Stress hormones such as glucocortocoids and epinephrine play a central role in the 'fight or flight' response evoked in conditioned fear learning experiences. Observations that post training injections of stress hormones enhance memory storage suggest that endogenous stress hormones released during fear conditioning are important for the formation of memories and amplify fear responses.
Research on PTSD and repressed memories also indicate that extreme stress may lead to an inability to consciously access memories although an emotional response still exists. The effect of high levels of stress resulting from traumatic emotional events, LeDoux notes, may result in damage to the hippocampus and, consequently, the inability to form explicit memories although the effects of the amygdala's memory consolidation prevail. It is interesting to note, however, that most of the research focusing on effects of stress have only dealt with prolonged exposure to stress instead of isolated traumatic experiences. It is unclear whether a single traumatic event can cause a great enough stress response to impair the hippocampus. Nevertheless, findings strongly suggest that the amygdala's capacity to consolidate experiences into powerful unconscious agents override the organism's ability to control their response.
I would have liked McGaugh, LeDoux, La Bar and Cabeza's explanations of memory extinction to describe the processes of extinction in greater detail (to be fair, LeDoux says he'll come back to it). From what I can gather, extinction is the process of learning to disassociate a stimulus with a conditioned response. In the case of auditory Pavlovian fear conditioning in rats, the animal would learn to associate an auditory cue with a neutral experience instead of a painful one. This idea is interesting for a number of reasons. First, I think it implies that extinction has nothing to do with forgetting a memory, but is actually a unique learning process. Secondly, in order for extinction to occur, the brain must associate the previously learned context with a new context, which is why animals are highly resistant to extinction when hippocampal damage exists. LeDoux mentions that extinction relies on the connections between the medial temporal cortex and amygdala. As the animal learns to consolidate a new context for the stimulus does this require recalling and comparing the memories? What would happen if an aversive emotional response was replaced with a pleasurable one?
And finally, our readings emphasized memories associated with negative emotions, such as fear. Does the amygdala play as influential a role in memory for pleasurable responses? Are memories associated with pain more powerful than pleasurable responses because there is a greater incentive for the survival of the organism? What about sex?
2 comments:
This week’s readings again focused on the brain structures responsible for the formation, storage, and retrieval of memories, particularly emotionally charged memories. Yet even after both weeks’ readings, I am still slightly confused as to the accuracy and enduring quality of memories of intensely emotional (in some cases traumatic) events. While the majority of psychologists seems to agree that emotions actually improve an individual’s ability to form, store, and ultimately recall most memories, LeDoux and other’s research on PTSD and repressed memories seems to indicate otherwise. If fact, LeDoux hypothesizes that the effect of high levels of stress resulting from traumatic emotional events can actually physically damage the hippocampus and therefore, an individual’s ability to form future explicit memories. As Meredith has noted, most of the research focusing on the [un]conscious repression of emotionally traumatic memories has been primarily concerned only with a prolonged exposure to stress instead of episodic experiences and it remains unclear whether a single traumatic event can physically damage the hippocampus, though I believe so. These findings seem to contradict each other. Perhaps the ultimate effect of emotion on memory depends on the individual. Whereas one person may vividly remember every detail (or so s/he believes they do) of say, a plane crash, another may completely repress this memory as is too traumatic for them to consciously recall. Or does every individual have a certain predetermined level of stress, that, once transgressed, activates certain brain structures programmed to repress memories?
To respond to Chess' question about the contradicting ideas about emotion improving memory versus eliminating it--Although the ultimate effect of of emotion on memory may depend partly on the individual, I think more likely, it has to do with the level of emotion and whether it is a positive or negative emotion. We have to remember Ledoux is specifically talking about mental disorders in chapter 8, which mainly involve extreme negative emotions in relation to memories.
What I have questions about (like my post last week) are the concepts of repression. Although it seems well known that 'aspects of traumatic experiences are sometimes stored in memory systems that are not directly accessible from consciousness'(246), the study of repression's involvement in memory is less certain. I found it interesting that the hippocampus can literally shut down from experiencing traumatic events, but what I wonder about is the conscious or unconcious control we have over shutting down our own hippocampus. What is the relationship between repression and denial?
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