The Neuroscience of Recalling Old Memories

The hippocampus encodes multiple aspects of a life event into a singular memory.

Posted Jul 03, 2015

Goa Novi/Shutterstock
Source: Goa Novi/Shutterstock

Neuroscientists have discovered that when someone recalls an old memory, a representation of the entire event is instantaneously reactivated in the brain that often includes the people, location, smells, music, and other trivia. Recalling old memories can have a cinematic quality. Memories often seem to play out in the mind's eye like an old Super 8 home movie or vintage Technicolor film, this new research explains why.

In a new study from University College London (UCL), neuroscientists discovered that when someone tries to remember a singular aspect of an event from his or her past—such as a recent birthday party—that a complete representation of the entire scene is reactivated in the brain like pieces of a jigsaw puzzle coming together to create a vivid recollection.

The July 2015 study, “Evidence for Holistic Episodic Recollection via Hippocampal Pattern Completion,” was published in Nature Communications. This research is the first to provide evidence for a pattern completion process in the human hippocampus, as it relates to the everyday experience of recalling previous life events and old memories.

In the new study, researchers were able to show how the hippocampus binds together the diverse elements from an event to form a singular and holistic memory. During memory recall, the brain recalls an old memory by piecing together various compenents via a pattern that forms a cohesive remembrance of things past. 

How are Memories Encoded for Retrieval? 

The new research reveals that humans remember life events using individual threads, that are coupled together into a tapestry of associations. During the neuronal encoding process, various element components activate distinct neocortical regions. 

When retrieving an old memory, neocortical activity occurs in areas linked to all the separate elements that create the memory. The degree to which someone can vividly remember a past memory correlates directly with the level of hippocampal activity. 

Courtesy of Dr. Aidan Horner
Hippocampus activity, circled in red, seen when forming event memories in fMRI. 
Source: Courtesy of Dr. Aidan Horner

In a press release, lead author Dr. Aidan Horner from UCL Institute of Cognitive Neuroscience explains, "When we recall a previous life event, we have the ability to re-immerse ourselves in the experience. We remember the room we were in, the music that was playing, the person we were talking to and what they were saying. When we first experience the event, all these distinct aspects are represented in different regions of the brain, yet we are still able to remember them all later on. It is the hippocampus that is critical to this process, associating all these different aspects so that the entire event can be retrieved."

The researchers showed that associations formed between the different aspects of an event allow one aspect to bring back a wave of memory that includes the other aspects. This process is known as "pattern completion."

Using fMRI, the researchers identified how various aspects of recalling an old memory are reflected in activity in different regions of the brain that hold components of the memory. When asked about one aspect of a previous event, activity in the hippocampus triggers the activation of each of these brain regions, this reactivation corresponds to an old memory coming to mind. Senior author of the study, Neil Burgess, explained this research saying, 

This work supports a long-standing computational model of how memory might work, in which the hippocampus enables different types of information to be bound together so that they can be imagined as a coherent event when we want to remember what happened. It provides a fundamental insight into our ability to recollect what has happened, and may help to understand how this process can go wrong in conditions such as Alzheimer's disease or post-traumatic stress disorder (PTSD).

Did You Happen to See Barack Obama in the Kitchen With a Hammer? 

The experiment involved 26 volunteers, who were asked to imagine and memorize a series of 'events' involving different locations, famous people, and random objects. Test subjects were asked to remember the details of the event based on a single cue. For example, one trial 'event' involved a scenario of President Barack Obama in a kitchen with a hammer.

Volunteers were then asked to remember details based on a single cue, such as "where was Obama?", "who was in the kitchen?" or "what object did Obama have?" While being asked to recall different aspects of events, volunteers underwent fMRI scans to measure their brain activity.

The results showed that different parts of the brain showed increased activity when encoding individual aspects of each event, and that the hippocampus later provides the critical links between them to form a complete memory that can be recalled. 

Using the Obama example, activity increased in one part of the brain when volunteers thought of Obama, another when they thought of the kitchen, and yet another when they thought of the hammer. The study showed that when asked "where was Obama?" activity also increased in the regions corresponding to Obama and Kitchen.

Courtesy University of Leicester
Seeing Clint Eastwood and the Leaning Tower of Pisa together instantly encodes a new memory that can later be recalled as a whole of its parts.
Source: Courtesy University of Leicester

Interestingly, this study mirrors the findings released yesterday by researchers at University of Leicester and UCLA who reported that new memories were formed by individual neurons in the hippocampus when a celebrity was photoshopped into an image with an iconic landmark. The photo of Clint Eastwood in front of the Leaning Tower of Pisa illustrates this phenomenon.  

If you'd like to read more about that study, check out my Psychology Today blog post, "The Neuroscience of Forming New Memories." 

Coincidentally, the UCL team also use the example of a celebrity and a famous location by referencing the association of Marilyn Monroe with New York City as an example of how two elements are married into a singular memory. 

The recollection of complex memories of life events is thought to be the hallmark of episodic memory. Due to the enriched aspects of memory encoding, having a flashback to a previous life event can feel like you are re-living the experience. This type of reminiscence can be nostalgic in a comforting way or harrowing if the old memory is linked to PTSD. 

Conclusion: The Hippocampus Connects the Dots to Recall Old Memories

Our brain is able to recall old memories by piecing together all of the various elements to create a vivid memory of the past. The hippocampus connects various neocortical regions, and brings them together into a holistic and cohesive ‘event engram’ or neural network that represents a specific life event of memory from your past. 

If you'd like to read more on this topic, check out my Psychology Today blog posts:

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