Odor stimulation (also called olfactory stimulation) is particularly important for maintaining the health of the brain’s memory centers because it is the only sensory system that takes a direct “superhighway” into those areas. The other senses take the brain’s “side streets” before reaching the memory centers, so they have much less impact on your memory. There are now several reports (see also Duoad et al., 2022 and Murphy, 1999) that describe the deterioration of the brain’s memory centers with the loss of olfaction.
But in our modern affluent world, we don’t get much odor stimulation. Most of us are probably deprived of the level of odor stimulation we need to keep our minds sharp. Moreover, we have gradually been damaging our ability to smell things by exposure to toxins, air pollution, smoking, having a chronically stuffed nose, having a head trauma, experiencing chronic childhood stress, taking any of a variety of medications, or going through menopause. All these experiences not only result in olfactory loss, but they also can result in memory loss.
Memory declines with age, starting at about age 60 and its decline mirrors the decline in one’s ability to smell things.
The COVID Effect
People don’t notice when they’ve sustained a gradual loss of olfaction. But many people who have had COVID-19, probably noticed a sudden loss of that ability. Researchers from Oxford University imaged the brains of hundreds of people before the pandemic, and then they brought them back to reimage their brains several months after they had recovered from mild COVID infections. They saw clear deterioration in their brains along the odor superhighway into the brain’s memory centers. They also reported continuing cognitive loss even after mild COVID infections.
Also, it’s possible to predict the level of memory loss from the severity of olfactory loss. As the olfactory superhighway is the first to deteriorate in Alzheimer’s disease, there has been a predictable increase in dementia due to the pandemic.
‘It’s possible to predict the level of memory loss from the severity of olfactory loss.’
In addition, the ability to smell things predicts which older adults are likely to develop mild cognitive impairment, the first step toward dementia. And it predicts which of those individuals will develop Alzheimer’s, and which of those individuals will descend quickly into dementia.
The inability to smell things accompanies all forms of dementia: semantic, frontotemporal, vascular, Lewy body, along with the dementia that can accompany the terminal stages of AIDS, alcoholism, Parkinson’s and schizophrenia.
It also accompanies virtually all brain disorders (at least 68 of them) including: anorexia, anxiety, ADHD, depression, epilepsy, stroke.
And olfactory loss accompanies all the diseases that are most likely to kill you: heart disease, cancer, lung disease, kidney disease and liver disease.
Remarkably, there are now several large studies (see also Pinto et al., 2014; Pinto et al., 2017) showing that, starting in middle age, you can predict the likelihood of dying from any cause within the next few years, based on your ability to smell things.
Stimulate Memory by Increasing Olfactory Stimulation
If decreasing olfactory stimulation impairs memory, can increasing olfactory stimulation improve memory? The good news is that it can. Smelling multiple odors each day can stimulate your memory centers and thereby improve your memory.
We call this kind of stimulation odor enrichment, using multiple odors each day to stimulate your brain. There are now several studies showing that it can improve your brain and cognitive ability.
For example, researchers from the University of Quebec looked at the brains of young adults training to be wine tasters (actually, wine smellers), who sniff dozens of odors each day. Brain imaging showed that odor enrichment enlarged a critical memory center in their brains.
Cynthia Woo and colleagues at the University of California Irvine gave older adults odor enrichment while they slept, and they remembered 226% more than older adults who weren’t enriched. To my knowledge this is the largest improvement in memory that has been reported for healthy older adults. Brain imaging showed that an important brain pathway for memory improved to a similar extent.
The most dramatic study comes from researchers at Namseoul University in South Korea, showing that when older adults with dementia sniffed 40 odors twice a day, their memory test scores improved by as much as 300% compared to other older adults with dementia. Hundreds of clinical trials that have tested drugs to treat dementia haven’t come close to that kind of success.
Of course, it’s a monumental task to open, sniff and close 80 bottles each day, but fortunately, we have developed a device that sits on your nightstand and automatically exposes you to those 40 odors, twice a night while you are sleeping. It’s called Memory Air and you can place your name on our list if you want to be contacted when it becomes available: www.memoryair.com.
Olfactory enrichment may also be able to prevent or treat other brain disorders. There are now two reports that it can decrease depression symptoms.
Who would have thought that the sense that you probably regard as your least important sense could turn out to be your most important sense?
Michael Leon, PhD, is professor emeritus in the Department of Neurobiology and Behavior at the Center for the Neurobiology of Learning and Memory, Institute for Memory Impairments and Neurological Disorders, at University of California Irvine.
Photo credit: Shutterstock/Bignai
