The Effects of Brain Modifications on Fear Conditioning
California State University Channel Islands
Brain Stimulation and Fear
Conditioned fear is a form of Ivan Pavlov’s classical conditioning in which a subject is conditioned to fear a neutral stimulus. This type of conditioning is most often done on animals and often uses an electric shock as the unconditioned stimulus and pairs that with a neutral stimulus in order to cause the animal to become afraid of the neutral stimulus. This association of fearing the neutral stimulus can be reversed through extinction, which is where only the neutral stimulus is presented without having an electric shock paired with it until the animal stops eliciting a fearful response. However it may take a long time for an animal to undergo extinction. There are now new studies out that have yielded results showing that stimulations or lesions in to the ventral medial prefrontal cortex can accelerate the extinction process (Mohammed & Quirk, 2002). There are other new studies that have shown other parts of the brain such as areas of the hippocampus help to speed up extinction (Maren and Holt, 2004; Keene and Bucci, 2008). All of these studies are of particular interest to researchers because their findings may help in treating people that are suffering from anxiety disorders such as post-traumatic stress disorder. Before these procedures can be used on human subjects it is important that researchers narrow down which area of the brain not only speeds up the extinction process but does it with the least amount of side effects.
Some researchers have proposed that the ventral medial prefrontal cortex plays a key component in fear conditioning (Baratta et al., 2000). Since the location of this cortex is in the frontal lobe it is involved in decision making and has also been implicated to show involvement in risk taking and assessing fear. In fear conditioning this part of the brain is shown to be most active during the extinction period in which the participant in the experiment is relearning not to fear the neutral stimulus. This part of the brain is thought to be most active because it is assessing whether or not the situation needs to be feared. An experiment by Baratta and his colleagues (2000), showed that inactivation of the infralimbic cortex, which is located in the ventral medial prefrontal cortex, prevented extinction occurring after rats were conditioned to fear a tone. Thus if under stimulation to this area inhibits extinction then overstimulation may have the opposite effect.
The hypothesis that stimulation of the ventral medial prefrontal cortex may decrease the fear response is what prompted the study done by Mohammed R. Milad and Gregory J. Quirk (2002). The researchers found that stimulating neurons in the medial prefrontal cortex after fear conditioning dramatically speeds up extinction. The study analyzed the relationship between neuron activity in the ventral medial prefrontal cortex and how often rats froze in fear to a conditioned stimulus. Prior to the initial conditioning a microelectrode was implanted in the ventral medial prefrontal cortex of the rats. The rats were then given one week to recuperate from the surgery. On the first day the rats were put through fear conditioning in which a tone that was simultaneously paired with a foot shock. The next day the rats were put through an extinction session in which only the tone played and no foot shock was administered. During the extinction session when the tone was played a group of rats received electric stimulation to the infralimbic cortex. The other group of rats did not receive any stimulation and were used as the control group. The group of rats that received stimulation stopped freezing to the tone significantly faster than the group of rats that did not receive any stimulation.
However the ventral medial…