Six years ago, on an early morning in September, Molly Birnbaum was out for her regular jog when she was hit by a car. Her pelvis was shattered, her skull fractured, her knee torn. Yet for her, the most serious damage was far less visible: she lost her sense of smell. Birnbaum, now 29, was an aspiring chef, and the loss meant the end of her career. It also meant something else, something that was potentially even more life-changing. “I felt like I lost a dimension of my memory,” she says. “It made me worried about the future. If I couldn’t smell ever again, was I losing this important layer?”

Memory comes in many forms. Every day we constantly receive and process sights, sounds, touches and smells from our surroundings, some of which will become our memories. The nature of those recollections, however, is inconstant. One memory can seem immediate and colorful, as if the event had just occurred, whereas another must be coaxed out of our brain little by little. Although a moment that excites our emotions is more likely to be recorded than a routine experience, the sensory qualities of the event we have buried in our brain also plays a part in how vividly and accurately we remember something.

Although sight dominates our daily life, it has long been thought that smell might have a privileged relation with memory. Until relatively recently, however, the precise nature of that connection remained largely unexplored. Now scientists are revealing that recollections tied to smell can be stronger than memory of other types. Olfaction can transport our thoughts back to some of our earliest experiences and tint these remembrances with feeling. On the flip side, its absence could be a sign—and potentially a cause—of cognitive decline. Scientists are at a very early stage of developing therapies to train people to smell better, which could one day stave off the deterioration of mental faculties.

Transported by Scent
Aristotle explored the apparent ties between odor and memory in his treatise from the fourth century B.C., On Sense and the Sensible. Since then, people have speculated that the memories elicited by smell are more intimate and immediate than other recollections. When we experience certain smells, we often find ourselves whisked back in time to a specific event or scene. For example, the smell of salsa reminded Birnbaum of watching James Bond movies on television with her dad while dipping chips in the spicy sauce. When she lost her sense of smell, she could still remember eating salsa with her father, but she could no longer quickly summon that long-ago scenario.

Psychology studies support the idea that memories associated with odors are unusually evocative. In a 2006 experiment psychologists Johan Willander and Maria Larsson of Stockholm University gave older adults one of three types of cues—visual, auditory or olfactory—and asked them to describe an autobiographical event that came to mind as a result. The participants also rated the event based on its emotionality, vividness and importance.

Although the volunteers came up with the same number of memories for each type of cue, odors elicited earlier memories, including far more from the first 10 years of life, than did sight or sound cues. Recollections emerging from scents were also associated with a stronger feeling of being brought back in time. The results suggest that memories tied to smell are both older and associated with a more time travel–like experience than are other types.

The use of odors to trigger memories has led researchers to reconsider the long-held notion that people recall more incidents from their teens and 20s than from any other time in their life. In 2000 psychologist Simon Chu, now at the University of Central Lancashire in England, and his colleagues discovered that although visual memories did peak between the ages of 11 and 25, odor-cued recollections crested between the ages of six and 10.

Rachel Herz, a cognitive neuroscientist at Brown University, sees olfaction as a potential key to a trove of past experiences that would otherwise remain locked. A whiff of a smell not encountered since childhood may bring us back to an event that we had all but forgotten existed, she theorizes.

Smell might have this power because odors themselves are relatively rare, compared with, say, visual stimuli. Every day our eyes are constantly bombarded with images, many of which are quite similar, creating confusing interference in the brain. In contrast, our nose detects distinct odors only infrequently, a fact that Richard L. Doty, director of the Smell and Taste Center at the University of Pennsylvania, surmises is key to the evocative power of scent. Because smells are encountered rarely, individual odors are often tied to a unique experience, enabling a strong and stable connection.

Smell has a privileged relation with memory on an anatomical level as well. It is the only sense that connects with the memory system without stopping over in the thalamus, a sensory relay station. Signals travel from the nose to the olfactory bulb and then directly to the hippocampus, an essential hub of memory formation, and the amygdala, which processes emotion. “Memory and odors are just sitting side by side,” says research psychiatrist Donald Wilson of New York University Langone Medical Center.

The connection does not end there. In a parallel track, the olfactory bulb passes information to the olfactory cortex, which sits at the surface of the brain just above the ears. Part of this region is involved in complex learning and memory tasks. The olfactory cortex, together with an adjacent decision-making area, the orbitofrontal cortex, processes the information contained in a smell and sends the data back to the hippocampus. This back-and-forth communication ties scents with remembrances.

Sniffs of Young Noses
To understand why odors seem to strongly evoke very early life experiences, scientists began to search for other differences in how the senses interact with memory. In 2009 neuroscientist Noam Sobel of the Weizmann Institute of Science in Rehovot, Israel, and his colleagues taught subjects to pair pictures of objects with a smell or a sound, or both. Subjects then viewed pictures of the objects while in an MRI scanner and were asked to recall either the smell or sound associated with each image. In a second round, the researchers paired every object with an opposing odor or sound or odor-sound pair: if the first stimulus had been pleasant, this time, it was unpleasant—and vice versa. Another brain scan and test of these memories followed.

One week later the researchers presented the pictures a third time and asked participants to name the odor or sound that popped into their mind. Overall, people recalled the memories from the first round slightly more than those in the second set. The brain scans, however, produced a more nuanced picture. When a person thought of  the first odor, the hippocampus became much more active than when he or she remembered the second smell, suggesting that the brain issues a special tag for first odor associations. In contrast, the hippocampus activity was the same for first and second sounds.

In addition, on the first memory test, the more the hippocampus responded during odor retrieval, the more likely a person was to later remember that first odor as opposed to the second. No such relation existed for sounds. Given the brain’s unique response to first odor memories, the smells of childhood may make early remembrances particularly durable.

Although its effect on our earliest recollections may be most pronounced, smell might also facilitate learning more broadly. In a study published in 2007 neuroendocrinologist Jan Born and his colleagues at the University of Lbeck in Germany asked people to inhale the smell of a rose while studying the locations of 15 pairs of cards on a computer screen. When the participants went to sleep that night in the lab, some of them were exposed to the rose odor, whereas others’ sleep was unscented. In the morning, all the participants were tested on their memory for the card locations. Those who had been exposed to the flower fragrance remembered 97 percent of them, compared with just 86 percent for those who had received an odorless stimulus, suggesting that odors can boost learning as memories are consolidated during sleep.

Waiting to Inhale
The memories that smell evokes also have a distinct emotional tint. In studies in which Herz and her colleagues asked people to rate the poignancy of various memories, those provoked by odors were steeped in more feeling than those brought to mind by visual, verbal, tactile and auditory cues. In these studies, the subjective responses of emotion jibed with physical changes, such as heart rate.

Consistent with the anatomical portrait of smell, odors also uniquely recruit brain regions that process both emotion and memory. In a 2004 study Herz’s team asked participants to identify a perfume that elicited a pleasant personal memory. One month later the people were shown a picture of the perfume as well as a photograph of a different perfume—and exposed to the odor of each—while inside a brain scanner. The researchers found that the odor related to the emotional memory generated more activity in the amygdala than did the pictures or the other odor. These chosen odors were also the only cues that boosted the neural response in memory-related regions. The brain’s response thus mirrors people’s subjective impressions that odors possess a unique power to summon emotional memories.

Accordingly, the loss of smell seems to have ripple effects on the integrity of memory and emotion centers. In studies published in 2010 and 2011 researchers at Friedrich Schiller University of Jena in Germany saw shrinkage of neural tissue in both the hippocampus and emotional brain structures in individuals with anosmia (the inability to perceive smells) and parosmia (the distortion of smells), as compared with people with no smell impairments, hinting that a loss of smell may impair memory or emotional processing, or a combination of both.

Such effects might explain Birnbaum’s impression that her anosmia, though not wiping out her memory, stripped her recollections of their poignancy. “I’d always had memories that came from smell that were really important to me,” Birnbaum recalls. After the accident, “I didn’t forget them, but the emotional potency wasn’t there.” Smell’s ties to emotion also become apparent in cases in which the loss of smell leads to depression—or depression leads to the loss of smell.

Although it is not clear whether olfactory deficits directly impair cognition, they are often an early sign of a declining mind. In 2009 research psychiatrist Monica Z. Scalco and her colleagues at the University of Toronto found that poor performance on a standard test of smell could serve as a very early indicator of cognitive decline in older people. Olfactory deficits in these individuals appear to precede cognitive impairment. Complete loss of smell is also a signature of incipient Alzheimer’s disease. In 2010 neurosurgeon Qing Yang and his colleagues at Pennsylvania State University reported that they could use functional MRI to detect subtle deviations in the activity of the olfactory system in Alzheimer’s patients that were not present in people without the disease. In the future, doctors might look for such changes to predict the onset of Alzheimer’s at a very early stage.

Exercise Your Nose
No one yet knows whether improved detection of odors can enhance cognition. Given that it might, however, scientists are looking at the possibility of shoring up people’s sense of smell. In some cases, exposure to an odor can improve its detection. Take androstenone, a steroid found in sweat and urine. About one third of us cannot smell it at all, and for the rest, it smells like either sweaty socks or vanilla, depending on an individual’s genetic makeup. In 2002 a group led by Joel Mainland, then at the University of California, Berkeley, demonstrated that exposing insensitive individuals to androstenone for 10 minutes daily for 21 days gave them the ability to pick up its scent. The researchers’ data suggest the changes occurred in olfactory brain systems rather than in the nose itself.

In findings published last November, Wilson and his colleagues revealed that rats could gain or lose the ability to smell the difference between two similar chemicals, depending on the circumstances. The results hint that, as with rats, humans may be able to learn or unlearn how to smell as a result of everyday experiences. If we then inadvertently lose our ability to distinguish among odors—say, as a result of inattention or lack of practice—data suggest that the loss may affect other parts of our brain. Those of us who end up with declining olfactory abilities may be at risk for a loss of mental acuity or changes in our memories.

As a remedy, some kind of smell training might help ward off such a decline. Doty believes that regular exposure to odors from childhood on—or more mindful attention to existing odors—might thwart a subtle erosion of cognition. In addition, people might be systematically tested for loss of smell just as they are examined for hearing and sight impairments now. “It could be a warning sign if olfaction falls off,” Doty says.

Although recovery from anosmia is rare, Birnbaum did regain her sense of smell. No one knows why, but her exposure to odors may have helped. She studiously inhaled odors—from jars of condiments, familiar foods and spices—wherever she went. Birnbaum’s brain has not totally rebuilt its memories surrounding these scents. The young woman’s early-life flashbacks remain devoid of the expressive color they once had, she says. But in time, as her brain restrings the wires between nose and brain, the salsa may viscerally summon the television once again.