In a presentation given by Dr. Stephanie Grella, Grella highlights her research on engrams within "Memory engrams: Recalling the past and imagining the future" by Sheena A. Josselyn and Susumu Tonegawa. Within this particular study, Grella and colleagues aimed to investigate the importance of engrams within memory formation and retrieval in response to external experiences. Engram populations within mice were observed, and observational, gain-of-function, loss-of-function, and mimicry studies were performed on these cells. Grella and her colleagues were able to gain a deeper understanding of engrams' role in memory, such as that they are active during memory retrieval and external experiences, they can be artificially activated to prompt memory retrieval, and they require a specific conditioned stimulus for recollection. Given these findings, there is much more to be understood about engrams, such as their mechanism of action at different speeds.
In the article, "Fast track to the neocortex: A memory engram in the posterior parietal cortex" (Brodt et al., 2018), Brodt and colleagues did just that. They attempted to gain a deeper understanding underlying the process of memory consolidation within fast-learning experiences--entailing the formation of a memory engram within a short time span.
The article first outlines that given recent research findings, it has been indicated that the posterior parietal cortex may be able to acquire a memory quickly in contrast to the presumption that it is a slow process. In regard to memory retrieval and encoding, functional magnetic resonance imaging, or FMRI's, can be used to visualize memory representations that are active. Thus, the researchers in this study used fMRI and diffusion-weighted MRI (DW-MRI) to depict neocortical areas of humans during two sessions of four encoding-recall repetitions of an object-location association task. This was done to find the location of the engram produced by a memory.
Changes in functional activity suggesting memory representations were first identified, as well as experience-dependent responses that increased with repetitive retrieval. The only regions that had a significant correlation with functional brain activity and the performance of memory were the posterior parietal areas. A multivariate pattern analysis was then conducted to determine if the precuneus were specific to content. It was concluded that this area can decode category information of stimuli and that it has the ability to have engrams of information retrieved. After further investigation of the precuneus, the memory representations were assessed as to whether they qualified as engrams. These memory representations were found to be engrams as they followed four criteria: having a relation to a specific experience, being able to be unused for a period of time, being able to produce change in neural substrate, and having the ability to facilitate the recalling of memories.
In short, Brodt and colleagues found that a neocortical engram was able to be made quickly solely by four rounds of memory rehearsal revolving vision. Additionally, it was consequently proposed that multiple instances of encoding are necessary for fast learning-induced neocortical plasticity. A deeper level of understanding was found in regard to timing of engram formation, which is essential to understanding the formation of memories in brief experiences--or fast learning. This information is fascinating considering that the systems memory consolidation has previously been known to occur slowly. In light of this challenged understanding of slower systems memory consolidation, should we continue to accept this idea or reject it? There is still much to be learned in terms of engrams and further research that can be done relating to the speed of engram formation, such as investigating engram formation speed and other perception tests.
References
- S. Brodt et al., Fast track to the neocortex: A memory engram in the posterior parietal
cortex. Science 362, 1045-1048 (2018). DOI: 10.1126/science.aau2528
Josselyn, S., Tonegawa, S. (2020). Memory engrams: Recalling past and imagining the future.
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