Fear conditioning

Fear conditioning is a behavioral paradigm in which organisms learn to predict aversive events.[1] 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 room) or neutral stimulus (e.g., a tone), resulting in the expression of fear responses to the originally neutral stimulus or context. This can be done by pairing the neutral stimulus with an aversive stimulus (e.g., a shock, loud noise, or unpleasant odor[2]). Eventually, the neutral stimulus alone can elicit the state of fear. In the vocabulary of classical conditioning, the neutral stimulus or context is the "conditional stimulus" (CS), the aversive stimulus is the "unconditional stimulus" (US), and the fear is the "conditional response" (CR).

Fear conditioning apparatus for mice equipped with a sound, a foot shock and an activity sensor with photobeams to measure freezing. Environment context can be changed. This apparatus is also used for PTSD studies.

Fear conditioning has been studied in numerous species, from snails[3] to humans.[4] In humans, conditioned fear is often measured with verbal report and galvanic skin response. In other animals, conditioned fear is often measured with freezing (a period of watchful immobility) or fear potentiated startle (the augmentation of the startle reflex by a fearful stimulus). Changes in heart rate, breathing, and muscle responses via electromyography can also be used to measure conditioned fear. A number of theorists have argued that conditioned fear coincides substantially with the mechanisms, both functional and neural, of clinical anxiety disorders.[5] Research into the acquisition, consolidation and extinction of conditioned fear promises to inform new drug based and psychotherapeutic treatments for an array of pathological conditions such as dissociation, phobias and post-traumatic stress disorder.[6]

Neurobiology

Amygdala

Fear conditioning is thought to depend upon an area of the brain called the amygdala. Electrophysiological recordings from the amygdala have demonstrated that cells in that region undergo long-term potentiation (LTP), a form of synaptic plasticity believed to underlie learning.[7] Additionally, inhibition of neurons in the amygdala disrupts fear acquisition, while stimulation of those neurons can drive fear-related behaviors, such as freezing behavior in rodents.[8] This indicates that proper function of the amygdala is both necessary for fear conditioning and sufficient to drive fear behaviors.

Joseph E. LeDoux has been instrumental in elucidating the amygdala's role in fear conditioning. He was one of the first to show that the amygdala undergoes long-term potentiation during fear conditioning, and that ablation of amygdala cells disrupts both learning and expression of fear, among many other achievements.[9]

Hippocampus

Some types of fear conditioning (e.g. contextual and trace) also involve the hippocampus, an area of the brain believed to receive affective impulses from the amygdala and to integrate those impulses with previously existing information to make it meaningful. Some theoretical accounts of traumatic experiences suggest that amygdala-based fear bypasses the hippocampus during intense stress and can be stored somatically or as images that can return as physical symptoms or flashbacks without cognitive meaning.[10]

Molecular mechanisms

One of the major neurotransmitters involved in conditioned fear learning is glutamate. It has been suggested that NMDA receptors (NMDARs) in the amygdala are necessary for fear memory acquisition, because disruption of NMDAR function disrupts development of fear responses in rodents.[11]

Epigenetics

Conditioned fear may be inherited transgenerationally. In one experiment, mice were conditioned to fear an acetophenone odor and then set up to breed subsequent generations of mice. Those subsequent generations of mice also showed a behavioral sensitivity to acetophenone, which was accompanied by neuroanatomical and epigenetic changes that are believed to have been inherited from the parents' gametes.[12]

Across development

The learning involved in conditioned fear, as well as the underlying neurobiology, changes dramatically from infancy, across childhood and adolescence, into adulthood and aging. Specifically, infant animals show an inability to develop fear associations, whereas their adult counterparts develop fear memories much more readily.[13]

See also

References

  1. ↑ Maren, Stephen (2001). "Neurobiology of Pavlovian fear conditioning". Annual Review of Neuroscience. 24: 897–931. doi:10.1146/annurev.neuro.24.1.897. PMID 11520922.
  2. ↑ Wallace, Karin J.; Rosen, Jeffrey B. "Predator odor as an unconditioned fear stimulus in rats: Elicitation of freezing by trimethylthiazoline, a component of fox feces". Behavioral Neuroscience. 114 (5): 912–922. doi:10.1037/0735-7044.114.5.912.
  3. ↑ Walters, E. T.; Carew, T. J.; Kandel, E. R. (1981-01-30). "Associative Learning in Aplysia: evidence for conditioned fear in an invertebrate". Science. 211 (4481): 504–506. doi:10.1126/science.7192881. ISSN 0036-8075. PMID 7192881.
  4. ↑ Critchley, Hugo D; Mathias, Christopher J; Dolan, Raymond J. "Fear Conditioning in Humans". Neuron. 33 (4): 653–663. doi:10.1016/s0896-6273(02)00588-3.
  5. ↑ Rosen, Jeffrey B.; Schulkin, Jay (1998). "From normal fear to pathological anxiety". Psychological Review. 105 (2): 325–50. doi:10.1037/0033-295X.105.2.325. PMID 9577241.
  6. ↑ VanElzakker, M. B., Dahlgren, M. K., Davis, F. C., Dubois, S. & Shin, L. M. (2014). From Pavlov to PTSD: The extinction of conditioned fear in rodents, humans, and anxiety disorders. Neurobiology of Learning and Memory 113: 3–18. | doi=10.1016/j.nlm.2013.11.014 | PMID 24321650
  7. ↑ Sah, P.; Westbrook, R. F.; Lüthi, A. (2008-05-01). "Fear Conditioning and Long-term Potentiation in the Amygdala". Annals of the New York Academy of Sciences. 1129 (1): 88–95. doi:10.1196/annals.1417.020. ISSN 1749-6632.
  8. ↑ Bocchio, Marco; Nabavi, Sadegh; Capogna, Marco (2017-05-17). "Synaptic Plasticity, Engrams, and Network Oscillations in Amygdala Circuits for Storage and Retrieval of Emotional Memories". Neuron. 94 (4): 731–743. doi:10.1016/j.neuron.2017.03.022. ISSN 0896-6273.
  9. ↑ LeDoux, Joseph E. (2000-03-01). "Emotion Circuits in the Brain". Annual Review of Neuroscience. 23 (1): 155–184. doi:10.1146/annurev.neuro.23.1.155. ISSN 0147-006X.
  10. ↑ Bromberg, Philip M. (2003). "Something wicked this way comes: Trauma, dissociation, and conflict: The space where psychoanalysis, cognitive science, and neuroscience overlap". Psychoanalytic Psychology. 20 (3): 558–74. doi:10.1037/0736-9735.20.3.558.
  11. ↑ Johansen, Joshua P.; Cain, Christopher K.; Ostroff, Linnaea E.; LeDoux, Joseph E. (2011-10-28). "Molecular Mechanisms of Fear Learning and Memory". Cell. 147 (3): 509–524. doi:10.1016/j.cell.2011.10.009. ISSN 0092-8674.
  12. ↑ Dias, Brian G; Ressler, Kerry J (January 2014). "Parental olfactory experience influences behavior and neural structure in subsequent generations". Nature Neuroscience. 17 (1): 89–96. doi:10.1038/nn.3594. ISSN 1546-1726. PMC 3923835.
  13. ↑ Ganella, Despina E; Kim, Jee Hyun (2014-10-01). "Developmental rodent models of fear and anxiety: from neurobiology to pharmacology". British Journal of Pharmacology. 171 (20): 4556–4574. doi:10.1111/bph.12643. ISSN 1476-5381. PMC 4209932.
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