Reconsolidation

The "life" of a memory trace

The previous posts discussed the basic processes of creating new memories and the way we can manipulate them in Working Memory and Long -Term Memory in order to create and maintain meaningful networks of useful knowledge.

Here I delve into what is known about the fate of the memory trace after it is created and stored when it is retrieved and then... what?

Learning in the brain

In this post I described and illustrated the learning process using a simplified model of dots and connectors. A similar model is used here (image below) to understand the basic processes that are involved in creating and using the memory traces. The dots represent neurons and the connectors are synapses. Grey stands for inactive state and color for the active state. The shapes are a highly simplified model of memory representations and the pathways between them. We explore these four stages of memory "life":

Four key stages of memory processing

Encoding is the stage where incoming information from the environment is processed in the brain. For example when you hear a new word for the first time. Neurons in the brain are activated to perceive and process the stimulus (represented by the orange dots). The incoming information is not processed by itself but rather in the context of other concepts that we already know (turquoise shapes). We make meaning of the new concept by explaining it in terms of concepts that we are already familiar with, as explained here.

Consolidation is the stage where what we were thinking about while encoding is stabilized. It means that connections are formed among that just active components of the new representation (orange) and the new and the existing knowledge (turquoise). Representations that were active simultaneously and connected meaningfully stand a better chance to undergo consolidation (refer to the idea by Daniel Willingham: "Memory is the residue of the thought"(1) e.g. discussed here). Consolidation is dependent on a biological process that is known to take hours and even days and is further enhanced by sleep (2,3).

After consolidation, the memory traces are considered stabilized and stored: the connections were formed, they are in an inactive state (grey), and may be potentially reactivated in the future upon retrieval.

The process of retrieval starts with an internal or external cue, that triggers an "associative chain reaction" of activating neural representations. Eventually, we may get to the concept that is recognized as the target, or not. In any way, this is an effortful multi-staged process that is crucially dependent on the existing associations (which pathway may lead to the sought-after concept) and their strength (a weak association will probably not activate another). Hence, the value of building these pathways of associations by understanding or "making meaning" and practicing them.

Retrieval accomplished! Now what?

When we retrieve something from memory and use it, new information may be presented: it may be an answer to a question, additional information, or even the mere confirmation that what we have retrieved is correct, or relevant or helpful, or maybe not... In other words, we are encoding new information (orange) in relation to the just retrieved prior knowledge (turquoise). This new complex can be stored for our future use.

The fate of the memory trace

Adding information to the original trace is one way to think about updating memory, but we should ask: what happens to the original trace? If we agree that the representation (turquoise) is a biological entity that depends on the biological process of consolidation after it is first formed, what is its fate after additional reactivation(s)?

A common view sees memory as stable and fixed after it is created: we can retrieve it and use it, add to it, and generally, what is once created is ours to use. A similar view was held by most neuroscientists for several decades, and a great deal of attention was devoted to understanding the biological mechanisms of consolidation.

However, a series of findings shifted this focus: an initial finding from 1968 (4) went largely unnoticed until the interest was renewed in 2000 (5) with a demonstration that an already consolidated fear memory in rats returns to a labile state upon retrieval, and is dependent again on the biological machinery of consolidation. Importantly, when this process is biologically disrupted the memory becomes irretrievable! This experiment and others that followed demonstrated the phenomenon of Reconsolidation: memory traces that were encoded and consolidated became malleable again upon reactivation at retrieval, and were again dependent on biological reconsolidation for long-term stabilization.

The life cycle of the memory trace

These findings shifted the focus from seeing encoding as the only opportunity for consolidation and long-term stabilization to realizing that activation is the critical event(6). When memories are activated they may be re-consolidated. Let's follow the figure on the right (based on 6) to summarize: after encoding (left, back), a new memory trace may be stabilized by the process of consolidation and stored in an inactive form for the long term. When it is reactivated by retrieval, the existing trace is destabilized and may be subject to Re-consolidation. It is noteworthy that anything that was activated, not just the target memory is theoretically prone to modifications. This new way of thinking about memory stabilization and re-stabilization opened up the field to new questions and research directions.

What are the possible implications?

Many experiments were conducted to further understand this process, the underlying biological mechanisms, and the conditions under which re-consolidation takes place. This accumulating information becomes relevant in understanding how consolidation and reconsolidation take place in a different life situation where memories are updated, changed and even erased or attenuated:

1. Attenuate maladaptive memories - what are the ways we can use this phenomenon to decrease the effects of disturbing memories or patterns of behavior in situations like phobias and post-traumatic stress disorders? This is a promising and developing avenue of research with some practical opportunities for non-invasive treatment (7).

2. Memory updating in eyewitness testimonies - how do memories change with every testimony and how are they are affected by interrogation and implicit suggestion (8)? Much more is known today about the potential unreliability of eyewitness testimonies, as a result, hundreds of prisoners that were wrongly convicted by eyewitness testimony were exonerated, sometimes years later, thanks to DNA evidence (9).

3. Updating and improving memory performance - if we can understand the conditions upon which memories are reconsolidated following retrieval, we can use these notions to design more effective learning and specifically practice activities.

In what follows, we focus on the last point and explore what is known about reconsolidation in human subjects, and what kind of insights we can take to educational contexts?

Reconsolidation in humans

The possible implications of the reconsolidation process on everyday memory are substantial, but as most of the research was performed with animal models using invasive methods, it is important to understand the research in humans that used behavioral (non-invasive) methods. I wish to review here a seminal study by Almut Hupbach and her distinguished colleagues (10).

In this study, 24 participants in two groups were presented with 20 real objects sequentially and were instructed to remember them. The procedure was repeated until the participants could remember most of the objects. Two days later, 20 new objects were presented simultaneously (different procedure) to the participants and they were required to study them, the procedure was repeated until they remembered most of them. The difference between the two groups was that participants in one group, called the Reminder group, were asked to recall the procedure from two days before (but not the specific objects) before learning the new items. The other group, the No-reminder group, did not receive any reminders. Two days later the two groups were asked to recall as many objects as they could from the first list. you can see an illustration of the procedure and the results below.

Results

The first thing to note is that there was no difference in the number of items correctly recalled from list A (turquoise squares on Friday). However, the reminder group incorrectly recalled more items from list B (orange squares on Friday), which were labeled "intrusions", as this is not what the participants were asked to remember. This suggests that the subtle reminder immediately prior to learning, reactivated the existing memory traces before learning the new ones, which distorted or updated the original trace, adding newly learned information to it.

Is this Reconsolidation?

Reconsolidation is a biological process that takes time (the critical phase takes several hours, night sleep matters, so 24 hrs can be considered long-term), so in order to claim that this phenomenon is in line with the reconsolidation assumption, it is important to look into the timeline of this phenomena. In another experiment, the authors repeated the procedure, but introduced the test right after learning the second list of objects (on Wednesday, rather than on Friday), in this case, there was no difference in the number of intrusions (very low rate, for both groups, please see the original study for further analysis, results, and discussion).

Conclusions and applications

This experiment demonstrates the reconsolidation effect in human declarative learning, and specifically that it is critically dependent on two components:

Reactivation of the existing trace, immediately prior to learning something new.
Enough time has passed for the trace to be consolidated in its updated form.

These results are an example to the intersections between neuroscience, cognitive science, and education, we can understand the same phenomenon across levels of investigation: from the biological level all the way to the classroom. These findings highlight the importance of activating prior knowledge before teaching new related information. This point may go unnoticed or taken for granted, but we have here yet another explanation for why it is important to assure that each and every student is actively retrieving their prior knowledge before learning something new. Then we need to ensure explicit and meaningful connections between the new and prior knowledge (as explained here). The next point that is highlighted here is that we should allow time for consolidation before attempting to verify that learning indeed took place. This is in line with applying a spaced schedule of revision and practice. In addition, this may point to the significance of rest and sleep for memory consolidation and reconsolidation (3).

Open Questions

This is one example of behavioral research with human subjects that was driven by the biological findings regarding the process of reconsolidation. Since its publication in 2007 additional studies further established the understanding that memories return to a labile state upon retrieval and may be subject to modification or updating (11). At the same time, many questions remain open, for example: does reconsolidation necessarily takes place after every reactivation? what are the conditions for reconsolidation? Is reconsolidation the core mechanism for memory updating? We can also ask what are the implications to education? What are the optimal conditions for associating existing knowledge and new information in a constructive way? What are the implications for practice schedules and types of feedback? On the next page: "The power of prediction", we will look into the first set of questions - what else can research tell us about the conditions for reconsolidation?

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