Extinction of conditioned responses happens when the conditioned stimulus occurs without the presence of the unconditioned stimulus

Extinction of conditioned responses happens when the conditioned stimulus occurs without the presence of the unconditioned stimulus (Phelps, Delgado, Nearing, & LeDoux, 2004). This results in a decreased or eliminated conditioned response. Conditioning is a learning method that works to create a wide array of responses, including fear.
Extinction of fear responses occurs across several areas of the brain. But one of the most prominent areas involved is the amygdala. The amygdala is a key brain area used in emotional memory formation. In addition to the formation of emotional memories (including fear memories), the amygdala works to extinguish fear. The amygdala’s role in extinction of classically conditioned fear responses is well established and widely excepted in psychological research. Research has shown that to extinguish fear responses, the amygdala must be active during the extinction process.
Previous works by LeBar, Gatenby, Gore, LeDoux, and Phelps (1998) sought to look at the amygdala during extinction. Their aim was to look at amygdala activity using fMRI to expand knowledge of amygdala activity and extinction. The researchers hypothesized that amygdala activity occurs during extinction. The researchers used visual images-the conditioned stimulus-and a mildly painful shock-the unconditioned stimulus-to condition fear in their participants. The extinction process involved presenting the conditioned stimulus (or CS) without the unconditioned stimulus (or UCS). The results, measured on fMRI, showed that the amygdala activity (specifically the right amygdala) during extinction was statistically significant (LeBar et. Al, 1998). These findings showed that the activity in the amygdala was significant enough to explain the extinction process. These results helped establish the basis for future research into the amygdala and extinction.
In addition to the work by LeBar et. al (1998), Knight, Smith, Cheng, Stein, and Helmstetter (2004) used similar methods to test amygdala activity in extinction. Like LeBar et. al (1998), Knight et. al (2004) also sought to measure amygdala activation during the extinction of fear responses. Their aim was to explore the brain regions active during extinction of fear responses. The researchers hypothesized that brain regions would include the amygdala Similarly, Knight et. al (2004) used fMRI to measure blood oxygen levels in the brain. The researchers used the same methods are LeBar et. al (1998) including a visual stimulus for the US and a shock for the UCS. However, the visual stimulus varied slightly as LeBar et. al (1998) used colored squares, while Knight et. al (2004) used a red light. Following the initial pairing of the light with the shock, the participants were presented with the shock alone as part of the extinction trials. The fMRI then measures BOLD (blood oxygen level dependent) images and the researchers analyzed these images for amygdala activity. Knight et. al (2004) found that amygdala activity did increase during the extinction trials. Their results further indicated that the right amygdala had heightened activity. Like LeBar et. al (1998), the results indicated that there was a hemispheric preference for activity during conditioning. The results Knight et. al (2004) study further strengthen the results of the LeBar et. al (1998) study because Knight et. al (2004) used a near identical method and found consistent results. While these two works used near identical methods and found the same results, other works have used the same methods without finding the same results.
Phelps, Delgado, Nearing, and LeDoux (2004) also looked at the role of the amygdala during fear extinction. Like the previously mentioned studies, they sought to look at amygdala activity during extinction. Their aim was to study the involvement of the amygdala in extinction. The researchers hypothesized that the amygdala was active during extinction. The methods used in the previous two studies, Phelps et. al (2004) used visual stimuli as the CS and a shock as the UCS. The exact visual stimulus used was colored squares-which was identical to Knight et. al (2004). These two studies used identical methods, however their results varied slightly. While LeBar et. al (1998) and Knight et. al (2004) found that the right amygdala was active, Phelps et. al (2004) found no hemispheric preference. They found that there was increased activity in the amygdala, but they could not pinpoint exactly where. Phelps et. al (2004) suggested that this could be since the amygdala was before the conditioned response of fear. While the results support the findings of LeBar et. al (1998) and Knight et. al (2004), Phelps et. al (2004) offered this alternate explanation for why they could not find a hemispheric preference. They stated that their results could be due to previous activation of the amygdala, prior to invoking the fear in the participants. While this explanation could account for why there was no hemispheric preference, the researchers concluded that the amygdala was active during extinction (Knight et. al, 2004). Despite the variation in hemispheric preference, all the studies used near identical methods and all the results supported that the amygdala is active during extinction. In addition to looking at the amygdala during extinction in general, other researchers have looked at the amygdala and extinction during specific times.
Schiller, Kanen, LeDoux, Monfils, and Phelps (2013) looked at amygdala activity during extinction when fear memory is being reconsolidated. Fear memory reconsolidation is when memories are retrieved for use and can be changed or disturbed. The researchers hypothesized that pairing extinction and reconsolidation would diminish the brain’s response-meaning that brain areas associated with extinction would be less active. Similarly, the methods used in Schiller et. al (2013)’s study was identical to both Phelps et. al (20040 and LeBar et. al (1998), and like Knight et. al (2004). The CS was a colored square while the UCS was a shock. After fear was conditioned in the subjects, extinction trails occurred during memory reconsolidation. In addition, the responses were measured using functional images and BOLD levels-like Knight et. al (2004). While the results did not indicate that memory reconsolidation had any effect on amygdala activity, the results showed that the amygdala was active during extinction (Schiller et. al, 2013). While the results did not find that reconsolidation had an impact, they did show amygdala activity. These results were the same as the previously mentioned studies. While Schiller et. al (2013) did not find any hemispheric preference like LeBar et. al (1998) and Knight et. al (2004), they still supported the general claim made by all of the aforementioned researchers. Overall, when looking at extinction in humans using fMRI, it was shown that the amygdala is active during extinction. Nevertheless, not all researchers chose to look at amygdala activity in humans. In fact, many other studies have utilized different methods to find similar results to the ones mentioned above.
Gunduz-Cinar et. al (2013) looked at amygdala activity in mice. The researchers tested the effects of the effects of the neurotransmitter anandamide on the amygdala during fear extinction. The researchers hypothesized that using a neurotransmitter that blocks the action of an anandamide inhibitor, FAAH, would increase the levels of anandamide in the amygdala. They hypothesized that if the anandamide inhibitor was blocked, extinction would be aided because anandamide aids fear regulation. The researchers first stated that anandamide is seen in high levels during extinction (Gunduz-Cinar et. al, 2013). In contrast to the works previously mentioned, Gunduz-Cinar et. al (2013) used mice as the subjects. The mice were conditioned using a tone (CS) and a shock (UCS). This conditioning paradigm matches the ones from the previously mentioned studies, as they all used a visual CS and a shock as the UCS. Following the acquisition phase, the mice went through the extinction phase. The mice were also injected with the anandamide inhibitor in the amygdala (pre and post extinction training) and then euthanized (Gunduz-Cinar et. al, 2013). Unlike the previous works, this study measured amygdala activity by looking at brain tissue that was removed after death. This varies because the works by LeBar et. al (1998), Knight et. al (2004), and Phelps et. al (2004) looked at amygdala activity in live participants. While the methods of the studies varied, the results found all supported that the amygdala is active during extinction. Gunduz-Cinar et. al (2013) found that the FAAH blocker increased the rate of extinction. They found that there were increased levels of anandamide in the amygdala. The results demonstrated how increased neurotransmitter activity aids extinction. The results of Gunduz-Cinar’s (2013) study looked at the amygdala activity in terms of neurotransmitters. The researchers’ results supported the conclusions found in the previous studies (LeBar et. al, 1998; Knight et. al, 2004; Phelps et. al, 2004), however Gunduz-Cinar et. al (2013) used neurotransmitters to explain amygdala activity. The ability to find amygdala activity during extinction using a vastly different method supports the relationship between the two.
Overall, the research consistently showed that the amygdala is active during extinction of classically conditioned fear. The studies have supported what was found in previous psychological works. Whether it was shown through fMRI or through neurotransmitter studies, the same conclusions have been reached. While some of the studies varied in the parts of the amygdala implicated-LeBar et.al (1998) and Knight et. al (2004) implicated the right amygdala, while Phelps et. al (2004) found no evidence for hemispheric preference-they still supported the overall relationship between the amygdala and extinction. This discrepancy in hemispheric preference can be the topic of further research. Future researchers should look into if the right amygdala is the one implicated. In addition to looking at hemispheric preference, future research should look at how amygdala activity and extinction could be affected. Schiller et. al (2013) wanted to see if consolidation of fear memories had any impact on the amygdala and the extinction process. While their results did not support their hypothesis (Schiller et. al, 2013), further research can see if any cognitive process disrupts the amygdala and extinction. Future research should investigate other neurotransmitters that can alter amygdala activity and extinction, like Gunduz-Cinar et. al (2013). However, the studies mentioned fail to revise any theories or apply the findings to the field. For example, while LeBar et. al (1998) and Knight et. al (1998) found hemispheric preference in amygdala activity, they did not apply these findings to the theory to revise it. Instead, they acknowledged that it occurred. In addition, they failed to offer an alternative explanation for why there was a hemispheric preference. Phelps et. al (2004) gave an alternative explanation when they did not find evidence to support the hemispheric preference. Because LeBar et. al (1998) and Knight et. al (2004) did not offer any other explanation, future research should look into finding one as well. In addition, none of the researchers applied their findings to the field of psychology. Because the works focused on fear extinction, the findings should be applied to findings ways to reduce fear in therapeutics sessions. Using these research findings can generate helpful techniques that reduce fear in patients that have PTSD, for example.
While the works by LeBar et. al (1998), Knight et. al (2004), Phelps et. al (2004), Gunduz-Cinar et. al (2013), and Schiller et al (2013) were not perfect, their results supported the claim that the amygdala is active during extinction. Although the methods varied, the results were consistent across experiments. Overall these works supported previous assumptions and expanded the field of psychology.