Scent back in Time

Scent back in Time

The ability to perceive the world with all five senses is vital to our quest for survival. The loss of any one of our senses can severely impact one's ability to function. To comprehend the abilities of our sensory system further, various scientific studies have explored the impacts of auditory, gustatory, visual, olfactory, and tactile stimuli on human behavior and physiology. 

In recent years, the neuroscientific community has led extensive research into how sensory stimuli and experiences interact with the human body and behavior. In particular, there has been extensive research on how sensory experiences, such as olfaction, can relate to our present-day and memories of the past (Toffolo et al 2012; Zucco et al 2012; Daniels and Vermetten 2016; Glachet and El Haj 2019; Grella et al. 2020). It has been well established that episodic memories and personal experiences rely upon the hippocampus (HPC) (Scoville and Milner 1957). Once an experience has been encoded, it gains permanence through the process of consolidation (Frankland and Bontempi 2005; Grella et al. 2020). 

Certain sensory stimuli, such as odors, can evoke highly distant yet contextual memories of the past. For instance, when you smell freshly-baked cookies, you might be reminded of your childhood or your favorite bakery. Alternatively, when you smell a particular perfume, you might be reminded of the day you first smelled it or the person who wore it. However, not all odors elicit pleasant memories. For instance, some scents may trigger painful memories of the past, especially for individuals with PTSD. When a particular sensory stimulus triggers a specific memory, it is referred to as the Proust phenomenon. To further explain the Proust phenomenon in the context of olfaction and its relation to the nervous system, Grella et al. 2020 explore how odor cues can trigger memories of the past, especially fear-related ones. 

The systems consolidation (SC) theory proposes that memories rely more on the PFC than the HPC as they age. Furthermore, during this transition, it is believed the memories begin to lose their contextual details (Tse et al., 2007; Kitamura et al., 2017; Grella et al., 2021). However, in the case of the Proust Phenomenon, sensory stimuli, such as odors, can retrieve distant yet highly contextual memories (Grella et al., 2020). For instance, in the case of PTSD, some scents may trigger harrowing memories. Grella et al. 2021 hypothesized that memories, especially those fear-related, become more PFC-dependent and less HPC-dependent over time in the quest to understand better how odor cues modulate memory consolidation. Additionally, Grella et al. hypothesized that odors would shift memory organization, causing it to be less reliant on the PFC and more on the HPC (Grella et al. 2020). In line with the SC theory, Grella et al. found in fear-conditioned mice that recent memories were more dorsal CA1-dependent, and later memories were more PL-dependent (a region associated with the PFC in humans) (Grella et al. 2021). The findings suggest that remote memories are subject to modulation by contextual cues, such as odor, and provide insight into how odors can guide therapeutic interventions, especially in memory disorders such as Alzheimer's, which have demonstrated deficits in olfactory memory relating to the HPC (Kesslak et al. 1988; Grella et al. 2020). 

Alzheimer's disease is a neurodegenerative disease associated with implicated beta-amyloid and neurofibrillary tangles containing hyperphosphorylated tau proteins and progressive memory deficits. Additionally, AD has marked neuronal atrophy in the medial temporal regions of the brain, specifically relating to the hippocampus (HPC) (Jack et al., 2010; Hajj 2021). Since the HPC plays an integral role in both AD pathology and odor-related memories, there has been significant interest in the effects of olfactory stimulation on memory retrieval. 

In a 2021 review, El Haj addresses studies conducted by his team and explores autobiographical deficits in patients with AD and how odor exposure may improve those deficits. In 2017, Hajj and colleagues conducted a study using odor exposure to retrieve autobiographical memories in patients with mild AD. Patients demonstrated better memories specificity during the odor-exposure condition than the odor-free condition. The study also indicated higher subjective experiences and emotional content after odor exposure (O. Glachet & El Haj, 2019; O. Glachet et al., 2018). Most recently, El Haj et al. 2021 conducted a study on the impacts of odor cues on retrieving self-defining memories, another feature implicated in AD. In odor-exposure conditions, patients with AD recalled a higher number of self-defining memories compared to odor-free conditions. Ultimately, the reviewed studies highlight the therapeutic potential of odor exposure, especially in the rehabilitation of individuals suffering from memory deficits or diminished senses of identity, particularly in AD. 

Odor cues and exposure have contributed to promising results in various studies, suggesting its ability to have therapeutic effects across different disease modalities. Altogether, these studies contribute and demonstrate the power harnessed within olfaction. As we continue to explore various neurological deficits and develop interventions aiding our survival, we may not have to look much farther than right under our noses. 

References: 

Grella, S.L., Fortin, A.H., McKissick, O.P., Leblanc, H., & Ramirez, S. (2020). Odor modulates the temporal dynamics of fear memory consolidation. Learning & memory, 27 4, 150-163 .

El Haj, Mohamad. (2021). Odor-evoked Autobiographical Memory in Alzheimer's disease?. Archives of Clinical Neuropsychology. 10.1093/arclin/acab074. 

El Haj, M., Glachet, O., Moustafa, A.A. et al. The smell of my self: Odor exposure increases the number of self-defining memories in Alzheimer's disease. Curr Psychol (2021). https://doi.org/10.1007/s12144-021-02028-4