Editing Fear (section)

==Mechanism==
{{anchor|Fear in animals}}
Often laboratory studies with rats are conducted to examine the acquisition and extinction of [[fear conditioning|conditioned fear]] responses.<ref>{{cite journal | vauthors = Morgan MA, LeDoux JE | title = Differential contribution of dorsal and ventral medial prefrontal cortex to the acquisition and extinction of conditioned fear in rats | journal = Behavioral Neuroscience | volume = 109 | issue = 4 | pages = 681–688 | date = August 1995 | pmid = 7576212 | doi = 10.1037/0735-7044.109.4.681 | s2cid = 3167606 }}</ref> In 2004, researchers conditioned rats (''Rattus norvegicus'') to fear a certain stimulus, through electric shock.<ref>{{cite journal | vauthors = Cammarota M, Bevilaqua LR, Kerr D, Medina JH, Izquierdo I | title = Inhibition of mRNA and protein synthesis in the CA1 region of the dorsal hippocampus blocks reinstallment of an extinguished conditioned fear response | journal = The Journal of Neuroscience | volume = 23 | issue = 3 | pages = 737–741 | date = February 2003 | pmid = 12574401 | pmc = 6741935 | doi = 10.1523/JNEUROSCI.23-03-00737.2003 }}</ref> The researchers were able to then cause an extinction of this conditioned fear, to a point that no medications or drugs were able to further aid in the extinction process. The rats showed signs of avoidance learning, not fear, but simply avoiding the area that brought pain to the test rats. The avoidance learning of rats is seen as a [[conditioned response]], and therefore the behavior can be unconditioned, as supported by the earlier research.

Species-specific defense reactions (SSDRs) or [[avoidance learning]] in nature is the specific tendency to avoid certain threats or stimuli, it is how animals survive in the wild. Humans and animals both share these species-specific defense reactions, such as the flight-or-fight, which also include pseudo-aggression, fake or intimidating aggression and freeze response to threats, which is controlled by the [[sympathetic nervous system]]. These SSDRs are learned very quickly through social interactions between others of the same species, other species, and interaction with the environment.<ref>{{cite book| vauthors = Davis S |title=21st Century Psychology: A Reference Handbook, Vol. 1|year=2008|publisher=Sage Publications, Inc.|location=Thousand Oaks, California|isbn=978-1-4129-4968-2|pages=282–286}}</ref> These acquired sets of reactions or responses are not easily forgotten. The animal that survives is the animal that already knows what to fear and how to avoid this threat. An example in humans is the reaction to the sight of a snake, many jump backwards before cognitively realizing what they are jumping away from, and in some cases, it is a stick rather than a snake.

As with many functions of the brain, there are various regions of the brain involved in deciphering fear in humans and other nonhuman species.<ref>{{cite web| vauthors = Robert P |title=The Amygdala and Its Allies|url=http://thebrain.mcgill.ca/flash/a/a_04/a_04_cr/a_04_cr_peu/a_04_cr_peu.html|website=2002|publisher=The Brain|access-date=2 October 2013|url-status=live|archive-url=https://web.archive.org/web/20130806074559/http://thebrain.mcgill.ca/flash/a/a_04/a_04_cr/a_04_cr_peu/a_04_cr_peu.html|archive-date=6 August 2013}}</ref> The [[amygdala]] communicates both directions between the [[prefrontal cortex]], [[hypothalamus]], the [[sensory cortex]], the [[hippocampus]], [[thalamus]], [[septum]], and the [[brainstem]]. The amygdala plays an important role in SSDR, such as the ventral amygdalofugal, which is essential for [[associative learning]], and SSDRs are learned through interaction with the environment and others of the same species. An emotional response is created only after the signals have been relayed between the different regions of the brain, and activating the sympathetic nervous systems; which controls the [[Fight, Flight or freeze|flight, fight, freeze, fright, and faint response]].<ref>{{cite journal | vauthors = Schmidt NB, Richey JA, Zvolensky MJ, Maner JK | title = Exploring human freeze responses to a threat stressor | journal = Journal of Behavior Therapy and Experimental Psychiatry | volume = 39 | issue = 3 | pages = 292–304 | date = September 2008 | pmid = 17880916 | pmc = 2489204 | doi = 10.1016/j.jbtep.2007.08.002 }}</ref><ref>{{cite journal | vauthors = Bracha HS | title = Freeze, flight, fight, fright, faint: adaptationist perspectives on the acute stress response spectrum | journal = CNS Spectrums | volume = 9 | issue = 9 | pages = 679–685 | date = September 2004 | pmid = 15337864 | doi = 10.1017/s1092852900001954 | url = http://cogprints.org/5014/1/2004_C.N.S_Five_Fs_of_FEAR%2D%2DFreeze_Flight_Fight_Fright_Faint.pdf | s2cid = 8430710 }}</ref> Often a damaged amygdala can cause impairment in the recognition of fear (like the human case of [[S.M. (patient)|patient S.M.]]).<ref>{{cite journal | vauthors = Adolphs R, Gosselin F, Buchanan TW, Tranel D, Schyns P, Damasio AR | title = A mechanism for impaired fear recognition after amygdala damage | journal = Nature | volume = 433 | issue = 7021 | pages = 68–72 | date = January 2005 | pmid = 15635411 | doi = 10.1038/nature03086 | url = https://authors.library.caltech.edu/55911/2/nature03086-s1.doc | s2cid = 2139996 | bibcode = 2005Natur.433...68A }}</ref> This impairment can cause different species to lack the sensation of fear, and often can become overly confident, confronting larger peers, or walking up to predatory creatures.

[[Robert C. Bolles]] (1970), a researcher at University of Washington, wanted to understand species-specific defense reactions and avoidance learning among animals, but found that the theories of avoidance learning and the tools that were used to measure this tendency were out of touch with the natural world.<ref>{{cite journal| vauthors = Bolles R |title=Species-Specific Defense Reactions and Avoidance Learning|journal=Psychological Review|year=1970|volume=77|issue=1|pages=32–48|doi=10.1037/h0028589}}</ref> He theorized the species-specific defense reaction (SSDR).<ref>{{cite journal | vauthors = Crawford M, Masterson FA | title = Species-specific defense reactions and avoidance learning. An evaluative review | journal = The Pavlovian Journal of Biological Science | volume = 17 | issue = 4 | pages = 204–214 | year = 1982 | pmid = 6891452 | doi = 10.1007/BF03001275 | s2cid = 142436039 | url = https://link.springer.com/article/10.1007/BF03001275 }}</ref> There are three forms of SSDRs: flight, fight (pseudo-aggression), or freeze. Even domesticated animals have SSDRs, and in those moments it is seen that animals revert to atavistic standards and become "wild" again. Dr. Bolles states that responses are often dependent on the reinforcement of a safety signal, and not the aversive conditioned stimuli. This safety signal can be a source of feedback or even stimulus change. Intrinsic feedback or information coming from within, muscle twitches, increased heart rate, are seen to be more important in SSDRs than extrinsic feedback, stimuli that comes from the external environment. Dr. Bolles found that most creatures have some intrinsic set of fears, to help assure survival of the species. Rats will run away from any [[Acute stress disorder|shocking]] event, and pigeons will flap their wings harder when threatened. The wing flapping in pigeons and the scattered running of rats are considered species-specific defense reactions or behaviors. Bolles believed that SSDRs are conditioned through [[Pavlovian]] conditioning, and not operant conditioning; SSDRs arise from the association between the environmental stimuli and adverse events.<ref>{{cite book| vauthors = Kiein S |title=Biological Influences on Learning|year=2002|publisher=McGraw-Hill Higher Education|location=Mississippi State University|url=http://highered.mcgraw-hill.com/sites/0072490462/student_view0/chapter10/chapter_outline.html|url-status=live|archive-url=https://web.archive.org/web/20081205080132/http://highered.mcgraw-hill.com/sites/0072490462/student_view0/chapter10/chapter_outline.html|archive-date=2008-12-05}}</ref> [[Michael S. Fanselow]] conducted an experiment, to test some specific defense reactions, he observed that rats in two different shock situations responded differently, based on instinct or defensive topography, rather than contextual information.<ref>{{cite journal| vauthors = Fanselow M |author-link=Michael Fanselow |journal=Learning and Motivation|date=1986|volume=17|issue=1|pages=16–39|title=Associative vs topographical accounts of the immediate shock-freezing deficit in rats: Implications for the response selection rules governing species-specific defensive reactions|doi=10.1016/0023-9690(86)90018-4}}</ref>

Species-specific defense responses are created out of fear, and are essential for survival.<ref>{{cite journal |vauthors=Crawford M, Masterson FA | title=Species-specific defense reactions and avoidance learning | journal=The Pavlovian Journal of Biological Science | publisher=Springer Science and Business Media LLC | volume=17 | issue=4 | date=October 1982 | issn=0093-2213 | doi=10.1007/bf03001275 | pages=204–214| pmid=6891452 | s2cid=142436039 }}</ref> Rats that lack the gene [[stathmin]] show no avoidance learning, or a lack of fear, and will often walk directly up to cats and be eaten.<ref>{{cite journal | vauthors = Brocke B, Lesch KP, Armbruster D, Moser DA, Müller A, Strobel A, Kirschbaum C | title = Stathmin, a gene regulating neural plasticity, affects fear and anxiety processing in humans | journal = American Journal of Medical Genetics. Part B, Neuropsychiatric Genetics | volume = 153B | issue = 1 | pages = 243–251 | date = January 2010 | pmid = 19526456 | doi = 10.1002/ajmg.b.30989 | s2cid = 14851460 }}</ref> Animals use these SSDRs to continue living, to help increase their chance of [[Fitness (biology)|fitness]], by surviving long enough to procreate. Humans and animals alike have created fear to know what should be avoided, and this fear can be learned through [[Learning|association]] with others in the community, or learned through personal experience with a creature, species, or situations that should be avoided. SSDRs are an evolutionary adaptation that has been seen in many species throughout the world including rats, [[Pan (genus)|chimpanzee]]s, [[prairie dog]]s, and even [[human]]s, an adaptation created to help individual creatures survive in a hostile world.

Fear learning changes across the lifetime due to natural developmental changes in the brain.<ref>{{Cite journal| vauthors = Kim JH, Ganella DE |date=2015-02-01|title=A Review of Preclinical Studies to Understand Fear During Adolescence|journal=Australian Psychologist|language=en|volume=50|issue=1|pages=25–31|doi=10.1111/ap.12066|s2cid=142760996|issn=1742-9544}}</ref><ref>{{cite journal | vauthors = Kim JH, Richardson R | title = New findings on extinction of conditioned fear early in development: theoretical and clinical implications | journal = Biological Psychiatry | volume = 67 | issue = 4 | pages = 297–303 | date = February 2010 | pmid = 19846065 | doi = 10.1016/j.biopsych.2009.09.003 | s2cid = 33444381 }}</ref> This includes changes in the [[prefrontal cortex]] and the [[amygdala]].<ref>{{cite journal | vauthors = Li S, Kim JH, Richardson R | title = Differential involvement of the medial prefrontal cortex in the expression of learned fear across development | journal = Behavioral Neuroscience | volume = 126 | issue = 2 | pages = 217–225 | date = April 2012 | pmid = 22448855 | doi = 10.1037/a0027151 }}</ref>

The visual exploration of an emotional face does not follow a fixed pattern but modulated by the emotional content of the face. Scheller et al.<ref>{{cite journal | vauthors = Scheller E, Büchel C, Gamer M | title = Diagnostic features of emotional expressions are processed preferentially | journal = PLOS ONE | volume = 7 | issue = 7 | pages = e41792 | date = 2012-07-25 | pmid = 22848607 | pmc = 3405011 | doi = 10.1371/journal.pone.0041792 | bibcode = 2012PLoSO...741792S | doi-access = free }}</ref> found that participants paid more attention to the eyes when recognising fearful or neutral faces, while the mouth was fixated on when happy faces are presented, irrespective of task demands and spatial locations of face stimuli. These findings were replicated when fearful eyes are presented<ref>{{cite journal | vauthors = Smith ML, Cottrell GW, Gosselin F, Schyns PG | title = Transmitting and decoding facial expressions | journal = Psychological Science | volume = 16 | issue = 3 | pages = 184–189 | date = March 2005 | pmid = 15733197 | doi = 10.1111/j.0956-7976.2005.00801.x | s2cid = 2622673 }}</ref> and when canonical face configurations are distorted for fearful, neutral and happy expressions.<ref>{{cite journal | vauthors = Elsherif MM, Sahan MI, Rotshtein P | title = The perceptual saliency of fearful eyes and smiles: A signal detection study | journal = PLOS ONE | volume = 12 | issue = 3 | pages = e0173199 | date = 2017-03-07 | pmid = 28267761 | pmc = 5340363 | doi = 10.1371/journal.pone.0173199 | bibcode = 2017PLoSO..1273199E | doi-access = free }}</ref>

===Neurocircuitry in mammals===
{{See also|Fear processing in the brain}}
* The thalamus collects sensory data from the senses
* Sensory cortex receives data from the thalamus and interprets it
* Sensory cortex organizes information for dissemination to the hypothalamus (fight or flight), amygdalae (fear), hippocampus (memory)

The brain structures that are the center of most neurobiological events associated with fear are the two [[amygdalae]], located behind the pituitary gland. Each amygdala is part of a circuitry of fear learning.<ref name=Olsson/> They are essential for proper adaptation to stress and specific modulation of emotional learning memory. In the presence of a threatening stimulus, the amygdalae generate the secretion of hormones that influence fear and aggression.<ref>Best, Ben (2004). [http://www.benbest.com/science/anatmind/anatmd9.html The Amygdala and the Emotions] {{webarchive|url=https://web.archive.org/web/20070309132748/http://www.benbest.com/science/anatmind/anatmd9.html |date=2007-03-09 }}. benbest.com</ref> Once a response to the stimulus in the form of fear or aggression commences, the amygdalae may elicit the release of hormones into the body to put the person into a state of alertness, in which they are ready to move, run, fight, etc. This defensive response is generally referred to in physiology as the [[fight-or-flight response]] regulated by the hypothalamus, part of the [[limbic system]].<ref>Gleitman, Henry; Fridlund, Alan J. and Reisberg, Daniel (2004). ''Psychology'' (6th ed.). W.W. Norton & Company. {{ISBN|0-393-97767-6}}.</ref> Once the person is in safe mode, meaning that there are no longer any potential threats surrounding them, the amygdalae will send this information to the medial [[prefrontal cortex]] (mPFC) where it is stored for similar future situations, which is known as [[memory consolidation]].<ref name=Travis/>

Some of the hormones involved during the state of fight-or-flight include [[adrenaline|epinephrine]], which regulates heart rate and metabolism as well as dilating blood vessels and air passages, [[norepinephrine]] increasing heart rate, blood flow to skeletal muscles and the release of glucose from energy stores,<ref>von Bohlen und Halbach, O; Dermietzel, R (2006). ''Neurotransmitters and neuromodulators: handbook of receptors and biological effects''. Wiley-VCH. p. 125. {{ISBN|978-3-527-31307-5}}.</ref> and [[cortisol]] which increases blood sugar, increases circulating neutrophilic leukocytes, calcium amongst other things.<ref>Hoehn K, Marieb EN (2010). ''Human Anatomy & Physiology''. San Francisco: Benjamin Cummings. {{ISBN|0-321-60261-7}}.</ref>

After a situation which incites fear occurs, the amygdalae and [[hippocampus]] record the event through synaptic [[neuroplasticity|plasticity]].<ref>{{cite journal | vauthors = Amunts K, Kedo O, Kindler M, Pieperhoff P, Mohlberg H, Shah NJ, Habel U, Schneider F, Zilles K | display-authors = 6 | title = Cytoarchitectonic mapping of the human amygdala, hippocampal region and entorhinal cortex: intersubject variability and probability maps | journal = Anatomy and Embryology | volume = 210 | issue = 5–6 | pages = 343–352 | date = December 2005 | pmid = 16208455 | doi = 10.1007/s00429-005-0025-5 | s2cid = 6984617 }}</ref> The stimulation to the hippocampus will cause the individual to remember many details surrounding the situation.<ref>Schacter, Daniel L.; Gilbert, Daniel T. and Wegner, Daniel M. (2011) ''Psychology Study Guide'', Worth Publishers, {{ISBN|1-4292-0615-2}}.</ref> Plasticity and memory formation in the amygdala are generated by activation of the neurons in the region. Experimental data supports the notion that synaptic plasticity of the neurons leading to the lateral amygdalae occurs with fear conditioning.<ref>{{cite journal | vauthors = LeDoux J | title = The emotional brain, fear, and the amygdala | journal = Cellular and Molecular Neurobiology | volume = 23 | issue = 4–5 | pages = 727–738 | date = October 2003 | pmid = 14514027 | doi = 10.1023/A:1025048802629 | s2cid = 3216382 | pmc = 11530156 }}</ref> In some cases, this forms permanent fear responses such as [[post-traumatic stress disorder]] (PTSD) or a [[phobia]].<ref>American Psychiatric Association (1994). Diagnostic and statistical manual of mental disorders: DSM-IV. Washington, DC. {{ISBN|0-89042-061-0}}.</ref> MRI and fMRI scans have shown that the amygdalae in individuals diagnosed with such disorders including [[bipolar disorder|bipolar]] or [[panic disorder]] are larger and wired for a higher level of fear.<ref>{{cite journal | vauthors = Cheng DT, Knight DC, Smith CN, Stein EA, Helmstetter FJ | title = Functional MRI of human amygdala activity during Pavlovian fear conditioning: stimulus processing versus response expression | journal = Behavioral Neuroscience | volume = 117 | issue = 1 | pages = 3–10 | date = February 2003 | pmid = 12619902 | doi = 10.1037/0735-7044.117.1.3 | url = http://www.uwm.edu/~fjh/cheng2003.pdf | url-status = dead | access-date = 2017-10-24 | citeseerx = 10.1.1.123.4317 | archive-url = https://web.archive.org/web/20081008020545/http://www.uwm.edu/~fjh/cheng2003.pdf | archive-date = 2008-10-08 }}</ref>

Pathogens can suppress amygdala activity. Rats infected with the [[toxoplasmosis]] parasite become less fearful of cats, sometimes even seeking out their urine-marked areas. This behavior often leads to them being eaten by cats. The parasite then reproduces within the body of the cat. There is evidence that the parasite concentrates itself in the amygdala of infected rats.<ref>{{cite journal | vauthors = Berdoy M, Webster JP, Macdonald DW | title = Fatal attraction in rats infected with Toxoplasma gondii | journal = Proceedings. Biological Sciences | volume = 267 | issue = 1452 | pages = 1591–1594 | date = August 2000 | pmid = 11007336 | pmc = 1690701 | doi = 10.1098/rspb.2000.1182 }}</ref> In a separate experiment, rats with lesions in the amygdala did not express fear or anxiety towards unwanted stimuli. These rats pulled on levers supplying food that sometimes sent out electrical shocks. While they learned to avoid pressing on them, they did not distance themselves from these shock-inducing levers.<ref>{{Cite journal | vauthors = Larkin M | title = Amygdala differentiates fear response | doi = 10.1016/S0140-6736(05)62234-9 | journal = The Lancet | volume = 350 | issue = 9073 | page = 268 | year = 1997 | s2cid = 54232230 }}</ref>

Several brain structures other than the amygdalae have also been observed to be activated when individuals are presented with fearful vs. neutral faces, namely the occipito[[cerebellum|cerebellar]] regions including the [[fusiform gyrus]] and the [[Inferior parietal lobule|inferior parietal]] / [[Superior temporal gyrus|superior temporal]] gyri.<ref name=radua2010>{{cite journal | vauthors = Radua J, Phillips ML, Russell T, Lawrence N, Marshall N, Kalidindi S, El-Hage W, McDonald C, Giampietro V, Brammer MJ, David AS, Surguladze SA | display-authors = 6 | title = Neural response to specific components of fearful faces in healthy and schizophrenic adults | journal = NeuroImage | volume = 49 | issue = 1 | pages = 939–946 | date = January 2010 | pmid = 19699306 | doi = 10.1016/j.neuroimage.2009.08.030 | url = https://zenodo.org/record/1066228 | url-status = live | access-date = 2019-08-14 | s2cid = 6209163 | archive-url = https://web.archive.org/web/20171201040641/https://zenodo.org/record/1066228 | archive-date = 2017-12-01 }}</ref> Fearful eyes, brows and mouth seem to separately reproduce these brain responses.<ref name=radua2010 /> Scientists from Zurich studies show that the hormone oxytocin related to stress and sex reduces activity in your brain fear center.<ref>Fear not." Ski Mar.–Apr. 2009: 15. Gale Canada In Context. Web. 29 Sep. 2011</ref>

====Pheromones and contagion====
In threatening situations, insects, aquatic organisms, birds, reptiles, and mammals emit odorant substances, initially called alarm substances, which are chemical signals now called alarm [[pheromones]]. This is to defend themselves and at the same time to inform members of the same species of danger and leads to observable behavior change like freezing, defensive behavior, or dispersion depending on circumstances and species. For example, stressed rats release odorant cues that cause other rats to move away from the source of the signal.

After the discovery of pheromones in 1959, alarm pheromones were first described in 1968 in ants<ref>{{cite journal | vauthors = Moser JC, Brownlee RC, Silverstein R | title = Alarm pheromones of the ant atta texana | journal = Journal of Insect Physiology | volume = 14 | issue = 4 | pages = 529–535 | date = April 1968 | pmid = 5649232 | doi = 10.1016/0022-1910(68)90068-1 | bibcode = 1968JInsP..14..529M }}</ref> and earthworms,<ref>{{cite journal | vauthors = Ressler RH, Cialdini RB, Ghoca ML, Kleist SM | title = Alarm pheromone in the earthworm Lumbricus terrestris | journal = Science | volume = 161 | issue = 3841 | pages = 597–599 | date = August 1968 | pmid = 5663305 | doi = 10.1126/science.161.3841.597 | s2cid = 30927186 | bibcode = 1968Sci...161..597R }}</ref> and four years later also found in mammals, both mice and rats.<ref>{{cite journal | vauthors = Rottman SJ, Snowdon CT | title = Demonstration and analysis of an alarm pheromone in mice | journal = Journal of Comparative and Physiological Psychology | volume = 81 | issue = 3 | pages = 483–490 | date = December 1972 | pmid = 4649187 | doi = 10.1037/h0033703 }}</ref> Over the next two decades, identification and characterization of these pheromones proceeded in all manner of insects and sea animals, including fish, but it was not until 1990 that more insight into mammalian alarm pheromones was gleaned.

In 1985, a link between odors released by stressed rats and [[pain perception]] was discovered: unstressed rats exposed to these odors developed opioid-mediated analgesia.<ref>{{cite journal | vauthors = Fanselow MS | title = Odors released by stressed rats produce opioid analgesia in unstressed rats | journal = Behavioral Neuroscience | volume = 99 | issue = 3 | pages = 589–592 | date = June 1985 | pmid = 3843728 | doi = 10.1037/0735-7044.99.3.589 }}</ref> In 1997, researchers found that bees became less responsive to pain after they had been stimulated with [[isoamyl acetate]], a chemical smelling of banana, and a component of bee alarm pheromone.<ref>{{cite journal | vauthors = Núñez J, Almeida L, Balderrama N, Giurfa M | title = Alarm pheromone induces stress analgesia via an opioid system in the honeybee | journal = Physiology & Behavior | volume = 63 | issue = 1 | pages = 75–80 | date = December 1997 | pmid = 9402618 | doi = 10.1016/s0031-9384(97)00391-0 | s2cid = 8788442 }}</ref> The experiment also showed that the bees' fear-induced [[pain tolerance]] was mediated by an [[endorphin]].

By using the [[Behavioural despair test|forced swimming test]] in rats as a model of fear-induction, the first mammalian "alarm substance" was found.<ref>{{cite journal | vauthors = Abel EL, Bilitzke PJ | title = A possible alarm substance in the forced swimming test | journal = Physiology & Behavior | volume = 48 | issue = 2 | pages = 233–239 | date = August 1990 | pmid = 2255725 | doi = 10.1016/0031-9384(90)90306-o | s2cid = 22994036 }}</ref> In 1991, this "alarm substance" was shown to fulfill criteria for pheromones: well-defined behavioral effect, species specificity, minimal influence of experience and control for nonspecific arousal. Rat activity testing with the alarm pheromone, and their preference/avoidance for odors from cylinders containing the pheromone, showed that the pheromone had very low [[Volatility (chemistry)|volatility]].<ref>{{cite journal | vauthors = Abel EL | title = Alarm substance emitted by rats in the forced-swim test is a low volatile pheromone | journal = Physiology & Behavior | volume = 50 | issue = 4 | pages = 723–727 | date = October 1991 | pmid = 1775546 | doi = 10.1016/0031-9384(91)90009-d | s2cid = 41044786 }}</ref>

In 1993 a connection between alarm chemosignals in mice and their [[immune system|immune response]] was found.<ref>{{cite journal | vauthors = Cocke R, Moynihan JA, Cohen N, Grota LJ, Ader R | title = Exposure to conspecific alarm chemosignals alters immune responses in BALB/c mice | journal = Brain, Behavior, and Immunity | volume = 7 | issue = 1 | pages = 36–46 | date = March 1993 | pmid = 8471798 | doi = 10.1006/brbi.1993.1004 | s2cid = 7196539 | title-link = BALB/c mice }}</ref> Pheromone production in mice was found to be associated with or mediated by the [[pituitary gland]] in 1994.<ref>{{cite journal | vauthors = Abel EL | title = The pituitary mediates production or release of an alarm chemosignal in rats | journal = Hormones and Behavior | volume = 28 | issue = 2 | pages = 139–145 | date = June 1994 | pmid = 7927280 | doi = 10.1006/hbeh.1994.1011 | s2cid = 11844089 }}</ref>

In 2004, it was demonstrated that rats' alarm pheromones had different effects on the "recipient" rat (the rat perceiving the pheromone) depending which body region they were released from: Pheromone production from the face modified behavior in the recipient rat, e.g. caused sniffing or movement, whereas pheromone secreted from the rat's anal area induced [[autonomic nervous system]] stress responses, like an increase in core body temperature.<ref>{{cite journal | vauthors = Kiyokawa Y, Kikusui T, Takeuchi Y, Mori Y | title = Alarm pheromones with different functions are released from different regions of the body surface of male rats | journal = Chemical Senses | volume = 29 | issue = 1 | pages = 35–40 | date = January 2004 | pmid = 14752038 | doi = 10.1093/chemse/bjh004 | doi-access = free }}</ref> Further experiments showed that when a rat perceived alarm pheromones, it increased its defensive and risk assessment behavior,<ref>{{cite journal | vauthors = Kiyokawa Y, Shimozuru M, Kikusui T, Takeuchi Y, Mori Y | title = Alarm pheromone increases defensive and risk assessment behaviors in male rats | journal = Physiology & Behavior | volume = 87 | issue = 2 | pages = 383–387 | date = February 2006 | pmid = 16337975 | doi = 10.1016/j.physbeh.2005.11.003 | url = https://zenodo.org/record/853489 | url-status = live | access-date = 2017-08-30 | s2cid = 12780994 | archive-url = https://web.archive.org/web/20170830233838/https://zenodo.org/record/853489 | archive-date = 2017-08-30 }}</ref> and its acoustic [[startle reflex]] was enhanced.

It was not until 2011 that a link between severe pain, neuroinflammation and alarm pheromones release in rats was found: real time [[RT-PCR]] analysis of rat brain tissues indicated that shocking the footpad of a rat increased its production of [[proinflammatory cytokines]] in deep brain structures, namely of [[IL-1β]], heteronuclear [[Corticotropin-releasing hormone]] and [[c-fos]] mRNA expressions in both the [[paraventricular nucleus]] and the bed nucleus of the [[stria terminalis]], and it increased stress hormone levels in plasma ([[corticosterone]]).<ref>{{cite journal | vauthors = Arakawa H, Arakawa K, Blandino P, Deak T | title = The role of neuroinflammation in the release of aversive odor cues from footshock-stressed rats: Implications for the neural mechanism of alarm pheromone | journal = Psychoneuroendocrinology | volume = 36 | issue = 4 | pages = 557–568 | date = May 2011 | pmid = 20888127 | doi = 10.1016/j.psyneuen.2010.09.001 | s2cid = 24367179 }}</ref>

The [[emotion#neurocircuitry|neurocircuit]] for how rats perceive alarm pheromones was shown to be related to the [[hypothalamus]], [[brainstem]], and [[amygdala]]e, all of which are evolutionary ancient structures deep inside or in the case of the brainstem underneath the brain away from the cortex, and involved in the [[fight-or-flight response]], as is the case in humans.<ref>{{cite journal | vauthors = Kiyokawa Y, Kikusui T, Takeuchi Y, Mori Y | title = Mapping the neural circuit activated by alarm pheromone perception by c-Fos immunohistochemistry | journal = Brain Research | volume = 1043 | issue = 1–2 | pages = 145–154 | date = May 2005 | pmid = 15862528 | doi = 10.1016/j.brainres.2005.02.061 | url = https://zenodo.org/record/854830 | url-status = live | access-date = 2017-08-31 | s2cid = 41186952 | archive-url = https://web.archive.org/web/20170831131924/https://zenodo.org/record/854830 | archive-date = 2017-08-31 }}</ref>

Alarm pheromone-induced anxiety in rats has been used to evaluate the degree to which [[anxiolytic]]s can alleviate anxiety in humans. For this, the change in the [[startle response#acoustic startle reflex|acoustic startle reflex]] of rats with alarm pheromone-induced anxiety (i.e. reduction of defensiveness) has been measured. Pretreatment of rats with one of five anxiolytics used in clinical medicine was able to reduce their anxiety: namely [[midazolam]], [[phenelzine]] (a nonselective monoamine oxidase (MAO) inhibitor), [[propranolol]], a nonselective [[beta blocker]], [[clonidine]], an [[alpha-2 adrenergic receptor#agonist|alpha 2 adrenergic agonist]] or [[CP-154,526]], a [[corticotropin-releasing hormone antagonist]].<ref>{{cite journal | vauthors = Inagaki H, Kiyokawa Y, Takeuchi Y, Mori Y | title = The alarm pheromone in male rats as a unique anxiety model: psychopharmacological evidence using anxiolytics | journal = Pharmacology, Biochemistry, and Behavior | volume = 94 | issue = 4 | pages = 575–579 | date = February 2010 | pmid = 19969015 | doi = 10.1016/j.pbb.2009.11.013 | s2cid = 28194770 }}</ref>

Faulty development of odor discrimination impairs the [[perception]] of pheromones and pheromone-related behavior, like [[aggression|aggressive behavior]] and mating in male rats: The enzyme [[MAPK7|Mitogen-activated protein kinase 7]] (MAPK7) has been implicated in regulating the development of the olfactory bulb and odor discrimination and it is highly expressed in developing rat brains, but absent in most regions of adult rat brains. [[Conditional gene knockout|Conditional deletion]] of the MAPK7gene in mouse neural stem cells impairs several pheromone-mediated behaviors, including aggression and mating in male mice. These behavior impairments were not caused by a reduction in the level of testosterone, by physical immobility, by heightened fear or anxiety or by depression. Using mouse urine as a natural pheromone-containing solution, it has been shown that the impairment was associated with defective detection of related pheromones, and with changes in their inborn preference for pheromones related to sexual and reproductive activities.<ref>{{cite journal | vauthors = Zou J, Storm DR, Xia Z | title = Conditional deletion of ERK5 MAP kinase in the nervous system impairs pheromone information processing and pheromone-evoked behaviors | journal = PLOS ONE | volume = 8 | issue = 10 | page= e76901 | year = 2013 | pmid = 24130808 | pmc = 3793934 | doi = 10.1371/journal.pone.0076901 | bibcode = 2013PLoSO...876901Z | doi-access = free }}</ref>

Lastly, alleviation of an acute fear response because a friendly peer (or in biological language: an affiliative [[conspecific]]) [[tend and befriend|tends and befriends]] is called "[[social buffering]]". The term is in analogy to the 1985 "buffering" hypothesis in psychology, where [[social support]] has been proven to mitigate the negative health effects of alarm pheromone mediated distress.<ref>{{cite journal | vauthors = Cohen S, Wills TA | title = Stress, social support, and the buffering hypothesis | journal = Psychological Bulletin | volume = 98 | issue = 2 | pages = 310–357 | date = September 1985 | pmid = 3901065 | doi = 10.1037/0033-2909.98.2.310 | s2cid = 18137066 }}</ref> The role of a "social pheromone" is suggested by the recent discovery that olfactory signals are responsible in mediating the "social buffering" in male rats.<ref>{{cite journal | vauthors = Takahashi Y, Kiyokawa Y, Kodama Y, Arata S, Takeuchi Y, Mori Y | title = Olfactory signals mediate social buffering of conditioned fear responses in male rats | journal = Behavioural Brain Research | volume = 240 | pages = 46–51 | date = March 2013 | pmid = 23183219 | doi = 10.1016/j.bbr.2012.11.017 | url = https://zenodo.org/record/854828 | url-status = live | access-date = 2017-08-31 | s2cid = 30938917 | archive-url = https://web.archive.org/web/20170831133047/https://zenodo.org/record/854828 | archive-date = 2017-08-31 }}</ref> "Social buffering" was also observed to mitigate the conditioned fear responses of honeybees. A bee colony exposed to an environment of high threat of predation did not show increased aggression and aggressive-like gene expression patterns in individual bees, but decreased aggression. That the bees did not simply [[habituation|habituate]] to threats is suggested by the fact that the disturbed colonies also decreased their foraging.<ref>{{cite journal | vauthors = Rittschof CC, Robinson GE | title = Manipulation of colony environment modulates honey bee aggression and brain gene expression | journal = Genes, Brain and Behavior | volume = 12 | issue = 8 | pages = 802–811 | date = November 2013 | pmid = 24034579 | pmc = 3863782 | doi = 10.1111/gbb.12087 }}</ref>

Biologists have proposed in 2012 that fear pheromones evolved as molecules of "keystone significance", a term coined in analogy to [[keystone species]]. Pheromones may determine [[species richness|species compositions]] and affect rates of energy and material exchange in an [[community (ecology)|ecological community]]. Thus pheromones generate structure in a [[food web]] and play critical roles in maintaining [[Ecosystem health|natural systems]].<ref>{{cite journal | vauthors = Ferrer RP, Zimmer RK | title = Community ecology and the evolution of molecules of keystone significance | journal = The Biological Bulletin | volume = 223 | issue = 2 | pages = 167–177 | date = October 2012 | pmid = 23111129 | doi = 10.1086/BBLv223n2p167 | s2cid = 592393 }}</ref>

====Humans====
Evidence of chemosensory alarm signals in humans has emerged slowly: Although alarm pheromones have not been physically isolated and their chemical structures have not been identified in humans so far, there is evidence for their presence. [[Androstadienone]], for example, a steroidal, endogenous odorant, is a pheromone candidate found in human sweat, axillary hair and plasma. The closely related compound [[androstenone]] is involved in communicating dominance, aggression or competition; sex hormone influences on androstenone perception in humans showed a high testosterone level related to heightened androstenone sensitivity in men, a high testosterone level related to [[unhappiness]] in response to androstenone in men, and a high estradiol level related to disliking of androstenone in women.<ref>{{cite journal | vauthors = Lübke KT, Pause BM | title = Sex-hormone dependent perception of androstenone suggests its involvement in communicating competition and aggression | journal = Physiology & Behavior | volume = 123 | pages = 136–141 | date = January 2014 | pmid = 24184511 | doi = 10.1016/j.physbeh.2013.10.016 | s2cid = 25729942 }}</ref>

A German study from 2006 showed when anxiety-induced versus exercise-induced human sweat from a dozen people was pooled and offered to seven study participants, of five able to olfactorily distinguish exercise-induced sweat from room air, three could also distinguish exercise-induced sweat from anxiety induced sweat. The [[startle reflex|acoustic startle reflex]] response to a sound when sensing anxiety sweat was larger than when sensing exercise-induced sweat, as measured by [[electromyography]] analysis of the orbital muscle, which is responsible for the eyeblink component. This showed for the first time that fear chemosignals can modulate the startle reflex in humans without emotional mediation; fear chemosignals primed the recipient's "defensive behavior" prior to the subjects' conscious attention on the acoustic startle reflex level.<ref>{{cite journal | vauthors = Prehn A, Ohrt A, Sojka B, Ferstl R, Pause BM | title = Chemosensory anxiety signals augment the startle reflex in humans | journal = Neuroscience Letters | volume = 394 | issue = 2 | pages = 127–130 | date = February 2006 | pmid = 16257486 | doi = 10.1016/j.neulet.2005.10.012 | s2cid = 23295966 }}</ref>

In analogy to the social buffering of rats and honeybees in response to chemosignals, induction of [[empathy]] by "smelling anxiety" of another person has been found in humans.<ref>{{cite journal | vauthors = Prehn-Kristensen A, Wiesner C, Bergmann TO, Wolff S, Jansen O, Mehdorn HM, Ferstl R, Pause BM | display-authors = 6 | title = Induction of empathy by the smell of anxiety | journal = PLOS ONE | volume = 4 | issue = 6 | page = e5987 | date = June 2009 | pmid = 19551135 | pmc = 2695008 | doi = 10.1371/journal.pone.0005987 | bibcode = 2009PLoSO...4.5987P | doi-access = free }}</ref>

A study from 2013 provided brain imaging evidence that human responses to fear chemosignals may be [[gender-specific]]. Researchers collected alarm-induced sweat and exercise-induced sweat from donors extracted it, pooled it and presented it to 16 unrelated people undergoing functional brain [[MRI]]. While stress-induced sweat from males produced a comparably strong emotional response in both females and males, stress-induced sweat from females produced markedly stronger arousal in women than in men. Statistical tests pinpointed this gender-specificity to the right amygdala and strongest in the superficial nuclei. Since no significant differences were found in the [[olfactory bulb]], the response to female fear-induced signals is likely based on processing the meaning, i.e. on the emotional level, rather than the strength of chemosensory cues from each gender, i.e. the perceptual level.<ref>{{cite journal | vauthors = Radulescu AR, Mujica-Parodi LR | title = Human gender differences in the perception of conspecific alarm chemosensory cues | journal = PLOS ONE | volume = 8 | issue = 7 | page = e68485 | date = Jul 2013 | pmid = 23894310 | pmc = 3722227 | doi = 10.1371/journal.pone.0068485 | bibcode = 2013PLoSO...868485R | doi-access = free }}</ref>

An [[approach–avoidance conflict|approach-avoidance task]] was set up where volunteers seeing either an angry or a happy cartoon face on a computer screen pushed away or pulled toward them a joystick as fast as possible. Volunteers smelling androstadienone, masked with clove oil scent responded faster, especially to angry faces than those smelling clove oil only, which was interpreted as androstadienone-related activation of the fear system.<ref>{{cite journal | vauthors = Frey MC, Weyers P, Pauli P, Mühlberger A | title = Androstadienone in motor reactions of men and women toward angry faces | journal = Perceptual and Motor Skills | volume = 114 | issue = 3 | pages = 807–825 | date = June 2012 | pmid = 22913022 | doi = 10.2466/07.16.22.28.PMS.114.3.807-825 | s2cid = 13194791 }}</ref> A potential mechanism of action is, that [[androstadienone]] alters the "emotional face processing". Androstadienone is known to influence the activity of the [[fusiform gyrus]] which is relevant for [[face perception|face recognition]].

===Cognitive-consistency theory===
[[cognitive dissonance|Cognitive-consistency]] theories assume that "when two or more simultaneously active cognitive structures are logically inconsistent, arousal is increased, which activates processes with the expected consequence of increasing consistency and decreasing arousal."<ref name=":Kampen">{{cite journal | vauthors = van Kampen HS | title = The principle of consistency and the cause and function of behaviour | journal = Behavioural Processes | volume = 159 | pages = 42–54 | date = February 2019 | pmid = 30562561 | doi = 10.1016/j.beproc.2018.12.013 | s2cid = 56478466 }}</ref> In this context, it has been proposed that fear behavior is caused by an inconsistency between a preferred, or expected, situation and the actually perceived situation, and functions to remove the inconsistent stimulus from the perceptual field, for instance by fleeing or hiding, thereby resolving the inconsistency.<ref name=":Kampen"/><ref>{{cite journal | vauthors = Hebb DO | title = On the nature of fear | journal = Psychological Review | volume = 53 | issue = 5 | pages = 259–276 | date = September 1946 | pmid = 20285975 | doi = 10.1037/h0061690 | s2cid = 5211697 }}</ref><ref name=":Archer"/> This approach puts fear in a broader perspective, also involving [[aggression]] and [[curiosity]]. When the inconsistency between perception and expectancy is small, learning as a result of curiosity reduces inconsistency by updating expectancy to match perception. If the inconsistency is larger, fear or aggressive behavior may be employed to alter the perception in order to make it match expectancy, depending on the size of the inconsistency as well as the specific context. Aggressive behavior is assumed to alter perception by forcefully manipulating it into matching the expected situation, while in some cases thwarted escape may also trigger aggressive behavior in an attempt to remove the thwarting stimulus.<ref name=":Kampen"/>