Role of Odors in Memory Consolidation

 Learning is the process of acquiring new information, whereas memory is the trace that’s left behind from learning. Memory can last from milliseconds to years. Three different types of memories are sensory memory (last ms to seconds), short-term/working memory (seconds to minutes), and long-term memory (minutes to years). Studies from different patients especially patient HM (Henry Molaison) have revealed that memory is heavily dependent on the brain structure called the hippocampus that is buried in the temporal lobe on each side of the brain. 

In the article “Odor modulates the temporal dynamics of fear consolidation”, Dr. Stephanie Grella in her paper talked about how with the passage of time memories are moved from the hippocampus and rely solely on the prefrontal cortex. This change in the organization is known as the “System consolidation”.

        

She also mentioned that the memories lose their contextual details in the prefrontal cortex. They performed an experiment where their goal was to see what happens to the organization of the memory when animals would be fear-conditioned in the presence of the odor. Dr. Grella et al. hypothesized that fear-conditioning in the presence of an odor would shift the organization of that memory, meaning that over time, the memory would be less reliant on the prefrontal cortex and more reliant on the hippocampus. The experiment was performed on the wild-type c57BL/6 mice. They found that more recent memories were more dCA1 dependent and remote memories were more prelimbic cortex (PL) dependent. They also were able to see that odors change the temporal dynamics so that memories are more reliant on the hippocampus than the PL. These results showed that odor has the ability to change the organization of fear memories. Odor has been used as a sensory cue in both rodents and humans. The hippocampus part, known as the dentate gyrus, plays a critical role in olfactory learning and memory.

Dr. Grella and colleagues also used animal fear conditioning techniques to study PTSD. This research aimed to make the negative fear memoryless negative by adding positive elements. The older memories can be brought back to the hippocampus using the odor cueing technique, where the memory can be recontextualized. They found out that the hippocampal cells active during the encoding of the positive experience can interfere with fear memory expression. They also found that original fear memory can be altered by adding some positive contextual elements that could help with PTSD. 

On the other hand, in the article "Odor cues during slow-wave sleep prompt declarative memory consolidation," Rasch et al. looked at whether the odor cues that were presented during the learning phase would enhance the declarative memory when the same cues are presented during sleep? They experimented on the human population. Researchers knew that the smells have high potency for various types of memory when used as retrieval cues. The fMRI data concluded that the hippocampus was activated during the SWS by exposure to the odor cues. They observed the enhanced activity in the anterior and posterior parts of the left hippocampus during the SWS. The left hippocampus was also activated during wakefulness but to a lesser extent compared to SWS. They also concluded that the covert reactivation of new hippocampal representations is critical for consolidating hippocampal memories during slow-wave sleep (SWS). The main finding was that odor cues enhance declarative memory retention (hippocampus-dependent) when presented during slow-wave sleep.   

Both studies show that the odor is an important cue that can help memory consolidation. Researchers manipulated the brain regions by using odor cues to alter the memory in both studies. Dr. Grella performed the experiment on the mice population, which made us think about whether we would observe the same results in humans. On the other hand, Rasch and the team were able to use the human participants that made it clear that odor cueing can improve the retention of hippocampus-dependent declarative memories. Rasch's research was important because the results show that hippocampal networks are sensitive to olfactory stimuli during sleep, reactivating hippocampus-dependent memories.  Dr. Grella's research on fear memory storage reorganization with the help of odors has opened doors for many in how we can help individuals with PTSD. Bringing the older memories from the prefrontal cortex back to the hippocampus and then contextualizing that fear memory with some positive elements can help reduce the negative effect of the fear memory. 

 

Reference:

Grella, Stephanie L., et al. “Odor Modulates the Temporal Dynamics of Fear Memory Consolidation.” Learning & Memory, 2020, pp. 150–163., doi:10.1101/2019.12.19.881615.

Rasch Björn, et al. “Odor Cues during Slow-Wave Sleep Prompt Declarative Memory Consolidation.” Science, vol. 315, no. 5817, 2007, pp. 1426–1429., doi:10.1126/science.1138581.