Memory: How It Works. Can It Be Edited?

Memory: How It Works. Can It Be Edited?

By David Glass MD 

As life events happen, significant or otherwise, we store some of them in our memory, to enjoy again years later (long-term memory), while others are gone in a day or two (short-term memory), and the rest are forgotten as quickly as they pass, like what you had for lunch yesterday, or what color shirt you wore the day before.

Long-term memory is said to be divided into two categories: Declarative Memory, which requires a conscious effort to recall; and Implicit Memory, which relates to skills and routines. Examples might be bringing to mind a first date or other special occasion, for the former; touch-typing or brushing one’s teeth, for the latter.

Think of the brain as the hard drive on a computer. Like the bits, bytes, and gigs of that electronic device, the brain has a network of billions of neurons that work together to create thoughts, enable speech, activate movement and—as it relates to this discussion—to store memories.

There is a combination of electrical and chemical processes that come together to make it possible for the human brain to store and recall memories. Understanding the mechanics of this complex system is pivotal to learning how to manipulate, erase or preserve memories.

Neurons, or Brain Cells

The core component of the nervous system in general, and the brain in particular, is the neuron, or nerve cell—the “brain cells” as we call them. Neurons are cells that carry messages between the brain and other parts of the body, and that are the basic units of the nervous system. A typical neuron consists of a cell body (soma), dendrites, and an axon. The cell body of a neuron frequently gives rise to multiple dendrites, but never to more than one axon, although the axon may branch hundreds of times before it terminates. At the majority of synapses, signals are sent from the axon of one neuron to a dendrite of another.

What Does What

Memories are stored primarily in the cerebral cortex, which is the outer layer of neurons in the brain. In the cortex’s intricate system, a synapse is a structure that permits a neuron (or brain cell) to pass an electrical or chemical signal to another neuron, via dendrites, which are short, branched extensions of nerve cells.

Small, membranous protrusions from these dendrites—dendritic spines—typically receive input from a single axon (a nerve fiber that is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body, or soma) at the synapse. It is the dendritic spine that is thought to serve as a storage site for memories.

Emotion and Memory Go Hand-In-Hand

Think about some of your most vivid memories. Most of them are about events with emotional impact. Right? The day you graduated from college and how happy you were … your joy at your wedding … the thrill of holding your new-born child … or the sorrow at the death of a loved one. You remember any or all of those like they were yesterday.

So, you probably won’t be surprised to learn that the portion of the emotion system of the brain (the “limbic system”) is in charge of converting the information and those feelings into memory. Key to this function is the temporal lobe, which is located beneath the front-to-back divider between the two cerebral halves of the brain. Years of experiments and surgical experience have determined that the main location for this transfer is a portion of the temporal lobe called the hippocampus. 

There is one hippocampus on each side of the brain. Long-term memories for facts and events are thought to be stored in one or the other, then some eventually are distributed via that complex network throughout the brain—much like the analogy to a computer hard drive, on which files are often fragmented and stashed in whatever “blocks” of space may be available to be written on, or over-written.

Two walnut-shaped clusters of gray matter that deal with emotion are called “amygdalae.” There is one in each cerebral hemisphere. The amygdalae are the reason we feel fear of things outside our control. The specific part of the central amygdalae that encodes fear memory has been identified as the lateral subdivision. The amygdalae also control the way we react to certain stimuli, or an event that causes an emotion, that we see as potentially threatening or dangerous—an event we are sure to tuck away as a memory.

Functioning as a hub in the widespread network for memory is the entorhinal cortex (EC), which is located in the medial temporal lobe, and is the main interface between the hippocampus and the neocortex. This interface plays an important role in Declarative Memories, and in particular spatial memories and sensory perception, including memory formation, memory consolidation, and memory optimization in sleep.

Keep Some, Discard Others?

Naturally, we want to hold onto all the fond memories we can. But what about the bad ones? Do we want those to hang around? Does the war veteran want to relive an especially violent battle, over and over? Does the drug addict really want to remember the euphoria of being high, which triggers the need and desire to use again? Can those memories be suppressed, so the trauma can be reduced or forgotten, and so bad habits can be eliminated? How?

“Researchers have used a three-stage model to describe how the brain learns and remembers, with impairment in any of these processes resulting in memory failure: acquisition, consolidation, and retrieval,” says Dr. David Vauzour, a research fellow at the University of East Anglia, in the United Kingdom.

One study that focused on the consolidation step determined that memories are periodically reassembled in the brain—much like defragmenting that computer hard drive. Electroconvulsive Therapy (ECT) appeared to prevent this defragging, or alters memories during the process. Study participants who were being treated for depression viewed a disturbing story and accompanying pictures. After being reminded of the story, they received ECT, and their recall of the troubling story was completely wiped out.

Using Pharmaceuticals

In a California research program, methamphetamine addicts were treated with inhibitors—“chemical cuts”—to target drug-related memories, because the brain is then using a different mechanism to store them, effectively rendering those memories “unavailable.”

“What makes this finding so exciting is that the inhibitors seem to be selective as to the memory type,” says Dr. Courtney Miller of the Scripps Research Institute in San Diego, where this program took place. Dr. Miller went on to say that she hopes to use a similar methodology to target traumatic memories in the future.

Another effort, at the Massachusetts Institute of Technology (MIT), used drugs known as histone deacetylase inhibitors (HDACIs) to increase the brain’s ability to replace old or traumatic memories with new ones … to over-write files on the hard drive.

In the first phase of this research, mice heard a tone, then received an electrical shock. Once the mice learned to connect the two events, they began to show fear the instant they heard the tone—even when it was not followed by the shock. Then, researchers kept sounding the tone, without the shock, to see if the mice could disconnect the two events. Mice who had experienced the one-two of tone-shock for a short period—just a day or so—were able to hear the tone without fear. The mice that had experienced the tone-shock sequence for a month or longer were not unable to dissociate the two. After these specimens were given HDACIs, they stopped freezing in fear when they heard the tone. Their memories of the bad experiences had been replaced with memories of pleasant ones.

This same study went on to discover a gene necessary for rendering memories extinct: TET1. By boosting TET1 activity, post-traumatic stress disorder (PTSD) sufferers could find it easier to replace fearful memories with happy or positive ones. Fear of a cage was instilled in two sets of mice, using electrocution. Then the mice were put in the cage without being electrocuted. The mice treated with an inhibited TET1 gene ceased to be afraid of the cage, because the fear memory was replaced with the new memory of a cage that was not electrified.

Movies and science fiction would have you believe that all recollection of a broken romance could be erased. Or that you could travel the galaxy through implanted false memories. Not yet, but some say it’s coming. “Memory is a very important aspect of cognition. It refers to what you can remember, along with the capacity for remembering. Some memories are retained for a short period of time and then discarded, but the most important ones are stored in the brain and can be retrieved at will,” says Dr. Vauzour.

We are not at a point where we can remove specific, individual memories yet, but these studies and their results indicate that there are ways to “massage” what you remember, so those recollections aren’t painful, traumatic, or harmful. 

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