Neuro Post-it 5: The Neuroscience of Memory
A new series of neuroscientific digestible insights! Written in collaboration with Cesca Centini.
Memory is such an indispensable part of our lives that it seems almost facile.
But while it may appear effortless on the outside, in reality there is a ton of complex neural processing going on inside the brain to make it all possible.
“Without memory, there is no culture. Without memory, there would be no civilization, no society, no future.” - Elie Wiesel
Distinguishing Learning and Memory
Before diving into memory, it’s important to make a distinction between memory and learning. These two cognitive processes that we are so incredibly adept at implementing are related but not exactly the same.
Learning involves changes in our nervous system that happen as a result of experience; memory entails how these changes are maintained over time and how they’re expressed → recall!
So…are they different?
Yes, but they are interconnected…
Learning is impossible without memory
Memory is impossible without learning
Learning and memory together contribute in many ways to who we are as human beings.
Deeply crucial to our existence, but susceptible to error
“The human memory is such a cruel, frustrating thing, the way it just discards things without asking permission, precious things.” - Lisa Jewell
While our immense capability for storing memories is by far one of our greatest assets as human beings, it can not be overstated that a lot of our memories are flawed.
As American psychologist Dr. Elizabeth Loftus describes perfectly in her TED talk, How reliable is your memory? :
“Many people believe that memory works like a recording device. Memory works a little but more like a wikipedia page. You can go in there and change it, but so can other people.”
But deep down this is a law of nature: everything in the biological world comes at a cost of energy, and storing memories in our brain is no exception. It is simply impossible to store 100% of our memories 100% of the time. As much as the human brain is unimaginably capable, it has been evolutionarily designed to prioritize certain bits of information while discarding others to optimize efficiency.
Yes, even the most complex object known to mankind runs on limited energy that must be allocated reasonably.
Of course, energy allocation isn’t the only limitation. There are other factors involved too such as a failure to retrieve info, weak initial encoding of info, and commonly, a phenomenon called interference in which we tend to forget something when trying to remember something else.
However, as you know, our brains are incredibly versatile and can be trained to minimize this. Through active recall and repetition of information we can enhance our memories and delay forgetting them quite substantially the more we review. Reviewing leads to changes in the synaptic strength of networks between neurons that underlie long-term memory, strengthening them over time.
Memory in synapses
As with many other processes, memory storage demands physical changes to synaptic connections in the brain.
Following a training period, there are multiple ways synapses could be modified:
More neurotransmitters are released into synapses
Postsynaptic neurons have a higher sensitivity to the neurotransmitters, often by an upregulation of receptors on their dendrites
Synapses increase in size
Input from neighbouring neurons can result in extra depolarization or hyperpolarization at the axon terminals, which alters how much neurotransmitter is released at the synapse. These changes in neurotransmitter release strengthen existing neural links, leading to the formation of memories over time.
As when exercise tones muscles, sometimes the communication between two neurons becomes strong enough that additional synapses form between them. If one neural pathway becomes more active than another due to repeated communication, the more active pathway takes over sites previously occupied by the less active one— a process that can lead to a phenomenon known as long-term potentiation (LTP). LTP enhances the efficiency of neurotransmission in the more active pathway, strengthening the associated memory over time.
Stages of Memory
Memory comes in three primary stages: Encoding, Storage, and Retrieval.
Encoding: Sensory information from all the senses enters sensory buffers, which is then encoded and placed into short-term memory (STM).
Storage: If the information gets rehearsed or used in some way, it could consolidate into long-term memory (LTM), which can last anywhere from minutes to a lifetime—depending on the amount of rehearsal and significance of the information.
Retrieval: Information from LTM is placed temporarily into STM for use.
But what areas of the brain do these processes actually take place in? Let’s learn some interesting neuroanatomy to find out…
There is one main region of the brain where this all takes place: the Medial Temporal Lobe.
This region of the brain isn’t just crucial to the formation of memories, but also plays a significant role in navigation and the perception of time which are all crucial in the development of long-term declarative memory. Unsurprisingly, this region of the brain has some of the highest and most complex levels of cognitive activity.
Let’s break down the medial temporal lobe to further understand the different cortical and subcortical structures involved:
Entorhinal cortex: connects the hippocampus to the neocortex—the site in the cerebral cortex where LTM is primarily stored—and is involved in encoding and transferring declarative memory between the two structures.
*think of the entorhinal cortex as an interface between the place where memory is encoded, the hippocampus, and where memory is stored and can be retrieved from, the neocortex.
Hippocampus: Probably the first structure that comes to mind when you think of the neuroanatomy of memory, this seahorse-shaped subcortical structure mediates the conversion of short-term declarative memory into long-term declarative memory during the storage stage. Also plays a big role in processing spatial relationships. (more below)
Parahippocampal gyrus: located adjacent to the hippocampus, this structure is a hub that combines sensory information to create a cohesive image of the environment and the memories associated with it.
*thanks to its proximity to the hippocampus, it is heavily involved in spatial memory and navigation.
A few captivating examples providing evidence that the hippocampus is important for spatial memory in species:
Birds that store seeds have larger hippocampi than non-food caching birds
In humans, there is hippocampal activation during the performance of virtual navigation tasks
London taxi cab drivers have much larger hippocampi than normal subjects
An endless quest of study
It’s fascinating how much we still don’t know about memory. We don’t know why some people have super amazing memories; or why certain individuals can sleep an hour a night and have perfectly normal cognitive functioning.
Neuroscience has studied people with endless semantic memory like Solomon Shereshevski, who could remember random equations shown to him years before yet had troubles with faces because “they change too much”. Or Kim Peek, a savant diagnosed with Asperger syndrome who could remember entire books after reading them once, but also had trouble remembering faces. On the other hand we’ve also seen cases of highly superior autobiographical memory (HSAM)—exhibited by people like Jill Price—who, when provided with a random date in history, could immediately specify what day of the week it was on with unparalleled accuracy.
The most fascinating thing, however, is that no two people have identical sets of memories. Each one of us lives life with unique combinations of experiences, regulated by our emotions, the places we’ve been to, and the people who we spend time with.
And it’s all made possible thanks to those little high-tech electrochemical cells nestled within our cranium.
Resources and interesting bits
Loftus, E. F. (2002). How reliable is your memory? [Video]. TED. https://www.ted.com/talks/elizabeth_loftus_how_reliable_is_your_memory
Squire, L. R., & Wixted, J. T. (2011). The cognitive neuroscience of human memory since H.M. Annual review of neuroscience, 34, 259–288. https://doi.org/10.1146/annurev-neuro-061010-113720
Lynch M. A. (2004). Long-term potentiation and memory. Physiological reviews, 84(1), 87–136. https://doi.org/10.1152/physrev.00014.2003
Maguire, E. A., Spiers, H. J., Good, C. D., Hartley, T., Frackowiak, R. S., & Burgess, N. (2003). Navigation expertise and the human hippocampus: a structural brain imaging analysis. Hippocampus, 13(2), 250–259. https://doi.org/10.1002/hipo.10087
Burgess, N., Maguire, E. A., & O'Keefe, J. (2002). The human hippocampus and spatial and episodic memory. Neuron, 35(4), 625–641. https://doi.org/10.1016/s0896-6273(02)00830-9
Gage FH. Neurogenesis in the adult brain. J Neurosci. 2002 Feb 1;22(3):612-3. doi: 10.1523/JNEUROSCI.22-03-00612.2002. PMID: 11826087; PMCID: PMC6758482.
https://www.newyorker.com/books/page-turner/the-mystery-of-s-the-man-with-an-impossible-memory
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