Odor-induced recall of emotional memories in PTSD–Review and new paradigm for research

Abstract

It is clinically well known that olfactory intrusions in PTSD can be a disabling phenomena due to the involuntary recall of odor memories. Odorants can trigger involuntary recall of emotional memories as well have the potential to help diminishing emotional arousal as grounding stimuli. Despite major advances in our understanding of the function of olfactory system, the study of the relation of olfaction and emotional memory is still relatively scarce. Odor memory is long thought to be different than other types of memories such as verbal or visual memories, being more strongly engraved and more closely related to strong emotions. Brain areas mediating smell memory including orbitofrontal cortex and other parts of medial prefrontal cortex, hippocampus and amygdala, have been implicated in learning and memory and are part of a neural circuitry that is involved in PTSD. The olfactory cortex itself also plays an important role in emotional processing. Clinical observations support the notion that odor-evoked memories can play a role in the symptomatology of PTSD. This paper reviews a re-emerging body of science linking odor processing to emotional processing in PTSD using the calming and grounding effect of odors as well as the use of odors in augmented exposure therapy. This results in converging evidence that olfaction is an excellent model for studying many questions germane to the field of human emotional memory processing.

Analysis of Genetically Regulated Gene Expression identifies a trauma type specific PTSD gene, SNRNP35

“…A number of the M-PTSD specific functional enrichments are also highly plausible; for example, enrichment H3K27 acetylation peaks from a gene-set derived from DLPFC neurons (Girdhar et al, 2018), known to have a role in stress models and neuropsychiatric disorders (McEwen et al, 2015), including PTSD (Maddox et al, 2018). Our results highlight a potential shared genetic basis between olfaction and M-PTSD, in line with previous findings of differential olfactory 5 identification in individuals with combat-related M-PTSD compared to healthy controls (Vasterling et al, 2000); olfactory triggers for PTSD intrusion symptoms (Daniels and Vermetten, 2016); olfactory-based treatments for PTSD (Aiken and Berry, 2015); and the key role of olfaction in fear conditioning in animal models (Morrison et al, 2015). Given the significant genetic overlap between olfaction and PTSD, our results support the hypothesis that 10 differential sensitivity to odors may predispose to development of PTSD.…”

Behavioral and Neurobiological Convergence of Odor, Mood and Emotion: A Review

“…Food odors can evoke cephalicphase digestive system responses as if food had actually been consumed (e.g., salivation), body odors can be used to guide social preferences (Li et al, 2007) and can serve as social buffers during stress in humans and animal models (Sullivan and Toubas, 1998), even in the absence of the individual, and odors can evoke relaxation or anxiety/fear dependent on their past associations. For example, odors associated with battle can evoke anxiety, fear, flight, and/or panic responses in individuals with PTSD (Daniels and Vermetten, 2016) while maternal odors can reduce crying in human infants (Sullivan and Toubas, 1998). Thus, given the role of odor cues in behavior, it should be clear that odors and emotion, and potentially mood, are closely aligned.…”

Odor Modulates the Temporal Dynamics of Fear Memory Consolidation

“…More specifically, odor seems to be a particularly powerful contextual cue that can evoke this experience long after memories are stabilized (Vermetten and Bremner, 2003;Arshamian et al, 2013). Odors can be potent triggers of memories, (Daniels and Vermetten, 2016) and as such, we believe that if they Grella et al,6 are present during encoding, they can shift the organization of arousing memories at the systems level, to a state of HPC-dependent processing.…”

“…As the French author Marcel Proust described long ago (Proust, 1927), referenced in his work A la Recherche du Temps Perdu which translates to In Search of Lost Time -"Yet a single scent already breathed long ago, may once again both be in the present and the past, be real without being present". Sensory experiences like olfaction and gustation have the ability to produce the evocation of vivid autobiographical memories of the distant past (Toffolo et al, Grella et al, 15 2012;Zucco et al, 2012;Daniels and Vermetten, 2016;Glachet and El Haj, 2019). This spontaneous process which we have all experienced at one time or another is often referred to as the Proust Phenomenon.…”