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 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). 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 has been studied in numerous species, from snails to humans. 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.
Fear conditioning is thought to depend upon an area of the brain called the amygdala. Ablation or deactivating of the amygdala can prevent both the learning and expression of fear. 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. A number of theorists have argued that conditioned fear coincides substantially with the mechanisms, both functional and neural, of clinical anxiety disorders. 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.
Joseph Ledoux's research 
Joseph E. LeDoux finds two amygdala pathways in the brain of the laboratory mouse by the use of fear conditioning and lesion study. He names them the "high road" and "low road". The low road is a pathway which is able to transmit a signal from a stimulus to the thalamus, and then to the amygdala, which then activates a fear-response in the body. This sequence works without a conscious experience of what comprises the stimulus, and it is the fast way to a bodily response. The high road is activated simultaneously. This is a slower road which also includes the cortical parts of the brain, thus creating a conscious impression of what the stimulus is. The low road only involves the sub-cortical part of the brain and is therefore regarded as a more primitive mechanism of defense, only existing in its separate form in lesser developed animals who have not developed the more complex part of the brain. In more developed animals, the high road and the low road work simultaneously to provide both fear-response and perceptual feedback.
See also 
- Classical conditioning
- Eyeblink conditioning
- Extinction (psychology)
- Measures of conditioned emotional response
- Fear processing in the brain
- 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.
- 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:10.1037/0736-97188.8.131.528.
- 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.
- LeDoux, Joseph (1996). The Emotional Brain: The Mysterious Underpinnings of Emotional Life. New York: Simon & Schuster.[page needed]