Age may have some effect on how the brain processes memories of fearful experiences, according to research published in The Journal of Neuroscience. In studying the process of fear extinction in developing rats, researchers from the University of New South Wales in Sydney found that young rats, the equivalent of human children, processed memories of a fearful experience differently than older rats, the equivalent of adolescents.
The study examined the role played by the amygdala during memory extinction, the process of extinguishing a conditioned response to fear. The researchers trained rats, aged 16 days and 24 days, to associate a certain sound with a mild electric shock. When they then repeatedly created the sound without the associated shock, the rats' fearful reaction to the sound dissipated. Fear extinction does not destroy the memory of the experience, and the feared response can reoccur spontaneously. This has not been observed in young rats, however. In this study, the researchers searched for a different mechanism underlying fear extinction in young rats by testing the role of the amygdala, an area of the brain involved in memory and emotion.
The researchers repeated the process of conditioning and extinction twice. Both times, they used anesthesia to temporarily deactivate the rats' amygdalae. In the first trial, the researchers observed that deactivating the amygdala hindered the process of fear extinction in 16 day old and 23 day old rats. In the second trial, rats went through the process of conditioning and extinction and were then re-conditioned to and re-extinguished from the same stimulus. The amygdala was deactivated in only some of the rats. This time, the researchers observed that in 17 day old rats, only those with an active amygdala successfully completed extinction training, a further indication of the amygdala's involvement at this age. In 24 day old rats, however, fear extinction occurred whether or not the amygdala had been deactivated. The researchers determined that the age at which the initial extinction training occurred was important for later extinction training. Re-extinction training depended upon the amygdala for rats that initially underwent extinction training at age 17 days. For rats that did not undergo extinction training until they were aged 24 days, however, subsequent extinction trainings did not depend upon the amygdala.
For young rats, the amygdala appears to play a critical role in processing memories of fear, but the mechanism changes as the brain matures. These results provide further understanding of how the brain processes and extinguishes fearful memories, and as we continue to study these processes, we will better understand the biological factors underlying fear and how we can treat those hindered by fear.
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