Summary: Exposure to unpleasant smells is associated with better memory recall 24 hours later.
Memories are stronger when the original experiences are accompanied by unpleasant odors, a team of researchers has found. The study broadens our understanding of what can drive Pavlovian responses and points to how negative experiences influence our ability to recall past events.
“These results demonstrate that bad smells are capable of producing memory enhancements in both adolescents and adults, pointing to new ways to study how we learn from and remember positive and negative experiences,” explains Catherine Hartley, an assistant professor in New York University’s Department of Psychology and the senior author of the paper, which appears in the journal Learning and Memory.
“Because our findings spanned different age groups, this study suggests that aversive odors might be used in the future to examine emotional learning and memory processes across development,” adds Alexandra Cohen, an NYU postdoctoral fellow and the paper’s lead author.
The impact of negative experiences on memory has long been shown–and is familiar to us. For example, if you are bitten by a dog, you may develop a negative memory of the dog that bit you, and your negative association may also go on to generalize to all dogs. Moreover, because of the trauma surrounding the bite, you are likely to have a better recollection of it than you would other past experiences with dogs.
“The generalization and persistence in memory of learned negative associations are core features of anxiety disorders, which often emerge during adolescence,” notes Hartley.
In order to better understand how learned negative associations influence memory during this stage of development, the researchers designed and administered a Pavlovian learning task to individuals aged 13 to 25. Mild electrical shocks are often used in this type of learning task. In this study, the researchers used bad smells because they can be ethically administered in studying children.
The task included the viewing of a series of images belonging to one of two conceptual categories: objects (e.g., a chair) and scenes (e.g., a snow-capped mountain). As the study’s participants viewed the images, they wore a nasal mask connected to an olfactometer. While participants viewed images from one category, unpleasant smells were sometimes circulated through the device to the mask; while viewing images from the other category, unscented air was used. This allowed the researchers to examine memory for images associated with a bad smell as well as for generalization to related images. In other words, if the image of a chair was associated with a bad smell, would memory be enhanced only for the chair or for objects in general?
What constitutes a “bad” odor is somewhat subjective. In order to determine which odors the participants found unlikable, the researchers had the subjects–prior to the start of the experiment–breathe in a variety of odors and indicate which ones they thought were unpleasant. The odors were blends of chemical compounds provided by a local perfumer and included scents such as rotting fish and manure.
As the subjects viewed the images, the scientists measured perspiration from the palm of the subjects’ hands as an index of arousal–a common research technique used to confirm the creation of a negative association (in this case, of a bad smell). A day later, researchers tested participants’ memory for the images.
Their findings showed that both adolescents and adults showed better memory specifically for images paired with the bad smell 24 hours after they saw these images. They also found that individuals with larger arousal responses at the point when they might experience either a bad smell or clean air while viewing the image, regardless of whether or not a smell was actually delivered, had better memory 24 hours later. This suggests that unpredictability or surprise associated with the outcome leads to better memory.
Funding: This research was supported by grants from the National Science Foundation (CAREER Award Grant No. 1654393 and SBE Postdoctoral Research Fellowship Grant No. 1714321)
The paper’s other authors included Nicholas Matese, Anastasia Filimontseva, and Xinxu Shen of NYU’s Department of Psychology as well as Tracey Shi of Columbia University Irving Medical Center and Ethan Livne of the Weizmann Institute of Science.
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Aversive learning strengthens episodic memory in both adolescents and adults
Adolescence is often filled with positive and negative emotional experiences that may change how individuals remember and respond to stimuli in their environment. In adults, aversive events can both enhance memory for associated stimuli as well as generalize to enhance memory for unreinforced but conceptually related stimuli. The present study tested whether learned aversive associations similarly lead to better memory and generalization across a category of stimuli in adolescents. Participants completed an olfactory Pavlovian category conditioning task in which trial-unique exemplars from one of two categories were partially reinforced with an aversive odor. Participants then returned 24 h later to complete a recognition memory test. We found better corrected recognition memory for the reinforced versus the unreinforced category of stimuli in both adults and adolescents. Further analysis revealed that enhanced recognition memory was driven specifically by better memory for the reinforced exemplars. Autonomic arousal during learning was also related to subsequent memory. These findings build on previous work in adolescent and adult humans and rodents showing comparable acquisition of aversive Pavlovian conditioned responses across age groups and demonstrate that memory for stimuli with an acquired aversive association is enhanced in both adults and adolescents.