How the Sense of Smell Triggers Old and Long-Forgotten Memories

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Conventional wisdom dictates that smells are particularly efficient cues for recalling long-buried personal memories, an effect that has become known as the Proust phenomenon.

The name of this phenomenon refers to a passage in Marcel Proust's Swann’s Way, where the narrator Marcel details how the taste of a madeleine pastry soaked in lime flower tea revives a long-buried memory. But if it is taste, and not smell, that triggers Marcel's childhood memories, why, then, does Proust's passage give its name to odor-evoked memories?

As philosopher Barry C. Smith (2016) notes, the answer to this question turns on intricacies about smells. We actually have two senses of smell. The odors we sniff in through the nose seem to us to come from the environment (e.g., the smell of garbage).

But smells can also reach the nose from the mouth via the nasopharynx. These so-called "retronasal" smells are not experienced as smells. In fact, they lack an independent phenomenology. But along with touch and taste, the "retronasal" smell makes up the flavor of foods and drinks. It is smell, taste, and touch—and not mere taste—that cues Marcel's childhood memories.

The Proust phenomenon raises interesting questions about the veracity of these spontaneously arising memories. Odor-evoked memories may not represent how things were but rather generate a conviction in us that this is how things were.

But the phenomenon itself is well-documented and has enjoyed sporadic surges of interest from researchers starting in the 1980s after posttraumatic stress disorder (PTSD) was first introduced (Berntsen, 2021).

Most of this research has focused on measuring the peculiar characteristics of odor-evoked memories without paying much attention to the causes of these peculiarities.

However, in a recent literature review of the Proust phenomenon, psychologists Ryan Hackländer and colleagues zoom in on the causes of the curious features of Proustian memories.

Autobiographical Memories Induced by Smells

Autobiographical memories are so-called because they relate to our personal history. Sometimes we consciously choose to retrieve these memories. But they can also come to mind spontaneously, without any cognitive effort on our part.

Such spontaneous recollections of past events can be elicited by a variety of cues: visual scenes, pictures, language, non-linguistic sounds, touches, flavors, and smells. However, the Proust phenomenon refers specifically to smell-evoked memories.

Larsson et al (2014) describe five characteristics of Proustian memories, using the acronym LOVER (Limbic, Old, Vivid, Emotional, Rarely Rehearsed):

• Limbic: Smell is closely tied to the brain's limbic system, which controls memory, emotion, attention, and hormone production.
• Old: Smells have the potential to evoke both long-buried childhood memories and traumatic memories we thought we had put behind us (Berntsen, 2021).
• Vivid: Odor-evoked autobiographical memories can appear more vivid, clear, and detailed than other memories.
• Emotional: Proustian memories are emotionally intense. Smell is the only sense that transmits sensory information directly to the amygdala, the brain region that processes fear and arousal.
• Rarely Rehearsed: Many of us are prone to rehearse upsetting events days on end. But the events Proustian memories bring to life are rarely rehearsed (Hackländer et al, 2019).

While odors have been found to be less effective memory cues than other sensory stimuli overall, odors are more effective for emotional memory recall than other types of cues (Chu & Downes, 2000).

How Can Odors Induce Long-Forgotten Memories?

To make headway on the question of what causes the peculiarities of Proustian memories, Hackländer et al (2019) assess several hypotheses.

One turns on the role odors play in memory formation in childhood. However, this hypothesis does not generalize to odor-evoked recollections of recent traumatic events (e.g., in war veterans) (Berntsen, 2021).

Among the remaining hypotheses ascertained by Hackländer et al, the consolidation hypothesis enjoys the strongest independent empirical support.

According to this hypothesis, the close and stronger connections between brain regions that process odor, emotion, and memory lead to faster and more effective memory consolidation.

Memory consolidation refers to a process of strengthening memory connections between neurons by depositing proteins in the synaptic gap between them. The result of this process is also known as long-term potentiation.

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Frequent recall, dream sleep, and repeated exposure to conjoint sensory stimuli (e.g., the co-occurrence of "dad" and your dad) can consolidate memories.

The tea-soaked madeleine that reminds Marcel of his aunt Léonie offering him crumbs of soaked cake on Sunday mornings is an example of repeated exposure to co-occurring sensory stimuli: his aunt Léonie and tea-soaked pastry.

Because of the powerful connections between brain regions dedicated to odor, emotion, and memory, emotions can enhance memory consolidation in the absence of frequent recall, dreams, and repeated exposure to co-occurring stimuli (e.g., Phelps & LeDoux, 2005).

Proustian memories anchored in emotions can thus explain their endurance in the absence of frequent recall. Smells that are inherently emotion-laden should likely be able to enhance memory consolidation (Yeshurun & Sobel, 2010).

The hypothesis that odor cues are specially effective only for triggering emotional memories also explains the older finding that odors are less effective for recall than other sensory stimuli. Indeed, mundane emotionless events like disinfecting a countertop or adding a slice of aged cheese to a sandwich involve strong odors but do not typically result in lasting memories.

However, a new study on Alzheimer's Disease challenges the earlier finding that odors generally are less effective cues for recall than other stimuli. The study, which was conducted by Glachet and El Haj (2021), revealed that odor is more effective than visual and verbal cues for autobiographical recall in Alzheimer's.

In Alzheimer's, plaque and tangles form in the hippocampus, which controls memory encoding, consolidation, and recall, rendering this brain region dysfunctional. Odors thus appear to enhance recall by enabling stored information to bypass the dysfunctional hippocampus.

If odor cues also turn out to enhance recall in "normal" individuals, then the Proust phenomenon may be far more prevalent than hitherto assumed.