At the beginning of this semester, Dr. Stephanie Grella presented on the topic of engrams, specifically how engrams influence behavior. To prepare for the lecture, we were required to read the article “Memory engrams: Recalling the past and imagining the future,” by Sheena Josselyn and Susumu Tonegawa. In this article the concept of engrams is introduced and described as groups of neurons responsible for encoding our memories, a physical representation of our memories in the brain. Josselyn and Tonegawa explain that neurons compete with one another to become part of an engram, and those with greater intrinsic excitability are more likely to succeed. In other words, different experiences lead to differing levels of excitability in neurons across brain regions, and neurons demonstrating higher excitability will be the ones recruited for the engram storing the memory of that experience. The authors provide an example of a finding that showed a fear memory leading to higher neuron excitability in the prefrontal cortex to demonstrate the mechanism of neuronal excitability in engram formation. Dr. Grella reinforced these concepts and provided more details on engram formation for fear memories. The article and lecture led me to consider the role of mood on neuronal excitability and whether depression can affect which neurons are chosen for an engram.
The article “Depression’s Impact on Memory” by Nicole Wetsman (BrainFacts/SfN, 2019) presents research studying the role of depression on memory recall and cognition. The article states that depressed individuals are less likely to remember fine details of events and struggle with carrying out tasks involving prospective memory, which is the ability to remember having to do something in the future. On the other hand, research shows that people suffering from depression have no issue remembering bad memories and, in fact, have a stronger recall for negative memories over positive ones. These findings suggest that depression, or mood, does play an important role in the storing and recalling of memories. In terms of neuronal excitability, the research and observations suggest that in depressed individuals, negative experiences result in higher neuronal excitability than positive experiences which would explain why depressed people have an easier timerecalling negative memories.
The article also describes that brain images of depressed individuals show smaller hippocampal volumes. Despite the correlation between hippocampal volume and depression, Wetsman notes that this finding can not be the sole explanation as to why depressed individuals experience memory issues. Wetsman emphasizes that there are many other factors, such as rumination, and the fact that smaller hippocampi can be present before depression, which proves that hippocampal volume is not a sufficient explanation for why depressed individuals face memory issues. However, if relating this finding back to the concept of neuronal excitability, one can argue that a smaller hippocampal volume would lead to a lower number of neurons in that region, which could ultimately lead to a lower chance of neuronal excitability or availability. Overall, depression increases one's risk for having a smaller hippocampal volume which could affect which neurons can or will be recruited for an engram. Understanding the mechanisms of engram formation and the influence of mood on their development can be an important discovery to develop methods of prevention, reversal, or treatment for individuals suffering from memory impairments due to mood-related disorders, and memory-related disorders in general.
References:
Josselyn, S. A., & Tonegawa, S. (2020). Memory engrams: Recalling the past and imagining the future. Science, 367(6473). https://doi.org/10.1126/science.aaw4325
Wetsman, N. (2019). Depression's Impact on Memory. Brainfacts.org.
https://www.brainfacts.org/diseases-and-disorders/mental-health/2019/depressions-impact-on-memory-022119
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