: amygdalae or amygdalas; also ';
Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...
from
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
, , ', 'almond', 'tonsil') is one of two almond-shaped clusters of nuclei located deep and medially within the
temporal lobe The temporal lobe is one of the four Lobes of the brain, major lobes of the cerebral cortex in the brain of mammals. The temporal lobe is located beneath the lateral fissure on both cerebral hemispheres of the mammalian brain. The temporal lobe ...
s of the brain's
cerebrum The cerebrum, telencephalon or endbrain is the largest part of the brain containing the cerebral cortex (of the two cerebral hemispheres), as well as several subcortical structures, including the hippocampus, basal ganglia, and olfactory bulb. In ...
in complex
vertebrates Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...
, including humans. Shown to perform a primary role in the processing of
memory Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remembered, ...
,
decision making In psychology, decision-making (also spelled decision making and decisionmaking) is regarded as the cognitive process resulting in the selection of a belief or a course of action among several possible alternative options. It could be either rati ...
, and emotional responses (including fear, anxiety, and aggression), the amygdalae are considered part of the
limbic system The limbic system, also known as the paleomammalian cortex, is a set of brain structures located on both sides of the thalamus, immediately beneath the medial temporal lobe of the cerebrum primarily in the forebrain.Schacter, Daniel L. 2012. ''Ps ...
. The term "amygdala" was first introduced by
Karl Friedrich Burdach Karl Friedrich Burdach (12 June 1776 – 16 July 1847) was a German physiologist. He was born in Leipzig and died in Königsberg. He was the first to use the word "biology" and was a pioneer of neuroanatomy. Life Burdach came from a family of ...
in 1822.


Structure

The regions described as amygdala nuclei encompass several structures of the
cerebrum The cerebrum, telencephalon or endbrain is the largest part of the brain containing the cerebral cortex (of the two cerebral hemispheres), as well as several subcortical structures, including the hippocampus, basal ganglia, and olfactory bulb. In ...
with distinct connectional and functional characteristics in humans and other animals. Among these nuclei are the basolateral complex, the cortical nucleus, the medial nucleus, the
central nucleus The central nucleus of the amygdala (CeA or aCeN) is a nucleus within the amygdala. It "serves as the major output nucleus of the amygdala and participates in receiving and processing pain information." CeA "connects with brainstem areas that con ...
, and the intercalated cell clusters. The basolateral complex can be further subdivided into the lateral, the basal, and the accessory basal nuclei. Anatomically, the amygdala, and more particularly its central and medial nuclei, have sometimes been classified as a part of the
basal ganglia The basal ganglia (BG), or basal nuclei, are a group of subcortical nuclei, of varied origin, in the brains of vertebrates. In humans, and some primates, there are some differences, mainly in the division of the globus pallidus into an extern ...
.


Hemispheric specializations

In one study, electrical stimulations of the right amygdala induced negative emotions, especially fear and sadness. In contrast, stimulation of the left amygdala was able to induce either pleasant (happiness) or unpleasant (fear, anxiety, sadness) emotions. Other evidence suggests that the left amygdala plays a role in the brain's
reward system The reward system (the mesocorticolimbic circuit) is a group of neural structures responsible for incentive salience (i.e., "wanting"; desire or craving for a reward and motivation), associative learning (primarily positive reinforcement and class ...
. Each side holds a specific function in how we perceive and process emotion. The right and left portions of the amygdala have independent memory systems, but work together to store, encode, and interpret emotion. The right hemisphere of the amygdala is associated with negative emotion. It plays a role in the expression of fear and in the processing of fear-inducing stimuli.
Fear conditioning Pavlovian fear conditioning is a behavioral paradigm in which organisms learn to predict aversive events. It is a form of learning in which an aversive stimulus (e.g. an electrical shock) is associated with a particular neutral context (e.g., a ...
, which occurs when a neutral stimulus acquires aversive properties, occurs within the right hemisphere. When an individual is presented with a conditioned, aversive stimulus, it is processed within the right amygdala, producing an unpleasant or fearful response. This emotional response conditions the individual to avoid fear-inducing stimuli and more importantly, to assess threats in the environment. The right hemisphere is also linked to
declarative memory Explicit memory (or declarative memory) is one of the two main types of long-term human memory, the other of which is implicit memory. Explicit memory is the conscious, intentional recollection of factual information, previous experiences, and c ...
, which consists of facts and information from previously experienced events and must be consciously recalled. It also plays a significant role in the retention of episodic memory.
Episodic memory Episodic memory is the memory of everyday events (such as times, location geography, associated emotions, and other contextual information) that can be explicitly stated or conjured. It is the collection of past personal experiences that occurred ...
consists of the autobiographical aspects of memory, permitting recall of emotional and sensory experience of an event. This type of memory does not require conscious recall. The right amygdala plays a role in the association of time and places with emotional properties.


Development and sex distinction

The amygdala is one of the best-understood brain regions with regard to differences between the sexes. The amygdala is larger in males than females in children aged 7 to 11, adult humans, and adult rats. There is considerable growth within the first few years of structural development in both male and female amygdalae. Within this early period, female limbic structures grow at a more rapid pace than the male ones. Amongst female subjects, the amygdala reaches its full growth potential approximately 1.5 years before the peak of male development. The structural development of the male amygdala occurs over a longer period than in women. Despite the early development of female amygdalae, they reach their growth potential sooner than males, whose amygdalae continue to develop. The larger relative size of the male amygdala may be attributed to this extended developmental period. Hormonal factors may contribute to these sex-specific developmental differences. The amygdala is rich in androgen receptors – nuclear receptors that bind to testosterone. Androgen receptors play a role in the DNA binding that regulates gene expression. Though testosterone is present within the female hormonal systems, women have lower levels of testosterone than men. The abundance of testosterone in the male hormonal system may contribute to development. In addition, the grey matter volume on the amygdala is predicted by testosterone levels, which may also contribute to the increased mass of the male amygdala. There are observable developmental differences between the right and left amygdala. The left amygdala reaches its developmental peak approximately 1.5–2 years prior to the right amygdala. Despite the early growth of the left amygdala, the right increases in volume for a longer period of time. The right amygdala is associated with response to fearful stimuli as well as face recognition. It is inferred that the early development of the left amygdala functions to provide infants the ability to detect danger. In childhood, the amygdala is found to react differently to same-sex versus opposite-sex individuals. This reactivity decreases until a person enters adolescence, where it increases dramatically at puberty. Other functional and structural differences between male and female amygdalae have been observed. Subjects' amygdala activation was observed when watching a horror film and
subliminal stimuli Subliminal stimuli (; the prefix ' literally means "below" or "less than") are any sensory stimuli below an individual's threshold for conscious perception, in contrast to stimuli (above threshold). A 2012 review of functional magnetic resonanc ...
. The results of the study showed a different lateralization of the amygdala in men and women. Enhanced memory for the film was related to enhanced activity of the left, but not the right, amygdala in women, whereas it was related to enhanced activity of the right, but not the left, amygdala in men. Similarly, a study of decision-making ability in patients with unilateral amygdala damage suggested that men with right (but not left) amygdala damage were more likely to be impaired in decision-making ability, while women with left (but not right) amygdala damage were more likely to be impaired in decision-making ability. One study found evidence that on average, women tend to retain stronger memories for emotional events than men.


Function


Connections

A simple view of the information processing through the amygdala follows as: the amygdala sends projections to the
hypothalamus The hypothalamus () is a part of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrine system via the pituitary gland. The hypothalamu ...
, the dorsomedial thalamus, the
thalamic reticular nucleus The thalamic reticular nucleus is part of the ventral thalamus that forms a capsule around the thalamus laterally. However, recent evidence from mice and fish question this statement and define it as a dorsal thalamic structure. It is separated fro ...
, the nuclei of the
trigeminal nerve In neuroanatomy, the trigeminal nerve ( lit. ''triplet'' nerve), also known as the fifth cranial nerve, cranial nerve V, or simply CN V, is a cranial nerve responsible for sensation in the face and motor functions such as biting and chewin ...
and the
facial nerve The facial nerve, also known as the seventh cranial nerve, cranial nerve VII, or simply CN VII, is a cranial nerve that emerges from the pons of the brainstem, controls the muscles of facial expression, and functions in the conveyance of tas ...
, the
ventral tegmental area The ventral tegmental area (VTA) (tegmentum is Latin for ''covering''), also known as the ventral tegmental area of Tsai, or simply ventral tegmentum, is a group of neurons located close to the midline on the floor of the midbrain. The VTA is the ...
, the
locus coeruleus The locus coeruleus () (LC), also spelled locus caeruleus or locus ceruleus, is a nucleus in the pons of the brainstem involved with physiological responses to stress and panic. It is a part of the reticular activating system. The locus coerule ...
, and the laterodorsal tegmental nucleus. The basolateral amygdala projects to the
nucleus accumbens The nucleus accumbens (NAc or NAcc; also known as the accumbens nucleus, or formerly as the ''nucleus accumbens septi'', Latin for " nucleus adjacent to the septum") is a region in the basal forebrain rostral to the preoptic area of the hypot ...
, including the medial shell. The medial nucleus is involved in the sense of smell and
pheromone A pheromone () is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the secreting individual, to affect the behavio ...
-processing. It receives input from the
olfactory bulb The olfactory bulb (Latin: ''bulbus olfactorius'') is a neural structure of the vertebrate forebrain involved in olfaction, the sense of smell. It sends olfactory information to be further processed in the amygdala, the orbitofrontal cortex ( ...
and olfactory cortex. The lateral amygdalae, which send impulses to the rest of the basolateral complexes and to the centromedial nuclei, receive input from the sensory systems. The centromedial nuclei are the main outputs for the basolateral complexes, and are involved in emotional arousal in rats and cats. Variability in amygdala connectivity has been related to a variety of behaviors and outcomes such as fear recognition and social network size.


Emotional learning

In complex vertebrates, including humans, the amygdalae perform primary roles in the formation and storage of memories associated with emotional events. Research indicates that, during
fear conditioning Pavlovian fear conditioning is a behavioral paradigm in which organisms learn to predict aversive events. It is a form of learning in which an aversive stimulus (e.g. an electrical shock) is associated with a particular neutral context (e.g., a ...
, sensory stimuli reach the basolateral complexes of the amygdalae, particularly the lateral nuclei, where they form associations with memories of the stimuli. The association between stimuli and the aversive events they predict may be mediated by
long-term potentiation In neuroscience, long-term potentiation (LTP) is a persistent strengthening of synapses based on recent patterns of activity. These are patterns of synaptic activity that produce a long-lasting increase in signal transmission between two neurons ...
, a sustained enhancement of signaling between affected neurons. There have been studies that show that damage to the amygdala can interfere with memory that is strengthened by emotion. One study examined a patient with bilateral degeneration of the amygdala. He was told a violent story accompanied by matching pictures and was observed based on how much he could recall from the story. The patient had less recollection of the story than patients with functional amygdala, showing that the amygdala has a strong connection with emotional learning. Emotional memories are thought to be stored in synapses throughout the brain. Fear memories, for example, are considered to be stored in the neuronal connections from the lateral nuclei to the central nucleus of the amygdalae and the bed nuclei of the stria terminalis (part of the
extended amygdala The extended amygdala is a macrostructure in the brain that is involved in reward cognition and defined by connectivity and neurochemical staining. It includes the central medial amygdala, sublenticular substantia innominata, and the bed nucleus o ...
). These connections are not the sole site of fear memories given that the nuclei of the amygdala receive and send information to other brain regions that are important for memory such as the hippocampus. Some
sensory neurons Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded potentials. This process is called sensory transduction. The ...
project their
axon terminals An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action po ...
to the
central nucleus The central nucleus of the amygdala (CeA or aCeN) is a nucleus within the amygdala. It "serves as the major output nucleus of the amygdala and participates in receiving and processing pain information." CeA "connects with brainstem areas that con ...
. The central nuclei are involved in the genesis of many fear responses such as defensive behavior (freezing or escape responses), autonomic nervous system responses (changes in blood pressure and heart rate/tachycardia), neuroendocrine responses (stress-hormone release), etc. Damage to the amygdalae impairs both the acquisition and expression of Pavlovian fear conditioning, a form of
classical conditioning Classical conditioning (also known as Pavlovian or respondent conditioning) is a behavioral procedure in which a biologically potent stimulus (e.g. food) is paired with a previously neutral stimulus (e.g. a triangle). It also refers to the learni ...
of emotional responses. Accumulating evidence has suggested that multiple neuromodulators acting in the amygdala regulates the formation of emotional memories. The amygdalae are also involved in appetitive (positive) conditioning. It seems that distinct neurons respond to positive and negative stimuli, but there is no clustering of these distinct neurons into clear anatomical nuclei. However, lesions of the central nucleus in the amygdala have been shown to reduce appetitive learning in rats. Lesions of the basolateral regions do not exhibit the same effect. Research like this indicates that different nuclei within the amygdala have different functions in appetitive conditioning. Nevertheless, researchers found an example of appetitive emotional learning showing an important role for the basolateral amygdala: The naïve female mice are innately attracted to non-volatile pheromones contained in male-soiled bedding, but not by the male-derived volatiles, become attractive if associated with non-volatile attractive pheromones, which act as unconditioned stimulus in a case of Pavlovian associative learning. In the vomeronasal, olfactory and emotional systems, Fos (gene family) proteins show that non-volatile pheromones stimulate the vomeronasal system, whereas air-borne volatiles activate only the olfactory system. Thus, the acquired preference for male-derived volatiles reveals an olfactory-vomeronasal associative learning. Moreover, the reward system is differentially activated by the primary pheromones and secondarily attractive odorants. Exploring the primary attractive pheromone activates the basolateral amygdala and the shell of nucleus accumbens but neither the ventral tegmental area nor the orbitofrontal cortex. In contrast, exploring the secondarily attractive male-derived odorants involves activation of a circuit that includes the basolateral amygdala, prefrontal cortex and ventral tegmental area. Therefore, the basolateral amygdala stands out as the key center for vomeronasal-olfactory associative learning.


Social


Reward

Glutamatergic Glutamatergic means "related to glutamate". A glutamatergic agent (or drug) is a chemical that directly modulates the excitatory amino acid (glutamate/aspartate) system in the body or brain. Examples include excitatory amino acid receptor agonis ...
neurons in the basolateral amygdala send projections to the
nucleus accumbens The nucleus accumbens (NAc or NAcc; also known as the accumbens nucleus, or formerly as the ''nucleus accumbens septi'', Latin for " nucleus adjacent to the septum") is a region in the basal forebrain rostral to the preoptic area of the hypot ...
shell and core. Activation of these projections drive motivational salience. The ability of these projections to drive
incentive salience Motivational salience is a cognitive process and a form of attention that ''motivates'' or propels an individual's behavior towards or away from a particular stimulus (psychology), object, perceived event or outcome. Motivational salience regulat ...
is dependent upon
dopamine receptor D1 Dopamine receptor D1, also known as DRD1. It is one of the two types of D1-like receptor family - receptors D1 and D5. It is a protein that in humans is encoded by the DRD1 gene. Tissue distribution D1 receptors are the most abundant kind of d ...
.


Memory modulation

The amygdala is also involved in the modulation of
memory consolidation Memory consolidation is a category of processes that stabilize a memory trace after its initial acquisition. A memory trace is a change in the nervous system caused by memorizing something. Consolidation is distinguished into two specific processe ...
. Following any learning event, the
long-term memory Long-term memory (LTM) is the stage of the Atkinson–Shiffrin memory model in which informative knowledge is held indefinitely. It is defined in contrast to short-term and working memory, which persist for only about 18 to 30 seconds. Long-t ...
for the event is not formed instantaneously. Rather, information regarding the event is slowly assimilated into long-term (potentially lifelong) storage over time, possibly via
long-term potentiation In neuroscience, long-term potentiation (LTP) is a persistent strengthening of synapses based on recent patterns of activity. These are patterns of synaptic activity that produce a long-lasting increase in signal transmission between two neurons ...
. Recent studies suggest that the amygdala regulates memory consolidation in other brain regions. Also,
fear conditioning Pavlovian fear conditioning is a behavioral paradigm in which organisms learn to predict aversive events. It is a form of learning in which an aversive stimulus (e.g. an electrical shock) is associated with a particular neutral context (e.g., a ...
, a type of memory that is impaired following amygdala damage, is mediated in part by long-term potentiation. During the consolidation period, the memory can be modulated. In particular, it appears that emotional arousal following the learning event influences the strength of the subsequent memory for that event. Greater emotional arousal following a learning event enhances a person's retention of that event. Experiments have shown that administration of
stress hormones Stress hormones are secreted by endocrine glands to modify one's internal environment during times of stress. By performing various functions such as mobilizing energy sources, increasing heart rate, and downregulating metabolic processes which a ...
to mice immediately after they learn something enhances their retention when they are tested two days later. The amygdala, especially the basolateral nuclei, are involved in mediating the effects of emotional arousal on the strength of the memory for the event, as shown by many laboratories including that of
James McGaugh James L. McGaugh (born December 17, 1931) is an American neurobiologist and author working in the field of learning and memory. He is a Distinguished Professor Emeritus in the Department of Neurobiology and Behavior at the University of Californ ...
. These laboratories have trained animals on a variety of learning tasks and found that drugs injected into the amygdala after training affect the animals' subsequent retention of the task. These tasks include basic
classical conditioning Classical conditioning (also known as Pavlovian or respondent conditioning) is a behavioral procedure in which a biologically potent stimulus (e.g. food) is paired with a previously neutral stimulus (e.g. a triangle). It also refers to the learni ...
tasks such as inhibitory avoidance, where a rat learns to associate a mild footshock with a particular compartment of an apparatus, and more complex tasks such as spatial or cued water maze, where a rat learns to swim to a platform to escape the water. If a drug that activates the amygdalae is injected into the amygdalae, the animals had better memory for the training in the task. If a drug that inactivates the amygdalae is injected, the animals had impaired memory for the task. In rats,
DNA damage DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA da ...
was found to increase in the amygdala immediately after exposure to stress. Stress was induced by 30 minutes of restraint or by forced swimming. By seven days after exposure to these stresses, increased DNA damage was no longer detectable in the amygdala, probably because of
DNA repair DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA dam ...
.
Buddhist monks A ''bhikkhu'' (Pali: भिक्खु, Sanskrit: भिक्षु, ''bhikṣu'') is an ordained male in Buddhist monasticism. Male and female monastics ("nun", ''bhikkhunī'', Sanskrit ''bhikṣuṇī'') are members of the Sangha (Buddhist c ...
who do
compassion meditation Compassion motivates people to go out of their way to relieve the physical, mental or emotional pains of others and themselves. Compassion is often regarded as being sensitive to the emotional aspects of the suffering of others. When based on n ...
have been shown to modulate their amygdala, along with their
temporoparietal junction The temporoparietal junction (TPJ) is an area of the brain where the temporal and parietal lobes meet, at the posterior end of the lateral sulcus (Sylvian fissure). The TPJ incorporates information from the thalamus and the limbic system as wel ...
and insula, during their practice. In an
fMRI Functional magnetic resonance imaging or functional MRI (fMRI) measures brain activity by detecting changes associated with blood flow. This technique relies on the fact that cerebral blood flow and neuronal activation are coupled. When an area ...
study, more intensive insula activity was found in expert meditators than in novices. Amygdala activity at the time of encoding information correlates with retention for that information. However, this correlation depends on the relative "emotionalness" of the information. More emotionally arousing information increases amygdalar activity, and that activity correlates with retention. Amygdala neurons show various types of
oscillation Oscillation is the repetitive or periodic variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. Familiar examples of oscillation include a swinging pendulum ...
during emotional arousal, such as theta activity. These synchronized neuronal events could promote
synaptic plasticity In neuroscience, synaptic plasticity is the ability of synapses to strengthen or weaken over time, in response to increases or decreases in their activity. Since memories are postulated to be represented by vastly interconnected neural circuit ...
(which is involved in memory retention) by increasing interactions between neocortical storage sites and temporal lobe structures involved in
declarative memory Explicit memory (or declarative memory) is one of the two main types of long-term human memory, the other of which is implicit memory. Explicit memory is the conscious, intentional recollection of factual information, previous experiences, and c ...
. Research using
Rorschach test The Rorschach test is a projective psychological test in which subjects' perceptions of inkblots are recorded and then analyzed using psychological interpretation, complex algorithms, or both. Some psychologists use this test to examine a pe ...
blot 03 finds that the number of unique responses to this random figure links to larger sized amygdalae. The researchers note, "Since previous reports have indicated that unique responses were observed at higher frequency in the artistic population than in the nonartistic normal population, this positive correlation suggests that amygdalar enlargement in the normal population might be related to creative mental activity."


Neuropsychological correlates of amygdala activity

Early research on primates provided explanations as to the functions of the amygdala, as well as a basis for further research. As early as 1888, rhesus monkeys with a lesioned temporal cortex (including the amygdala) were observed to have significant social and emotional deficits.
Heinrich Klüver Heinrich Klüver (; May 25, 1897 – February 8, 1979) was a German-American biological psychologist and philosopher born in Holstein. After having served in the Imperial German Army during World War I, he studied at both the University of Ha ...
and Paul Bucy later expanded upon this same observation by showing that large lesions to the anterior temporal lobe produced noticeable changes, including overreaction to all objects, hypoemotionality, loss of fear,
hypersexuality Hypersexuality is extremely frequent or suddenly increased libido. It is controversial whether it should be included as a clinical diagnosis used by mental healthcare professionals. Nymphomania and satyriasis were terms previously used for the c ...
, and hyperorality, a condition in which inappropriate objects are placed in the mouth. Some monkeys also displayed an inability to recognize familiar objects and would approach animate and inanimate objects indiscriminately, exhibiting a loss of fear towards the experimenters. This behavioral disorder was later named Klüver-Bucy syndrome accordingly, and later research proved it was specifically due to amygdala lesions. Monkey mothers who had amygdala damage showed a reduction in maternal behaviors towards their infants, often physically abusing or neglecting them. In 1981, researchers found that selective radio frequency lesions of the whole amygdala caused Klüver-Bucy syndrome. With advances in
neuroimaging Neuroimaging is the use of quantitative (computational) techniques to study the structure and function of the central nervous system, developed as an objective way of scientifically studying the healthy human brain in a non-invasive manner. Incr ...
technology such as
MRI Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to form pictures of the anatomy and the physiological processes of the body. MRI scanners use strong magnetic fields, magnetic field gradients, and radio waves ...
, neuroscientists have made significant findings concerning the amygdala in the human brain. A variety of data shows the amygdala has a substantial role in mental states, and is related to many
psychological disorders A mental disorder, also referred to as a mental illness or psychiatric disorder, is a behavioral or mental pattern that causes significant distress or impairment of personal functioning. Such features may be persistent, relapsing and remitti ...
. Some studies have shown children with anxiety disorders tend to have a smaller left amygdala. In the majority of the cases, there was an association between an increase in the size of the left amygdala with the use of
SSRI Selective serotonin reuptake inhibitors (SSRIs) are a class of drugs that are typically used as antidepressants in the treatment of major depressive disorder, anxiety disorders, and other psychological conditions. SSRIs increase the extracellul ...
s (antidepressant medication) or psychotherapy. The left amygdala has been linked to
social anxiety disorder Social anxiety disorder (SAD), also known as social phobia, is an anxiety disorder characterized by sentiments of fear and anxiety in social situations, causing considerable distress and impaired ability to function in at least some aspects ...
, obsessive and compulsive disorders, and
posttraumatic stress disorder Post-traumatic stress disorder (PTSD) is a mental and behavioral disorder that can develop because of exposure to a traumatic event, such as sexual assault, warfare, traffic collisions, child abuse, domestic violence, or other threats on ...
, as well as more broadly to separation and
generalized anxiety disorder Generalized anxiety disorder (GAD) is a mental and behavioral disorder, specifically an anxiety disorder characterized by excessive, uncontrollable and often irrational worry about events or activities. Worry often interferes with daily function ...
. In a 2003 study, subjects with
borderline personality disorder Borderline personality disorder (BPD), also known as emotionally unstable personality disorder (EUPD), is a personality disorder characterized by a long-term pattern of unstable interpersonal relationships, distorted sense of self, and strong ...
showed significantly greater left amygdala activity than normal control subjects. Some borderline patients even had difficulties classifying neutral faces or saw them as threatening. Individuals with
psychopathy Psychopathy, sometimes considered synonymous with sociopathy, is characterized by persistent antisocial behavior, impaired empathy and remorse, and bold, disinhibited, and egotistical traits. Different conceptions of psychopathy have bee ...
show reduced autonomic responses to instructed fear cues than otherwise healthy individuals. In 2006, researchers observed
hyperactivity Attention deficit hyperactivity disorder (ADHD) is a neurodevelopmental disorder characterised by excessive amounts of inattention, hyperactivity, and impulsivity that are pervasive, impairing in multiple contexts, and otherwise age-inapp ...
in the amygdala when patients were shown threatening faces or confronted with frightening situations. Patients with severe
social phobia Social anxiety disorder (SAD), also known as social phobia, is an anxiety disorder characterized by sentiments of fear and anxiety in social situations, causing considerable distress and impaired ability to function in at least some aspects ...
showed a correlation with increased response in the amygdala.Lay summary
available from
Science Daily ''Science Daily'' is an American website launched in 1995 that aggregates press releases and publishes lightly edited press releases (a practice called churnalism) about science, similar to Phys.org and EurekAlert!. The site was founded by mar ...
.
Similarly, depressed patients showed exaggerated left amygdala activity when interpreting emotions for all faces, and especially for fearful faces. This hyperactivity was normalized when patients were administered antidepressant medication. By contrast, the amygdala has been observed to respond differently in people with
bipolar disorder Bipolar disorder, previously known as manic depression, is a mental disorder characterized by periods of depression and periods of abnormally elevated mood that last from days to weeks each. If the elevated mood is severe or associated with ...
. A 2003 study found that adult and adolescent bipolar patients tended to have considerably smaller amygdala volumes and somewhat smaller
hippocampal The hippocampus (via Latin from Greek , 'seahorse') is a major component of the brain of humans and other vertebrates. Humans and other mammals have two hippocampi, one in each side of the brain. The hippocampus is part of the limbic system, an ...
volumes. Many studies have focused on the connections between the amygdala and
autism The autism spectrum, often referred to as just autism or in the context of a professional diagnosis autism spectrum disorder (ASD) or autism spectrum condition (ASC), is a neurodevelopmental condition (or conditions) characterized by difficulti ...
. Studies in 2004 and 2006 showed that normal subjects exposed to images of frightened faces or faces of people from another race will show increased activity of the amygdala, even if that exposure is
subliminal Subliminal may refer to: * Subliminal stimuli, sensory stimuli below an individual's threshold for conscious perception * Subliminal channel, in cryptography, a covert channel that can be used over an insecure channel * Subliminal (rapper) (born ...
.Lay summary
available from
Science Daily ''Science Daily'' is an American website launched in 1995 that aggregates press releases and publishes lightly edited press releases (a practice called churnalism) about science, similar to Phys.org and EurekAlert!. The site was founded by mar ...
.
However, the amygdala is not necessary for the processing of fear-related stimuli, since persons in whom it is bilaterally damaged show rapid reactions to fearful faces, even in the absence of a functional amygdala.


Sexual orientation

Recent studies have suggested possible correlations between brain structure, including differences in hemispheric ratios and connection patterns in the amygdala, and sexual orientation.
Homosexual men Gay men are male homosexuals. Some bisexual men, bisexual and homoromantic men may also dually identify as gay, and a number of young gay men also identify as queer. Historically, gay men have been referred to by a number of different terms, ...
tend to exhibit more feminine patterns in the amygdala than heterosexual males do, just as homosexual females tend to show more masculine patterns in the amygdala than heterosexual women do. It was observed that amygdala connections were more widespread from the left amygdala in homosexual males, as is also found in heterosexual females. Amygdala connections were more widespread from the right amygdala in homosexual females, as in heterosexual males.


Social

Increased activity in the amygdala following compassion-oriented meditation may contribute to social connectedness. Similarly, the structural white matter connectivity to other brain regions is also associated with social network size. Amygdala volume correlates positively with both the size (the number of contacts a person has) and the complexity (the number of different groups to which a person belongs) of
social network A social network is a social structure made up of a set of social actors (such as individuals or organizations), sets of dyadic ties, and other social interactions between actors. The social network perspective provides a set of methods for an ...
s. Individuals with larger amygdalae had larger and more complex social networks. The amygdala is responsible for facial recognition and allows others to respond appropriately to different emotional expressions. They were also better able to make accurate social judgments about other persons' faces. The amygdala's role in the analysis of social situations stems specifically from its ability to identify and process changes in facial features. It does not, however, process the direction of the gaze of the person being perceived. The amygdala is also thought to be a determinant of the level of a person's
emotional intelligence Emotional intelligence (EI) is most often defined as the ability to perceive, use, understand, manage, and handle emotions. People with high emotional intelligence can emotion recognition, recognize their own emotions and those of others, use em ...
. It is particularly hypothesized that larger amygdalae allow for greater emotional intelligence, enabling greater societal integration and cooperation with others. The amygdala processes reactions to violations concerning
personal space Proxemics is the study of human use of space and the effects that population density has on behaviour, communication, and social interaction. Proxemics is one among several subcategories in the study of nonverbal communication, including haptics ...
. These reactions are absent in persons in whom the amygdala is damaged bilaterally. Furthermore, the amygdala is found to be activated in
fMRI Functional magnetic resonance imaging or functional MRI (fMRI) measures brain activity by detecting changes associated with blood flow. This technique relies on the fact that cerebral blood flow and neuronal activation are coupled. When an area ...
when people observe that others are physically close to them, such as when a person being scanned knows that an experimenter is standing immediately next to the scanner, versus standing at a distance.


Aggression

Animal studies have shown that stimulating the amygdala appears to increase both sexual and aggressive behavior. Likewise, studies using brain lesions have shown that harm to the amygdala may produce the opposite effect. Thus, it appears that this part of the brain may play a role in the display and modulation of aggression.


Fear

There are cases of human patients with focal bilateral amygdala lesions due to the rare genetic condition Urbach-Wiethe disease. Such patients fail to exhibit fear-related behaviors, leading one, S.M., to be dubbed the "woman with no fear". This finding reinforces the conclusion that the amygdala "plays a pivotal role in triggering a state of fear".


Alcoholism and binge drinking

The amygdala appears to play a role in
binge drinking Binge drinking, or heavy episodic drinking, is drinking alcoholic beverages with an intention of becoming intoxicated by heavy consumption of alcohol over a short period of time, but definitions ( see below) vary considerably. Binge drinking ...
, being damaged by repeated episodes of intoxication and withdrawal. Protein kinase C-epsilon in the amygdala is important for regulating behavioral responses to
morphine Morphine is a strong opiate that is found naturally in opium, a dark brown resin in poppies (''Papaver somniferum''). It is mainly used as a analgesic, pain medication, and is also commonly used recreational drug, recreationally, or to make ...
,
ethanol Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an Alcohol (chemistry), alcohol with the chemical formula . Its formula can be also written as or (an ethyl ...
, and controlling anxiety-like behavior. The protein is involved in controlling the function of other proteins and plays a role in development of the ability to consume a large amount of ethanol. The duration of chronic alcohol consumption and abstinence may affect dynamic brain network adaptations. When excessive drinking occurs, the amygdala is affected through behavioral changes and reduces the brain's plasticity. Brain plasticity is how our brain grows and develops; it is also how our neurons can make connections with other neurons. This ultimately increases our neural pathways allowing us to increase our knowledge of the world around us. When our brain plasticity decreases, it makes it difficult for neurons to make connections to other neurons. Often when binge drinking, or alcoholism occurs, our amygdala is affected and leads to behavior damage. These behavioral damages can be lack of control, inability to conduct oneself in a mature manner, aggressive behavior, loss of conduct, anxiety, depression, personality disorders, excessive drug intake, bi-polar disorder, confusion, higher tolerance levels, irritability, and inappropriate sexual behaviors with others and self.


Anxiety

There may also be a link between the amygdala and
anxiety Anxiety is an emotion which is characterized by an unpleasant state of inner turmoil and includes feelings of dread over anticipated events. Anxiety is different than fear in that the former is defined as the anticipation of a future threat wh ...
. In particular, there is a higher prevalence of females that are affected by
anxiety disorders Anxiety disorders are a cluster of mental disorders characterized by significant and uncontrollable feelings of anxiety and fear such that a person's social, occupational, and personal function are significantly impaired. Anxiety may cause physi ...
. In an experiment, degu pups were removed from their mother but allowed to hear her call. In response, the males produced increased serotonin receptors in the amygdala but females lost them. This led to the males being less affected by the stressful situation. The clusters of the amygdala are activated when an individual expresses feelings of fear or aggression. This occurs because the amygdala is the primary structure of the brain responsible for fight or flight response. Anxiety and panic attacks can occur when the amygdala senses environmental stressors that stimulate fight or flight response. The amygdala is directly associated with conditioned fear. Conditioned fear is the framework used to explain the behavior produced when an originally neutral stimulus is consistently paired with a stimulus that evokes fear. The amygdala represents a core fear system in the human body, which is involved in the expression of conditioned fear. Fear is measured by changes in autonomic activity including increased heart rate, increased blood pressure, as well as in simple reflexes such as flinching or blinking. The central nucleus of the amygdala has direct correlations to the hypothalamus and brainstem – areas directly related to fear and anxiety. This connection is evident from studies of animals that have undergone amygdalae removal. Such studies suggest that animals lacking an amygdala have less fear expression and indulge in non-species-like behavior. Many projection areas of the amygdala are critically involved in specific signs that are used to measure fear and anxiety. Mammals have very similar ways of processing and responding to danger. Scientists have observed similar areas in the brain – specifically in the amygdala – lighting up or becoming more active when a mammal is threatened or beginning to experience anxiety. Similar parts of the brain are activated when rodents and humans alike observe a dangerous situation, the amygdala playing a crucial role in this assessment. By observing the amygdalae's functions, it can determined why one rodent may be much more anxious than another. There is a direct relationship between the activation of the amygdala and the level of anxiety the subject feels. Feelings of anxiety start with a catalyst – an environmental stimulus that provokes stress. This can include various smells, sights, and internal sensations that result in anxiety. The amygdala reacts to this stimuli by preparing to either stand and fight or to turn and run. This response is triggered by the release of adrenaline into the bloodstream. Consequently, blood sugar rises, becoming immediately available to the muscles for quick energy. Shaking may occur in an attempt to return blood to the rest of the body. Apart from initiation of stress, long-term changes in amygdala neurons may also increase anxiety after long-term or traumatic stress, led by the action of stress-related hormones within the amygdala. On the flip side, blocking the action of stress hormones in the amygdala reduces anxiety. A better understanding of the amygdala and its various functions may lead to a new way of treating clinical anxiety.


Posttraumatic stress disorder

There seems to be a connection with the amygdalae and how the brain processes
posttraumatic stress disorder Post-traumatic stress disorder (PTSD) is a mental and behavioral disorder that can develop because of exposure to a traumatic event, such as sexual assault, warfare, traffic collisions, child abuse, domestic violence, or other threats on ...
. Multiple studies have found that the amygdalae may be responsible for the emotional reactions of PTSD patients. One study in particular found that when PTSD patients are shown pictures of faces with fearful expressions, their amygdalae tended to have a higher activation than someone without PTSD.


Bipolar disorder

Amygdala dysfunction during face emotion processing is well-documented in
bipolar disorder Bipolar disorder, previously known as manic depression, is a mental disorder characterized by periods of depression and periods of abnormally elevated mood that last from days to weeks each. If the elevated mood is severe or associated with ...
. Individuals with bipolar disorder showed greater amygdala activity (especially the amygdala/medial-prefrontal-cortex circuit).


Additional images

File:Amygdala.png, Frontal and side view of amygdala File:Amygdala_-_DK_ATLAS.png, Amygdala along with other subcortical regions, in glass brain. File:Amygdala dorsal view.png, Dorsal view of the amygdala in an average human brain File:Amygdala frontal view.png, Frontal view of the amygdala in an average human brain File:Amygdala left lateral slice.png, Left lateral view of the amygdala in an average human brain File:Amygdala coronal sections.gif, Amygdala highlighted in green on coronal T1 MRI images File:Amygdala sagittal sections.gif, Amygdala highlighted in green on sagittal T1 MRI images File:Amygdala transversal sections.gif, Amygdala highlighted in green on transversal T1 MRI images


See also

* Accessory olfactory cortical areas * Amygdala hijack *
BELBIC In recent years, the use of biologically inspired methods such as the evolutionary algorithm have been increasingly employed to solve and analyze complex computational problems. BELBIC (Brain Emotional Learning Based Intelligent Controller) is one s ...
*
Intercalated cells of the amygdala The intercalated cells of the amygdala (ITC or ICC) are a group of GABAergic neurons situated between the basolateral and central nuclei of the amygdala that play a significant role in inhibitory control over the amygdala. They play important regula ...
*
List of regions in the human brain The human brain anatomical regions are ordered following standard neuroanatomy hierarchies. Functional, connective, and developmental regions are listed in parentheses where appropriate. Hindbrain (rhombencephalon) Myelencephalon * Med ...
*
Triune brain The triune brain is a model of the evolution of the vertebrate forebrain and behavior, proposed by the American physician and neuroscientist Paul D. MacLean in the 1960s. The triune brain consists of the reptilian complex (basal ganglia), the p ...
*
Amygdalotomy Amygdalotomy is a form of psychosurgery which involves the surgical removal or destruction of the amygdala, or parts of the amygdala. It is usually a last-resort treatment for severe aggressive behavioral disorders and similar behaviors including ...
*
Amygdalohippocampectomy Amygdalohippocampectomy is a surgical procedure for the treatment of epilepsy. It consists of the removal of the hippocampus, which has a role in memory, spatial awareness, and navigation, and the amygdalae, which have a role in the processing and ...
* S.M. (patient)


References


Further reading

*


External links

* {{Authority control Neuropsychology