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Science & Technology

Where are memories stored in the brain ?

January 11, 2022 | Comment icon 49 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 (49)

Recent comments on this story
Comment icon #40 Posted by psyche101 13 hours ago
After the amount of times I've corrected you on this imaginative idea of yours based on old information, you either cannot retain information or refuse to. Again. The brain takes hours to die. Near death is not death.† Human brain may stay active for hours after death Study leader Dr. Sam Parnia said that the patients could describe in details what happened around them. He explained that the time death is declared is the one when the heart stops beating. As the heart stops beating, it stops pumping blood to the brain and slowly the brain begins to shut down, he explains. He added that this pro... [More]
Comment icon #41 Posted by papageorge1 13 hours ago
I have heard dozens of cases that make no sense in your memory storage theory. Here would be how I'd humorously and seriously appraise your argument style for your theory (oh, excuse me, your non-controversial certain fact ): If you ignore all data that doesn't fit, the data fits nicely
Comment icon #42 Posted by psyche101 13 hours ago
I don't care. I've offered actual data. Old wives tales aren't data. They are fairytales. All you did was illustrate your belief again, so I repeat, who cares what you believe? Everything you have illustrated as your belief is rather silly.† What data? I'm the only one who has provided any and it doesn't support your fantasy. It supports the real world information that I have posted. You have referred to useless anecdotes.
Comment icon #43 Posted by Golden Duck 12 hours ago
Do you like Assasins Creed?
Comment icon #44 Posted by papageorge1 12 hours ago
Sorry donít know of it.
Comment icon #45 Posted by Golden Duck 12 hours ago
Just remember it then.
Comment icon #46 Posted by onlookerofmayhem 3 hours ago
https://www.britannica.com/biography/Eric-Kandel "Kandelís 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 ba... [More]
Comment icon #47 Posted by onlookerofmayhem 3 hours 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 th... [More]
Comment icon #48 Posted by moonman 1 hour ago
Remember kids, the only one who loses an argument against stupid is you.
Comment icon #49 Posted by onlookerofmayhem 59 minutes ago
Guilty as charged.

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