Have you ever lost your wallet? Think back a bit. Let the experience really come back to you. I'd wager that you can remember a good deal about it. You can probably remember a good deal about where you were, the people you were with, maybe even some of the things that were running through your head. There is an evolutionary advantage to the preferential recall of emotionally intense memories. In the wild, remembering if the big bear you saw in that cave was scary might mean the difference between death and survival. So whether the memory involves the fear of losing your wallet, the excitement of opening a big present on Christmas morning, or the warm comfort of a bed after a long day, your brain has a vested interest in storing these emotionally charged memories for later recall.
As Dr. Radulovic covered in her lecture, there are several areas of the brain that work together to mediate the consolidation and recall of emotionally-related memories, but chief among them are the hippocampus, the prefrontal cortex, and the amygdala. The exact nature of the roles that these structures play in emotionally charged memories is continuing to be teased out, thanks to a variety of recent advances in biotechnology. In her research, Dr. Radulovic makes use of genetic modifications, chemo- and optogenetic techniques, and targeted designer neurotransmitters to unravel causal relationships between networks of neurons in the brain. The most prominent experimental technique that integrates these practices in the aim of measuring emotional memory consolidation and retrieval is the floor shock paradigm. Rodents have an innate freezing reflex that occurs during moments of fear -- an easily measurable, unambiguous representation of emotion that can be used to measure increases or decreases in consolidation or retrieval, depending on when the rodents are exposed to the stimulus that preceded the shock a second time. By blocking the action connections from one area to another during different parts of the experiment, a causal role can be established for specific circuitry in the brain.
A few years ago, a study lead by Andrew Holmes at the National Institutes of Health found that afferent connections from the amygdala were responsible for the extinction of fear response. By stimulating the connections from the amygdala to the hippocampus, rodents would effectively "purge" their memory of the foot shock, freezing far less often than their control counterparts in subsequent trials. Conversely, disrupting the connections between the amygdala and hippocampus prevented the extinction of the fear response, suggesting a relationship between the two. However, as Holmes points out, the amygdala is only part of the picture. Connections from the prefrontal cortex go just about everywhere, and they modulate and are modulated by just about every structure they connect to. However, pinpointing the precise role of the prefrontal cortex is far from easy, since it's near impossible to know which neurons carry what information. However, recent advances like optogenetics and transcranial magnetic stimulation provide opportunities to research and treat the problematic human correlates of overactive fear-related memories, anxiety and PTSD being the big ones. Hopefully, once we can better unravel the nature of these connections, we can begin to develop therapies to effectively treat these potentially debilitating diseases. Or at least, we can help you forget all about the time that you lost your wallet.
Scientific American - "How the Brain Purges Bad Memories"
"Differential Contributions of Glutamatergic Hippocampal to Retrosplenial Cotrical Projections to the Formation and Persistence of Context Memories" N. Yamawaki, K. A. Corcoran, A. L. Guedea, G. M. G. Shepherd, J. Radulovic. 2018
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