Learning the associations between aversive events and the environmental stimuli that predict such events is an essential survival skill in animals. Fear conditioning paradigm is an epitome of this form of associative learning. Conditional fear can be powerful and long lasting for some individuals.
Classical conditioning, also known as Pavlovian conditioning, is learning through the association of a neutral stimulus with a biologically potent aversive stimulus.
Pavlov observed that his dogs would salivate whenever he entered the room, even when he did not bring food, because the dogs associated his entrance with being fed.
Before conditioning, Pavlov’s entrance was a neutral stimulus (NS) that did not trigger a response in the dogs. However, after repeatedly bringing food, an unconditioned stimulus (US), with him whenever he entered the room, the entrance became a conditioned stimulus (CS) that could trigger the dogs to salivate involuntarily, which was called the conditioned response (CR).
What Is Fear Conditioning
Fear conditioning, a form of classical conditioning, involves learning that certain environmental stimuli (CS) can predict the occurrence of aversive events (CR)1.
It is the mechanism we learn to fear people, objects, places and events. In evolution, this form of fear learning can promote our survival in the face of future threats2.
Although meant to be a survival mechanism, unchecked conditioned fear may contribute to disorders of fear and anxiety in humans, such as panic disorder, specific phobias or post-traumatic stress disorder (PTSD)3.
The “Little Albert” Experiment
In the 1920s, John Watson and Rosalie Rayner devised a controversial experiment with baby Albert4.
The goal was to condition the baby with fear of a white rat. Before the experiment, little Albert didn’t fear the rat.
In the experiment, a loud and averse noise was sounded after presenting the rat. Watson made the loud, aversive noise by striking a hammer on a suspended steel bar to generate a fear response in little Albert. The baby was startled violently and broke into a sudden crying fit. After repeating this procedure several times, little Albert came to fear the rat itself.
Even when the rat was presented without the aversive sound, little Albert showed involuntary fear responses, such as falling over, crying and crawling away from the rat.
Note: This cruel experiment would certainly not be acceptable by current ethical standards.
Little Albert had learned that an innocuous conditioned stimulus (CS, the white rat) predicted the occurrence of a noxious unconditional stimulus (US, the loud noise) which would trigger a conditioned response (CR, crying). This experiment exemplifies the classical conditioning of fear psychology.
Cued and Contextual Fear Conditioning
A context is the set of circumstances around an event. They are typically multisensory, diffuse and continuously present. In addition to places, context can include relationships between spatial, temporal (time), interoceptive (e.g. hunger, stress), cognitive (how info is encoded and retrieved), social and cultural5.
In conditioning, when a CS is a static context of the environment, such as a room, the result is called contextual fear conditioning. The conditioned fear in this case is associated with the context. When the conditioned person is returned to the same context, CR is triggered.
When the Pavlovian fear conditioning is created between a discrete cue, i.e. CS added to the environment, such as a brief signal or a loud sound, cued fear conditioning is created. In this case, the conditioned fear is paired with a cue.
When a conditioned person is repeatedly exposed to the fear-eliciting CS in the absence of the aversive US, the conditioned fear response will decline and eventually become extinct. This procedure is called fear extinction, in which the predictive value of the CS as to the occurrence of the US is reduced.
Extinction is not the same as unlearning or forgetting. Rather, it’s a new associated learning of fear inhibition6.
Human studies have found that fear memories can last for years with little forgetting. So extinction does not remove the fear memory. But it creates a new memory that pairs the CS with the absence of the US.
Development of Fear Experiments
To study the phenomenon, behavioral neuroscience researchers use fear conditioning in rats extensively.
In typical fear conditioning studies, a rat or rodent is not presented the aversive stimulus in the home cage. The animal is then placed in a novel environment, provided an aversive stimulus, e.g. foot shock, and subsequently removed. When it is returned to the same “novel” environment, it generally will demonstrate freezing behavior or changes in heart rate if it becomes fear conditioned8.
Scientists have found that amygdala lesions, especially the brain areas in basolateral amygdala, can prevent the learning and expression of fear.
Fear and the Brain
Being able to use neutral cues to predict threat is crucial in survival. However, when this type of conditioned fear fails to extinguish when it’s no longer a threat, anxiety disorders such as posttraumatic stress disorder (PTSD) results. In those who suffer from PTSD, the conditioning memory of traumatic events can cause debilitating conditioned fear responses for decades after the danger has passed.
Not everyone who is exposed to the conditioning experiences will develop disorders. Lesion studies have found that the neural circuits involved in the fear acquisition and extinction include the brain regions in amygdala, hippocampus and medial prefrontal cortex. Dysfunction in these networks can cause the individuals to develop fear7.
- 1.Maren S. Neurobiology of Pavlovian Fear Conditioning. Annu Rev Neurosci. Published online March 2001:897-931. doi:10.1146/annurev.neuro.24.1.897
- 2.Fanselow MS. Neural organization of the defensive behavior system responsible for fear. Psychon Bull Rev. Published online December 1994:429-438. doi:10.3758/bf03210947
- 3.Rosen JB, Schulkin J. From normal fear to pathological anxiety. Psychological Review. Published online 1998:325-350. doi:10.1037/0033-295x.105.2.325
- 4.Watson JB, Rayner R. Conditioned emotional reactions. Journal of Experimental Psychology. Published online 1920:1-14. doi:10.1037/h0069608
- 5.Maren S, Phan KL, Liberzon I. The contextual brain: implications for fear conditioning, extinction and psychopathology. Nat Rev Neurosci. Published online May 2, 2013:417-428. doi:10.1038/nrn3492
- 6.Phelps EA, Delgado MR, Nearing KI, LeDoux JE. Extinction Learning in Humans. Neuron. Published online September 2004:897-905. doi:10.1016/j.neuron.2004.08.042
- 7.Meunier M, Bachevalier J, Murray EA, Málková L, Mishkin M. Effects of aspiration versus neurotoxic lesions of the amygdala on emotional responses in monkeys. European Journal of Neuroscience. Published online December 1999:4403-4418. doi:10.1046/j.1460-9568.1999.00854.x
- 8.Curzon P, Rustay NR, Browman KE. Cued and contextual fear conditioning for rodents. In: Methods of Behavior Analysis in Neuroscience. 2nd ed. NCBI; 2009:.