Rumination on Memory and Emotion, Part 2: Physiology

Parts of the Brain Involved in Memory (Image from Queensland Brain Institute)

Different types of memories are registered and stored in different parts of the brain. Episodic and semantic memories are formed and cataloged in the hippocampus, which is regarded as the center for memory in the brain. Though it is crucial for the initial creation of memories, the hippocampus does not permanently store them. Over time, information from these temporarily stored memories can transfer to the neocortex, the neural tissue that forms the outside of the brain. The process of converting short-term memories to long-term memories is called consolidation. This explanation of consolidation has been widely accepted, but a recent Massachusetts Institute of Technology study in 2017 suggests the process may differ from our initial beliefs. We know episodic and semantic memories begin with the hippocampus then end up in the neocortex for long-term storage. However, “the engrams [change in neural tissue postulated in order to account for persistence of memory] and circuits [an arrangement of neurons and their interconnections] that support neocortical memory consolidation have thus far been unknown." (Kitamura 1) The researchers at the Massachusetts Institute of Technology “found that neocortical prefrontal memory engram cells, which are critical for remote contextual fear memory, were rapidly generated during initial learning through inputs from both the hippocampal–entorhinal cortex network [the major gateway between the hippocampal formation and the neocortex] and the basolateral amygdala. After their generation, the prefrontal engram cells, with support from hippocampal memory engram cells, became functionally mature with time. Whereas hippocampal engram cells gradually became silent with time, engram cells in the basolateral amygdala, which were necessary for fear memory, were maintained.” (Kitamura 1) In other words, this study of the underlying neural circuits crucial for consolidation of memory revealed that memories are formed simultaneously in the hippocampus and prefrontal cortex (the frontmost portion of the neocortex), with input from the hippocampus and amygdala. Researchers also learned that long-term memories in the prefrontal cortex are silent for about two weeks before becoming functionally mature, while the memories in the hippocampus slowly become silent. 

The left and right amygdalae, commonly referred to as the amygdala, give input to our memories. The amygdala is the part of the brain that gives prominence to emotionally significant memories, modifying their strength and emotional content. In doing so, it has an important role: determining which memories are stored and how robustly.

Like the amygdala, the hippocampus is an emotional processing center. After the amygdala filters memories, the hippocampus encodes emotional context from the amygdala. In doing so, the hippocampus processes our explicit long-term memories. Knowing that our memories are processed through the same parts of the brain as our emotions, we can see why memory and emotion are so closely linked, and it comes as no surprise that emotion has an influential role in memory. Our previous and current findings consistently show that the hippocampus, amygdala, and prefrontal cortex, all parts of the limbic system, play central roles in memory. (op cit.)

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Dumper, Kathryn, William Jenkins, Arlene Lacombe, Marilyn Lovett, and Marion Perimutter. “Parts of the Brain Involved in Memory.” Introductory psychology. Accessed November 27, 2021. https://opentext.wsu.edu/psych105/chapter/8-3-parts-of-the-brain-involved-in-memory.

Merriam-Webster. Merriam-Webster. Accessed December 6, 2021. https://www.merriam-webster.com/dictionary/engram.

Trafton, Anne | MIT News Office. “Neuroscientists Identify Brain Circuit Necessary for Memory Formation.” MIT News | Massachusetts Institute of Technology. Accessed November 27, 2021. https://news.mit.edu/2017/neuroscientists-identify-brain-circuit-necessary-memory-formation-0406.

“Where Are Memories Stored in the Brain?” University of Queensland, Queensland Brain Institute, July 23, 2018. https://qbi.uq.edu.au/brain-basics/memory/where-are-memories-stored.

Witter, Menno. “Entorhinal Cortex.” Scholarpedia 6, no. 10 (2011): 4380. https://doi.org/10.4249/scholarpedia.4380.