The Link Between Odor Cues and Memory

Many of us have experienced the rush of memories that can be brought forth by a simple scent, like a family member’s perfume or the aroma of a favorite food. Unlike other sensations, the odor pathway bypasses the thalamus, goes directly to the olfactory bulb and, subsequently, the amygdala and hippocampus, where much of our emotional processing and long-term memories reside. In an experiment by Dr. Stephanie Grella et al., this phenomenon and the learning associated with it was studied. The researchers theorized that the organization of salient memories could be transformed by odor cues, delaying their de-contextualization in the cortex, and allowing their retrieval to remain dependent on the hippocampus.

To test the hypothesis that fear memories become less dependent on the hippocampus over time, and that odor could shift the organization of such memories back to the hippocampus, Grella et al. used chemogenetics and viral transduction to label sets of engrams during fear memory formation. They used doxycycline (DOX) to induce control of the cells. When present, DOX prevents expression of a fluorescent reporter protein, and when absent, it has the opposite effect. After being placed back on DOX, the mice who’d been fear-conditioned were tested for memory recall either a day or three weeks later. Half the mice were exposed to an almond extract odor cue at the conditioning and recall stage. They were then given CNO, and the researchers measured activity in specific areas of the hippocampus and pre-frontal cortex. Their results demonstrated that odor does, indeed, delay translocation.

Another lab also studied odor-cued salient memories and found that a different part of the brain could be responsible for the strong link between our memories and smells: the piriform cortex, a part of the olfactory bulb projections. In a report by Discovery’s Ashley Hamer, researchers Christina Strauch and Denise Manahan-Vaughan at the Ruhr University Bochum in Germany studied memory formation in rats. They electrically stimulated the animals and measured field excitatory postsynaptic potentials in the anterior piriform cortex. Their results demonstrated that the piriform cortex, in conjunction with activity in the orbitofrontal cortex, allowed for synaptic plasticity—and, therefore, the storage of odor-cued long-term memories. Together, this research by Grella et al. and Strauch & Manahan-Vaughan could prove especially important for future developments in treating emotional processing in post-traumatic stress disorder, where strong contextual cues—like odor—can bring back difficult memories.

References

Christina Strauch, Denise Manahan-Vaughan, In the Piriform Cortex, the Primary Impetus for Information Encoding through Synaptic Plasticity Is Provided by Descending Rather than Ascending Olfactory Inputs, Cerebral Cortex, Volume 28, Issue 2, February 2018, Pages 764–776, https://doi.org/10.1093/cercor/bhx315

Hamer, A. (2019). Here's Why Smells Trigger Such Vivid Memories | Latest Science News and Articles. Discovery. Retrieved December 15, 2021, from https://www.discovery.com/science/Why-Smells-Trigger-Such-Vivid-Memories

Ruhr-Universitaet-Bochum. (2017, December 22). How odors are turned into long-term memories. ScienceDaily. Retrieved December 15, 2021 from www.sciencedaily.com/releases/2017/12/171222092552.htm