The Danger of Remembering

Edited by Alexandra Logerfo1

^{Based on an article published in Frontiers in Behavioral Neuroscience in May of 2014 (Link to the article)}

The Comical Introduction:

The ability to forget and the ability to remember are two seemingly opposing processes. Ask anybody about the two and they will generally say that they would like to forget less and remember more. The strength of a memory is sometimes coupled to the strength of the emotions tied to it: stronger emotion, stronger memory. This concept is what led me to write about an article discussing the relationship between memory and emotion.²

Memory	Emotion
What if you were unable to forget every single bad thing that your best friend did to you? Though you tried to forgive, every time you see his face you remember that time he puked Mudslides all over the floor of a certain Washington University residence hall because he decided funneling them would be a good idea, and left you to clean it up	The memory itself wouldn’t be that horrible if it didn’t come with emotion, the fear that you were going to get caught, the irritation that he couldn’t help you, and of course the stomach turning that accompanies a task like that.

Background and the Real Problem:

While the situation above is completely made up³, and somewhat comical, it highlights the relationship between memory and emotion. Traumatic experiences such as rape, abuse, or shell shock result in what is called “aversive learning.” This aversive learning can result in cue or context based expression of fear. This means that items and/or sounds associated with the event (cues) or similar situations (contexts) can trigger the expression of fear. It can be debilitating, forcing people to experience the terror of an already horrible incident over and over again. This is one of the key aspects of post-traumatic stress disorder (PTSD). People experience intense fear/anxiety when exposed to cues and contexts.

The hippocampus is a part of the brain that is often associated with memory, and the amygdala is a part of the brain that is associated with the emotion of anxiety. Neuroscience is all about finding the connections between parts of the brain and that is what Dennis Sparta and Jim Smithuis set out to do in an article titled

“

Before I can get into the heart of the article and the research findings, I need to explain two things: the process of experiencing a traumatic memory and the technique of optogenetics.

The process of experiencing a traumatic memory involves two basic parts. Let’s use the example of a mugging. You first have to gain the information; that is termed acquisition. Seeing a masked man, experiencing fear, and then storing the experience and the paired emotion of fear in your brain is acquisition. The second part, expression, might involve you seeing a completely different masked man and then stopping in your tracks because of that fear.

Optogenetics has been around for a little while now, but its usage is permeating more and more of neuroscience. This technique allows a scientist to put a light sensitive switch in a specific subset of neurons. By shining light on these specific neurons, scientists can stimulate the neuron or shut it down. It is a step forward in solving one of the toughest problems in neuroscience: resolution. Before optogenetics, shutting down neurons involved using direct injections or lesioning parts of the brain. Neither of these are terribly accurate and affect neurons in the magnitude of thousands - a large section of neurons is damaged and the role of specific neurons cannot be established. Metaphorically, this would be like knocking out a school, seeing that graffiti in an area decreased and then concluding that schools are responsible for graffiti, when in reality, only a couple of kids are responsible for the graffiti.

The Experiment

This experiment attempts to identify a specific connection between fear/anxiety (Basolateral amygdala, BLA) and memory (hippocampal formation) through a brain region called the entorhinal cortex (EC). Specifically, the authors, Sparta and Smithius, inhibit the BLA-EC pathway to look at its role in both the acquisition and the expression of contextual fear.

They start by virally “installing” the optogenetic channels into the BLA. This will allow them to control neurons. The two channels they installed, halorhodopsin and channelrhodopsin-2, were attached to a yellow fluorescent protein (YFP). Halorhodopsin allowed the scientists to shut the neurons down and

Once placement was verified, they ran a fairly standard contextual fear acquisition paradigm. On Day 1, two groups of mice were placed in a room and then shocked. The first group had the BLA-EC pathway inhibited, and the second had the pathway activated. The mice were placed in the room for a short amount of time on Day 2 and then again on Day 3, which was a week later. On Day 2, scientists looked to see if the mice froze when placed in the room without being shocked. Freezing indicated that the mice remembered the context, the room, and associated it with fear. They did the same thing on Day 3.

Mice with inactive BLA-EC pathways on Day 1 did not freeze as much as mice with active BLA-EC pathways. This indicated that the non-freezing mice, with inhibited BLA-EC pathways, weren’t associating the room with fear even though they had been shocked.

I am glad that Sparta and Smithius went a step further. They repeated the experiment but only inhibited the BLA-EC pathway in one of two groups when the mice were RE-EXPOSED to the room on Day 2. Shutting down the BLA-EC at this time would mean that the BLA-EC was removed during the expression of contextual fear and not acquisition of it. To clarify, the BLA-EC was not shut down on Day 1, as it was in the previous experiment.

Both groups of mice froze equally when re-exposed to the room. That means that even mice with inactive BLA-EC froze, which indicates they were experiencing fear to the same degree as mice with the active BLA-EC.

Let’s summarize everything here. If the BLA-EC pathway was blocked when mice were learning to associate the room with shocks (Day 1), then the mice did not express fear when re-exposed to the room (Day 2 and 3). If the BLA-EC pathway was unblocked when the mice were learning to associate the room with shocks, then the mice did express fear when re-exposed to the room. When the BLA-EC pathway was unblocked during learning, but blocked during re-exposure the mice did express fear.

Discussion

Sparta and Smithius demonstrated in their experiment that the hippocampus and the BLA communicate while a subject is learning about or experiencing painful stimuli. This means that fear and visual memory are linked during the storage process. I’m interested to know if the mice with the blocked BLA-EC pathway remembered the room. It is possible that they did not freeze simply because they did not remember the room?

As of right now, I haven’t been able to find any optogenetic studies done on humans. I think the FDA would currently frown on injecting viruses into a person’s head and attaching flashlights that shine into the brain. But this type of technology, in which we voluntarily manipulate very specific neurons, is coming.

PTSD isn’t the only disease involving memory that could benefit from this technology and additional research using optogenentics. Aspects of addiction also involve very powerful learning. Could there be a soldier in whom we could inhibit PTSD? Could there be criminals who could erase memories of their crimes? I have no clue, but certain things are for sure: Our understanding of the brain is changing with new technologies, and our evolved understanding is going to force us to reconsider how we deal with human behavior.

Footnotes

[1] Logerfo can be contacted at aclogerfo02@gmail.com, if you need help with science writing.

[2] The other reason is that one of my 12th graders presented this article in Journal Club, and I was confused by it. Thanks Elizabeth Krauetler for explaining it to me.

[3] By completely made up I mean that it is a completely true story.