Neurons that are responsible for new experiences interfere with the signals of neurons that contain memories and thereby disturb the recall of memories – at least in mice.
The research group of Martin Fuhrmann of the German Center for Neurodegenerative Diseases (DZNE) reports this phenomenon in the scientific journal Nature Neuroscience.
The results of this study potentially shed new light on memory impairment in Alzheimer‘s disease.
The hippocampus is a brain region responsible for memories and is early affected during Alzheimer’s disease. Neurons in the hippocampus respond to our experiences and build networks to store these memories.
Thereby, experiences and learned content can be recalled: we can for example remember our way home or to work. Individuals suffering from dementia have problems retrieving this kind of memories – as a specific region in the hippocampus, the so-called CA1 area, responsible for spatial memory, is strongly affected by Alzheimer pathology.
So far it was thought that neurons that “contain” a memory are impaired by the disease in a way that they fail to be reactivated and eventually lose the memory. Apparently, the process of forgetting during Alzheimer’s disease – at least in a mouse model – works in a different way: a research group at the DZNE investigated mice with similar protein deposits in their brains (so-called amyloid-beta plaques) as people with Alzheimer’s disease.
The deposits resulted in symptoms in these mice similar to those seen in Alzheimer’s disease. The researchers found that the neurons responsible for the memory were still active in the diseased mice. However, recall of the memory failed.
Signals of other neurons interfere with the memory
“The reason is novel experience encoding neurons disturbing the signals of memory-containing neurons and superimposing them with their signal,” says Dr. Martin Fuhrmann, group leader at the DZNE.
“It is like a noisy TV signal: the picture becomes diffuse and distorted; you might even see pixels or stripes. Something similar happened inside the mice’s brain: Interfering signals suppressed their memories. This disturbance is obviously a result of the pathological changes in the brain.”
When healthy mice remember a situation, like learning a new path or exploring a novel environment, the neuronal network will be reactivated that was active during encoding the initial experience.
To find out what actually happens to this neuronal network, the researchers performed an experiment: They let healthy, as well as mice with Alzheimer-like pathology explore a novel environment.
With the help of a special microscopy method – two-photon in vivo microscopy – the researchers were able to follow the activity of individual neurons in the hippocampus.
When the mice were exposed to the very same environment a few days later both groups behaved differently: the healthy mice remembered the environment; mice with Alzheimer-like pathology did not.
They explored the environment as if it was their first experience. This was accompanied by differences in brain activity. Dr. Stefanie Poll, postdoc in the lab of Martin Fuhrmann and first author of this study explained:
“In the diseased mice we not only found active neurons encoding the memory, but also a group of active neurons that contained novel environmental information. The signal of these novelty-containing neurons caused a superimposition disturbing signal of the memory encoding neurons.”
To verify this, the researchers employed a technique based on the combination of chemical molecules and genetics: “chemogenetics.”
Thereby, neurons encoding novelty were made responsive to a specific chemical molecule. “Applying this molecule, we were able to modulate the activity of these neurons. It works like a switch, the molecule presses the switch,” says Stefanie Poll.
On and off switching of novelty-containing neurons
“Like that we were able to specifically target neurons encoding novel information and switch these neurons on and off—controlling their activity,” explains Martin Fuhrmann. “In the diseased mice we switched these neurons off, in the healthy group we did the opposite.”
Thereby, it was possible to on the one hand to reduce and on the other hand to induce the disturbing noise artificially.” This was evident in the mice’s behavior: “Mice with Alzheimer-like pathology now recognized the environment again, their memory was restored.
The memory of healthy mice, however, was impaired by the artificial noise,” says Stefanie Poll.
“The results of this study indicate a previously unknown mechanism that may contribute to the memory impairment in Alzheimer’s disease,” explains Martin Fuhrmann. “Imagining future therapies, we might be able to rescue memories of individuals suffering from Alzheimer’s disease or other diseases impacting memory recall.
We might achieve this by lowering the activity of these noise-inducing neurons with future methods. Furthermore, it could be possibly helpful for individuals suffering from post-traumatic stress disorders.
Here, noise-inducing neurons could be artificially activated to interfere with the traumatic memory aiming to overwrite it. The remaining question, whether our results can be translated to humans, has to be answered by future studies.”
MEMORY
Our memories define our character and have a completely unique perspective than everyone else’s experiences. Creating a memory involves three stages. The first, encoding, occurs when a stimulus results in the formation of a new memory (11–13).
This new formation is often referred to as an engram, which is thought to be a physical memory trace in the brain (14). This trace is very susceptible to decay until the next stage occurs, consolidation (14).
Consolidation is the process in which a memory becomes stable and is assimilated into previously acquired knowledge (14). The final stage, retrieval, occurs when the memory is recollected (14).
Sometimes when we create a memory, it only lasts for a short period of time, when other memories last a lifetime (15, 16). Memories are temporally defined as long term or short term depending on the length of time the memory lasts. Short-term memories last less than 2 min, whereas long term memories can last 2 min to a lifetime (15, 16).
Short-term memories usually are comprised of working memory (15, 16). Working memory typically involves small amounts of information that we only need to retain for a couple seconds, for example, the prices two items when comparing costs while shopping.
Types of memory
There are multiple types of memory and subdivisions within each type. The main two types are explicit (declarative) and implicit (nondeclarative) (15). When a memory is explicit, the individual is consciously aware of the memory, whereas with implicit memory, the individual is not (15).
There are two subsets within explicit memory: episodic and semantic. Episodic, or autobiographical memories, include a what, where, and when aspect of the memory (16–18). Semantic memories are facts about the world around us (15).
An example of semantic memory may be knowing that baseball is a sport, whereas an episodic memory may be remembering the first time you went to a baseball game. Implicit memories also have two subdivisions: procedural and priming (15).
Procedural memories include motor skills and other actions we complete automatically without conscious thought (e.g., walking and writing) (15). Priming occurs when an individual is exposed to a stimulus that influences their response to a later stimulus (15). An example of priming may be seeing a flash of an image on a computer while taking a computer-based memory task.
Forgetting
Why are some memories retained yet others are lost?
There are many reasons that we forget information we have learned or events we have experienced (19–21). The act of forgetting can occur actively or passively.
Passive forgetting occurs through natural decay, or biological degradation, of neurons within a memory engram (19). Partial decay of an engram can make it challenging to activate the memory during retrieval (19).
Active forgetting can occur through several mechanisms: interference, motivated forgetting, or retrieval-induced forgetting (19). Interference, which will be detailed later in this chapter, occurs when competing information makes it difficult to retrieve the correct memory (19).
Motivated forgetting often occurs when an individual actively suppresses a memory due to some unpleasant quality (e.g., guilt, shame, and embarrassment) (19). Finally, retrieval-induced forgetting occurs when only parts of a memory are normally recalled, causing the other parts to degrade over time (19).
Memory Interference
As stated in the previous section, memory interference (MI) is a cause of forgetting. There are two types of MI, proactive and retroactive. Proactive interference (PI) occurs when previously acquired information interrupts the recall of newly learned information (old → new).
For example, calling your new boyfriend by your old boyfriend’s name. Retroactive interference (RI) occurs in the opposite direction, newly acquired information interrupts the recall of old information (old ← new).
Following the previous example, this would be calling your old boyfriend by your new boyfriend’s name. Interference can be benign to serious memory disruption depending on the situation. Interference is also linked to similarity of the content. If the competing material is similar, interference is more likely to occur.
In experiments, MI is measured using paired associate learning tasks. These tasks are typically comprised of lists of word pairs or figure pairs (e.g., “bread knife” or ♦♣) that a participant is asked to memorize as a pair.
Research participants memorize lists of the word or figure pairs and subsequently recall them (e.g., bread – _ or ♦ – _). There are multiple models of paired associate tasks, but ones commonly used include AB–CD, AB–AC, AB–ABr, and AB–DE AC–FG.
For the models, the letter pairs (e.g., AB and CD) signify one list each (e.g., AB = List 1, CD = List 2), each letter in the name (e.g., AB–CD) stands for one word (A = bread B = knife), and the combined letters (e.g., AB) represent one word-pair (breadknife). Examples of measuring MI using various models are summarized in tables 1–4.
Table 1AB–CD example
| List 1 (AB) | List 2 (CD) |
|---|---|
| BABY HUNTER | SPIDER CANDLE |
| SUPPER SHERIFF | ARROW THEATER |
| WEDDING MOVIE | CHERRY MONEY |
| APPLE DIAMOND | TIGER HOTEL |
| MONKEY GARDEN | CANNON HAMMER |
| FOREST BATTLE | LADY BUTTER |
Table 4AB–DE AC–FG example
| List 1 (AB, DE) | Cued recall 1 A__, D__ | List 2 (AC, FG) | Cued recall 2 A__, F__ | MMFR A__ __ D__ __ F__ __ |
|---|---|---|---|---|
| BABY HUNTER | SPIDER _______ | FOREST CITY | ARROW ______ | BABY ___ ___ |
| SUPPER SHERIFF | FOREST _______ | ARROW THEATER | FOREST ______ | CHERRY ___ ___ |
| WEDDING MOVIE | BABY ________ | BABY SALAD | TIGER ______ | ARROW ___ ___ |
| SPIDER CANDLE | CHERRY ______ | TIGER HOTEL | SUPPER ______ | SUPPER ___ ___ |
| MONKEY GARDEN | MONKEY ______ | MONKEY ENGINE | LADY _______ | TIGER ___ ___ |
| FOREST BATTLE | SUPPER _______ | LADY BUTTER | CANNON ____ | WEDDING ___ ___ |
| CHERRY MONEY | APPLE ________ | CANNON HAMMER | BABY _______ | MONKEY ___ ___ |
| APPLE DIAMOND | WEDDING ____ | SUPPER JACKET | MONKEY _____ | LADY ___ ___ |
| SPIDER ___ ___ | ||||
| FOREST ___ ___ | ||||
| APPLE ___ ___ | ||||
| CANNON ___ ___ |
Table 2AB–AC example
| List 1 (AB) | List 2 (AC) |
|---|---|
| BABY HUNTER | MOVIE WEDDING |
| SUPPER SHERIFF | APPLE CANNON |
| MONKEY GARDEN | BABY SALAD |
| FOREST BATTLE | MONKEY ENGINE |
| MOVIE CHERRY | FOREST CITY |
| APPLE TIGER | SUPPER JACKET |
Table 3AB–ABr example
| List 1 (AB) | List 2 (ABr) |
|---|---|
| BABY HUNTER | SHERIFF CHERRY |
| SUPPER SHERIFF | DIAMOND BABY |
| WEDDING MOVIE | BATTLE SUPPER |
| SPIDER CANDLE | FOREST HUNTER |
| MONKEY GARDEN | MONEY MOVIE |
| FOREST BATTLE | SPIDER GARDEN |
| CHERRY MONEY | APPLE WEDDING |
| APPLE DIAMOND | CANDLE MONKEY |
Alzheimer’s disease and MI
Alzheimer’s disease causes the decay of neurons which eventually leads to memory impairment (1, 4, 5, 9). Although there are a lot of research fields focusing on how Alzheimer’s disease damages memory, there is less research focusing directly on MI effects on patients with Alzheimer’s disease.
When investigating patients with Alzheimer’s disease and those with mild cognitive impairments without Alzheimer’s disease, Dewer et al. found that memory retention is much higher in these patients when there is minimal interference compared to a normal MI paradigm (22).
Their findings align with previous literature demonstrating that memory dysfunction in patients with Alzheimer’s disease is associated with an increased susceptibility to MI (22). The authors hypothesize that this may be due to a decline in the ability to consolidate new memories (22).
The interference paradigm utilized in this experiment is strong at predicting which patients with mild cognitive impairments will or will not progress to Alzheimer’s disease within 2 years, with 80% sensitivity and 100% specificity (22).
In term of semantic memory, or facts about the world (e.g., baseball is a sport), patients with Alzheimer’s disease perform worse on these memory tasks, which may be due to a deficit in working memory and attention (23).
As stated previously, working memory is short-term memory that lasts for a very short period of time with a concurrent interfering stimulus. Hartman describes how, in her experiment, there was no evidence that the patients with Alzheimer’s disease utilized semantic knowledge (relatedness) of word pairs during recall (23).
When patients’ working memory is impaired, as is typical in Alzheimer’s disease, it is more difficult to retain relevant information about the relationships of words in paired associate tasks (23).
Despite this experiment’s focus on working memory, its results shed light on MI impairments since semantically relating words is a typical strategy utilized when memorizing word pairs in paired associate tasks. When detailing the symptoms of Alzheimer’s disease, we noted that difficulty remembering and following instructions is common (23).
This may also influence performance on memory tasks. Repeatedly needing external cues or verbal instructions may alter outcome scores, as mentioned elsewhere (23). Another study that focused specifically on proactive and RI compared mildly demented Alzheimer’s disease patients, patients with mild cognitive impairment without Alzheimer’s disease, and healthy elderly patients on interference tasks (24).
When controlling for overall memory impairment, mild Alzheimer’s disease patients demonstrated higher rates of PI, but equal amounts of RI when compared to the cognitively impaired patients (24). As expected, the healthy elderly participants experienced the least amount of interference (24). Vulnerability to semantic interference may reflect early signs of the onset of Alzheimer’s disease (24).
EXERCISE AND ALZHEIMER’S DISEASE
As stated previously, physical inactivity is a risk factor for developing Alzheimer’s disease and is even considered the highest population attributable risk (25). In one systematic review, the majority of experiments demonstrated that physical activity was inversely associated with risk of developing Alzheimer’s disease (26).
Exercise has also been demonstrated to improve multiple types of memory, including long-term and short-term memory (27–33). Exercise may even prevent the onset of Alzheimer’s disease by decreasing the risk of cardiovascular disease, increasing cerebral blood flow, increasing hippocampal volume, and improving neurogenesis (34).
Higher levels of physical activity are associated with a reduced risk of developing the disease (34), with long-term prospective studies demonstrating that walking regularly is associated with a twofold reduced risk in cognitive impairment (25).
During exercise, many chemicals are released in the body, including brain-derived neurotrophic factor, which is directly associated with learning and memory (35).
Research also demonstrates that regular physical activity prevents mental decline and improves thinking in populations with vascular cognitive impairment (34). It has been used clinically in the treatment of preclinical and late-stage Alzheimer’s disease, as well as a prevention strategy (34).
Recent prospective work, that compared sedentary individuals to those who were the most active, demonstrated a 38% reduction in incidence of Alzheimer’s disease (36). In animal studies, mice with Alzheimer’s disease that completed 16 weeks of treadmill exercise have been shown to elicit changes in therapeutic parameters at the cellular and molecular level, providing biological plausibility to exercise as a therapy (37).
In order to reduce the risk of developing Alzheimer’s disease, to lessen the effects for those already suffering from memory loss, individuals should participate in regular physical activity. The American national guidelines suggest at least 150 min per week of moderate to vigorous physical activity (38).
Meta-analyses have demonstrated mixed findings on which mode of exercise is best for improving specific types of memory; however, walking, cycling, and jogging are three of the most common exercises implemented, all of which have demonstrated beneficial effects (33, 39).
Multicomponent exercise programs have also been effective in improving cognitive function in institutionalized older adults with mild to moderate Alzheimer’s disease (40). One particular study found that incorporating a program that included supervised aerobic, muscular resistance, flexibility, and postural exercises for 45–55 min sessions twice per week for 6 months significantly improved patients’ cognitive function when compared to a control group (40). These findings suggest that incorporating a variety of physical activities may be an effective non-pharmacological meth
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More information: Stefanie Poll et al, Memory trace interference impairs recall in a mouse model of Alzheimer’s disease, Nature Neuroscience (2020). DOI: 10.1038/s41593-020-0652-4
