Human beings are information seekers. We are constantly taking in details—big and small—from our environment. But the majority of the stuff we encounter in a given day we rarely need to remember. For instance, what are the chances that you need to remember where you ate lunch with a friend last Wednesday?
But what if later on you learned that there was something important to remember about that lunch? The brain has a remarkable ability to store information that seems inconsequential at the time.
So, if you learn that your friend got sick from what they ordered at lunch last week, then details from the meal become relevant: which restaurant was it and what did your friend order? Did you get the same thing? Now those not-so-important details from lunch aren’t so trivial.
Given new and relevant information, human beings have an amazing capacity to strengthen weak memories. This points to the adaptive nature of human memory.
Over the past several years, we’ve been interested in understanding how the brain stores memories for emotionally neutral events that gain significance through subsequent experience. How does the brain store all of this information? And how does emotion strengthen mundane memories?
The study of emotional enhancement of memory largely focuses on how we remember emotionally arousing stimuli or events, like evocative imagery or traumatic events, like 9/11, which is the subject of a long-term study on what affects memory retention.
We take for granted that we remember highly emotional events (like 9/11) better than we remember neutral events, (like that lunch date).
Emotion increases our ability to remember by affecting activity in brain regions involved in emotional processing, particularly the amygdala and striatum, and also the regions involved in encoding new experiences, like the hippocampus. Emotion also increases the strength of our memory over time, a process called consolidation.
Strong emotion can increase memory for positive events, like a surprise birthday party thrown by your closest friends, and for negative events, like making an embarrassing faux pas in front of your boss at the office holiday party.
Of course many details are not intrinsically emotionally arousing. But they can gain emotional significance through our experiences.
For example, the memory of a surprise birthday party includes details like what you were wearing and who was there. On the face of it, these details are not emotionally significant, but you remember them because of the context in which they were experienced.
In some of our earlier studies, we found that people selectively remember neutral pictures if the pictures had been associated with an electrical shock the previous day, even when the volunteers were unaware that we would later test their memory.
We have also shown that people remember neutral pictures if they are warned that if they forget them, they will receive a shock the next day. Likewise, being rewarded with money for remembering certain pictures the next day can boost memory for those pictures as well.
These experiments focus on emotional factors at the time the original memory is created and the findings show how seemingly trivial information associated with a meaningful event can be selectively preserved in memory.
But what happens when the emotional event happens after the original memories were formed? In a recent study, we found that an emotional experience can enhance memory for neutral information encountered previously.
Volunteers viewed a series of trivial pictures from two categories, either animals or tools. After a delay, volunteers were presented with a new set of animal and tool pictures—only this time, when the volunteer saw the pictures they received an electrical shock to the wrist.
We already knew that memory would be strengthened for the pictures paired with the electrical shock. But here we found that if we paired shocks with pictures of animals, memory was strengthened for pictures of animals volunteers saw before any shocks were delivered. If we shocked volunteers when they where shown pictures of tools, memory for the earlier pictures of tools was strengthened.
Like remembering details from lunch last Wednesday after you discovered your friend got sick, the negative experience selectively increased memory for related information that was completely trivial when it was originally experienced.
We use our memory not only to remember the past, but to guide our decisions in the future. Emotion helps us remember relevant information to determine our choices. But without the ability to strengthen seemingly trivial past experiences with new important information, we might end up missing out on future rewards or repeating the same mistakes.