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Where are memories stored in the brain ?

January 11, 2022 · Comment icon 55 comments

There is still much we don't understand about the brain. Image Credit: CC BY 2.0 Allan Ajifo
Prof Don Arnold and colleagues have been attempting to solve the long-running mystery of where our memories are stored.
All memory storage devices, from your brain to the RAM in your computer, store information by changing their physical qualities. Over 130 years ago, pioneering neuroscientist Santiago Ramón y Cajal first suggested that the brain stores information by rearranging the connections, or synapses, between neurons.

Since then, neuroscientists have attempted to understand the physical changes associated with memory formation. But visualizing and mapping synapses is challenging to do. For one, synapses are very small and tightly packed together. They're roughly 10 billion times smaller than the smallest object a standard clinical MRI can visualize. Furthermore, there are approximately 1 billion synapses in the mouse brains researchers often use to study brain function, and they're all the same opaque to translucent color as the tissue surrounding them.

A new imaging technique my colleagues and I developed, however, has allowed us to map synapses during memory formation. We found that the process of forming new memories changes how brain cells are connected to one another. While some areas of the brain create more connections, others lose them.

Mapping new memories in fish

Previously, researchers focused on recording the electrical signals produced by neurons. While these studies have confirmed that neurons change their response to particular stimuli after a memory is formed, they couldn't pinpoint what drives those changes.

To study how the brain physically changes when it forms a new memory, we created 3D maps of the synapses of zebrafish before and after memory formation. We chose zebrafish as our test subjects because they are large enough to have brains that function like those of people, but small and transparent enough to offer a window into the living brain.

To induce a new memory in the fish, we used a type of learning process called classical conditioning. This involves exposing an animal to two different types of stimuli simultaneously: a neutral one that doesn't provoke a reaction and an unpleasant one that the animal tries to avoid. When these two stimuli are paired together enough times, the animal responds to the neutral stimulus as if it were the unpleasant stimulus, indicating that it has made an associative memory tying these stimuli together.

As an unpleasant stimulus, we gently heated the fish's head with an infrared laser. When the fish flicked its tail, we took that as an indication that it wanted to escape. When the fish is then exposed to a neutral stimulus, a light turning on, tail flicking meant that it's recalling what happened when it previously encountered the unpleasant stimulus.

To create the maps, we genetically engineered zebrafish with neurons that produce fluorescent proteins that bind to synapses and make them visible. We then imaged the synapses with a custom-built microscope that uses a much lower dose of laser light than standard devices that also use fluorescence to generate images. Because our microscope caused less damage to the neurons, we were able to image the synapses without losing their structure and function.

When we compared the 3D synapse maps before and after memory formation, we found that neurons in one brain region, the anterolateral dorsal pallium, developed new synapses while neurons predominantly in a second region, the anteromedial dorsal pallium, lost synapses. This meant that new neurons were pairing together, while others destroyed their connections. Previous experiments have suggested that the dorsal pallium of fish may be analogous to the amygdala of mammals, where fear memories are stored.
Surprisingly, changes in the strength of existing connections between neurons that occurred with memory formation were small and indistinguishable from changes in control fish that did not form new memories. This meant that forming an associative memory involves synapse formation and loss, but not necessarily changes in the strength of existing synapses, as previously thought.

Could removing synapses remove memories?

Our new method of observing brain cell function could open the door not just to a deeper understanding of how memory actually works, but also to potential avenues for treatment of neuropsychiatric conditions like PTSD and addiction.

Associative memories tend to be much stronger than other types of memories, such as conscious memories about what you had for lunch yesterday. Associative memories induced by classical conditioning, moreover, are thought to be analogous to traumatic memories that cause PTSD. Otherwise harmless stimuli similar to what someone experienced at the time of the trauma can trigger recall of painful memories. For instance, a bright light or a loud noise could bring back memories of combat. Our study reveals the role that synaptic connections may play in memory, and could explain why associative memories can last longer and be remembered more vividly than other types of memories.

Currently the most common treatment for PTSD, exposure therapy, involves repeatedly exposing the patient to a harmless but triggering stimulus in order to suppress recall of the traumatic event. In theory, this indirectly remodels the synapses of the brain to make the memory less painful. Although there has been some success with exposure therapy, patients are prone to relapse. This suggests that the underlying memory causing the traumatic response has not been eliminated.

It's still unknown whether synapse generation and loss actually drive memory formation. My laboratory has developed technology that can quickly and precisely remove synapses without damaging neurons. We plan to use similar methods to remove synapses in zebrafish or mice to see whether this alters associative memories.

It might be possible to physically erase the associative memories that underlie devastating conditions like PTSD and addiction with these methods. Before such a treatment can even be contemplated, however, the synaptic changes encoding associative memories need to be more precisely defined. And there are obviously serious ethical and technical hurdles that would need to be addressed. Nevertheless, it's tempting to imagine a distant future in which synaptic surgery could remove bad memories.

Don Arnold, Professor of Biological Sciences and Biomedical Engineering, USC Dornsife College of Letters, Arts and Sciences

This article is republished from The Conversation under a Creative Commons license.

Read the original article. The Conversation

Source: The Conversation | Comments (55)

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Recent comments on this story
Comment icon #46 Posted by onlookerofmayhem 2 years ago "Kandel’s award-winning research centred on the sea slug Apylsia, which has relatively few nerve cells, many of them very large and easy to study. The sea slug also has a protective reflex to guard its gills, which Kandel used to study the basic learning mechanisms. These experiments, combined with his later research on mice, established that memory is centred on the synapses, as changes in synaptic function form different types of memory. Kandel showed that weak stimuli give rise to certain chemical changes in synapses; these changes are the... [More]
Comment icon #47 Posted by onlookerofmayhem 2 years ago
That is one of the most inane, ironic, hypocritical and ridiculous things you have ever said on here. 99.999999999999999999% of the data says that memories are created by and stored in the brain. And you come in, with absolutely no actual data, to say that Deepak Chopra says it's not. And posting links about non-locality and refusing to explain what it has to do with anything.  You are the one dismissing pretty much all of the data and science to contend otherwise. Where is all if this scientific data and studies relating to people knowing things they couldn't have possibly known? Where are t... [More]
Comment icon #48 Posted by moonman 2 years ago
Remember kids, the only one who loses an argument against stupid is you.
Comment icon #49 Posted by onlookerofmayhem 2 years ago
Guilty as charged.
Comment icon #50 Posted by papageorge1 2 years ago
Remember all science can do at this time is observe physical activity that occurs. The resultant experience of consciousness and memory they cannot directly detect. But anyway, in my mind the physical storage theory in the brain cannot be the final answer unless it can explain things such as: 1. How people can experience memory during Near Death Experiences when all higher level functioning of the brain is not occurring. 2. Documented cases of verifiable reincarnation memories.  3. Mediumistic and telepathic communication from the long departed that shows their memories as their former selves... [More]
Comment icon #51 Posted by onlookerofmayhem 2 years ago
Prove that this is actually occurring. Prove that this is actually occuring. Prove that this is actually occurring. That's what amounts to a 98 page blog post that contains hundreds of hours of YouTube videos. You've posted it before and it's mostly unverifiable hearsay stories. They are nowhere near well documented. They are stories. I asked for scientific studies. The ones that show brain dead people and the resultant information they obtained. This is the main problem. You are claiming that these stories are true, but at the same time impossible to provide evidence for. And all I hear from ... [More]
Comment icon #52 Posted by papageorge1 2 years ago
We each have to go through the body of cases and fairly ask ourselves if all this fits in a materialistic model of consciousness and memory. My rational position is that that this body of cases cannot be explained by a materialist concept of consciousness and memory but is quite consistent with explanations given in other spiritual wisdom traditions.  If you honestly feel the materialist model of consciousness best explains this body of cases, then hold to that model. Personally, I think this body of data/evidence/anecdotes/experiences/etcetera cannot be explained by a materialist model beyon... [More]
Comment icon #53 Posted by psyche101 2 years ago
You're right though
Comment icon #54 Posted by Grey Area 2 years ago
They don’t experience memory during near death experience.  Upon awaking they have a memory of a supposed experience.  An experience I would hasten to add occurring during a period of abnormal chemical. sensory and traumatic conditions.  NDE is in no way an experience that supports this remote memory hypothesis.   If we were able to 100% verify past life memory it would change the world, shocker, it remains as unverified as this remote memory theory.  But again, that a person may be reincarnated and keep some memories wouldn’t necessarily be confirmation of remote memory storage, just... [More]
Comment icon #55 Posted by papageorge1 2 years ago
I guess we have a different position on what occurs during an NDE. I believe the astral/mental body separates from the physical body. In that state the experiencer has memory of who they are and who those around them are even though no higher physical brain functioning is occurring. I feel some past life memories have been verified by researchers (such as Dr. Ian Stevenson) as well as can be expected. If memories only existed in the brain of the deceased person, then one would expect verifiable reincarnation memories to be impossible. Yet, they seem to occur. It is possible too to speculate on... [More]

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