Sniff test

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Scent can be very powerful, often evoking memories and positive associations more clearly than by looking at a photograph or listening to a song. When we think of our sense of smell and its link to memory, however, we tend to focus on pleasant scents – the aroma of a favourite dish or a loved one’s signature perfume.

While unpleasant smells and negative experiences warn us of dangers and hazards, could they also play an important role in how we remember things? According to a new paper, memories are actually strengthened if a bad smell was part of the original experience.

The study, published in the journal Learning and Memory, and with authors from New York University, Columbia University Irving Medical Center and the Weizmann Institute of Science, highlights the link between aversive odours and episodic memory. Aiming to build on previous research that has shown how adults’ memories are boosted by emotional associations, the researchers decided to focus on adolescents, and examine whether emotional learning influences memory in the same way for people aged 13 to 25.

Their primary aim was to test whether ‘acquired aversive associations’, using bad smell as a reinforcer, improved adolescents’ memories, and whether such associations generalise across a category. For example, if someone associated a bad smell with one object, would this translate to improved memory for objects in general?

Emotional learning

Why use bad smells? Some previous studies examining the link between learning and experience have used electrical shocks, but it would not have been ethical to take this approach in an experiment involving children. “We used aversive odours in this study, as they can be ethically administered in developmental populations,” says Alexandra Cohen, postdoctoral fellow at New York University and the paper’s first author. “We aimed to show that odours might be useful in studying emotional learning and memory across development.”

As what constitutes a ‘bad’ or unpleasant odour can be subjective, the researchers asked each participant to complete an ‘odour selection’ procedure before the experiment. Participants were asked to rate eight aversive smells, supplied using air-puff canisters. Each smell was rated three times, with the average scores for each calculated to determine the four smells deemed most aversive.

The four ‘worst’ smells were then rated again, using a nasal mask, which meant they were experienced as they would be during the experiment. The process allowed the researchers to “identify an odour that the person rated as very bad to use during learning”, says Cohen.

In a Pavlovian-category conditioning task developed by the researchers, participants then viewed images presented on a screen while breathing through a nasal mask that was connected to an olfactometer – sometimes breathing in bad smells and sometimes unscented air. The researchers chose neutral images of objects (such as a chair) and scenes, and participants were asked if they noticed any associations between them and smells they experienced.

Stronger memories

A day later, the second part of the experiment tested participants’ ability to remember the pictures, by showing all the images from the day before, plus some new ones. It was conducted a day later because of evidence from previous studies, which suggests that emotional memory enhancement effects emerge over time.

The researchers used skin-conductance response during the category conditioning task to serve as a measure of learning. After the recognition memory test, participants were asked to complete self-reported measures of anxiety and intolerance of uncertainty via Qualtrics surveys.

For adults and adolescents, memories were stronger a day later for the images that had been paired with bad smells, compared with images they had viewed while breathing unscented air.

In addition, people’s anticipation of a bad smell was enough to improve their memories of an image, whether or not they experienced the smell. “We were able to examine arousal during learning by measuring sweat from the participant’s palms,” says Cohen. “We found that larger arousal responses at the time when an individual might experience a bad small or clean air while viewing an image were related to better memory for that image 24 hours later, regardless of whether the bad smell was actually delivered. This suggests that unpredictability or surprise associated with an outcome can also lead to better memory across age.”

The study findings support a large body of evidence highlighting how emotional associations change the way memories are stored, leading to stronger memories that are forgotten more slowly. Further research is needed, however, to understand more about other factors that influence memory strength, explains Cohen.

“Remembering experiences associated with unpleasant outcomes can help us learn to avoid experiencing something unpleasant in the future. While this is often useful, strong memories of negative experiences can sometimes interfere with one’s daily life, as is often the case in anxiety disorders.

“More research is needed for us to understand the many factors that influence the strength of our memories, so that we can uncover specific ways in which to use emotion-related memory enhancements to our advantage. We can then begin to uncover ways of preventing emotion-related memory enhancements from having detrimental effects.”

Aversive learning strengthens episodic memory in both adolescents and adults, Alexandra Cohen, Nicholas Matese, Anastasia Filimontseva, et al, Learning and Memory, 2019