Understanding how the brain forms, stores, and retrieves memories is one of the most fascinating challenges in the field of neuroscience. It is fascinating how something so perceivably simple and crucial to our everyday being is so complex in its true nature. Memory is not a single event, but a process involving widespread communication between neurons that both physically and chemically adapt in response to our experiences. These adaptations create patterns of neural activity that represent specific memories. The recent work being done in the field that we were informed about before Dr. Grella’s presentation showed how these neural representations, or engrams, can be reactivated to bring a memory back into conscious awareness. Apart from acknowledging this, it is important to note how, daily, memory formation and retrieval are deeply influenced by emotional states. Using this basic knowledge, recent advances in the field have built upon previous research by demonstrating how emotional arousal alters neural activity and can strengthen synaptic connections, which can enhance the likelihood of certain events being remembered over others. These findings emphasize that memory is not purely cognitive but also an affective process that is rooted in the interaction between our emotions and neural plasticity.
In their study presented to us by Dr. Grella, “Memory engrams: Recalling the past and imagining the future,” Josselyn and Tonegawa explore the biological foundations of how memories are physically stored in the brain. They describe engrams as networks of neurons that undergo long-lasting chemical and structural changes when a memory is formed. It is also explained how, although the sizes of engrams remain constant across varying memory strengths, a stronger memory engages a greater number of synapses between engram cells. This has to do with neuronal competition, which occurs when it comes to which neurons end up being allocated to an engram. These engram cells can later be reactivated to bring that memory back to one’s current consciousness, effectively serving as the physical representation of experience. Using modern tools like optogenetics and chemogenetics, their research shows that activating or silencing specific engram cells can either trigger or “erase” a memory in animal models. Their groundbreaking work demonstrates that memory is not just a psychological construct, but a tangible biological process dependent on neuronal competition, excitability, and connectivity.
The emotional dimension of this process is further illuminated in the study by Gärtner et al. Participants were placed in either a positive or a negative mood before encoding word lists, while EEG activity was recorded. The researchers found that memory performance was better in the positive mood condition compared to the negative mood condition. On the neural level, successful encoding during a positive mood enhanced memory performance and was linked to increased delta band power (slow oscillations below 8 Hz), while a negative mood decreased beta band power (8-30 Hz) in frontal regions of the brain. These oscillatory differences offer insight into how mood might bias which neuronal networks are recruited during encoding. During a positive mood, the enhanced delta activity suggests a more globally synchronized, integrative mode of processing.
This relationship suggests that mood is not just a backdrop to memory, but an active contributor in shaping its biological foundation. Positive emotional states facilitate stronger and more coherent neural representations or engrams, while negative states may limit or distort them. This could also affect the clarity of memory retrieval when those engrams are tagged or stimulated. Together, the work of Gärtner et al. and Josselyn and Tonegawa highlights that emotion and neural competition jointly determine how memoirs are formed, stabilized, and later recalled. This links our internal emotional landscape directly to the physical imprint of the experience in the brain.
References
Josselyn, S.A., & Tonegawa, S. (2020). Memory engrams: Recalling the past and imagining the future. Science (New York, N.Y.), 367(6473), eaaw4325. https://doi.org/10.1126/science.aaw4325
Gärtner, M., & Bajbouj, M. (2014). Encoding-related EEG oscillations during memory formation are modulated by mood state. Social Cognitive and Affective Neuroscience, 9(12), 1934-1941. https://doi.org/10.1093/scan/nst184
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