Sweet smell of success: Simple fragrance method produces major memory boost

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It is common to see news of various foods that are purported to stimulate the brain and potentially stave off dementia or slow down cognitive decline associated with old age. For example, foods rich in antioxidants, whole grains, meats like chicken, turkey, or fish, olive oil, and a little wine. But what if it were simply sniffing odors that did it? Researchers at the University of California - Irvine have proposed that this just might work.

From the Harvard Gazette article, What the Nose Knows

It has already been shown that losing your sense of smell (olfactory capacity) can sometimes predict the onset of almost 70 neurological and psychiatric diseases. These include Alzheimer's and other dementias, Parkinson's, schizophrenia and alcoholism. The COVID-19 virus is also linked to a loss of smell and cognitive ability, but research on that is still young. What's the back story on sense of smell and diseases?

It all starts with how we smell, and how that gets processed in the brain. The other five senses send their signals directly to a part of the brain called the thalamus first, and then it gets interpreted and sent to other regions of the brain. With olfaction (smell), that's not exactly the case. (You can skip all this biology stuff if you like and get a preview with the 5-minute video at the end of the blog.)

Thalamus (in red) (Wikipedia)

Air enters the nose, and back inside the nasal cavity, there is a patch of several types of cells on the top. This is the olfactory epithelium. It's only 5 square cm (0.78 square inches) big. Sticking out through the mucus of the epithelium and into the nasal cavity are hair-like extensions of nerve cells. The extensions are called cilia, and on the ends they have  about 6 million specialized receptors for detecting certain smells. 

Cross section of nasal cavity and olfactory epithelium (from Cerveny et al., 2022)

Dogs have 100-300 million, and their receptors are more sensitive. Humans have 400-1,000 types of these receptors, giving us the capability of detecting about 10,000 different odors, although some research says it's really a trillion! When you think that a dog's brain has an area 40 times bigger than a human's for interpreting smells, just imagine how well they do it, from 1,000 to 10,000 times better!

The nerve cells extend up through a bony plate and into an area called the olfactory bulb. From there, the signal for an odor goes to the brain.

Route of odor signals from cilia receptors in the nasal passage (bottom) to the olfactory bulb (top)

But where exactly in the brain do the odor signals go? Signals from other senses (sight, sound, touch, taste) go directly to the thalamus, but in the olfactory bulb, two types of nerve cells carry the odor signals to a variety of other places in the brain, then the thalamus, all in a complex system that sometimes feedbacks on itself. See the diagrams below.

Odor signals from the olfactory bulb to places in the brain

Parts of these areas of the brain are associated in different ways with memories. In fact, the sense of smell is the only one that is directly linked to the memory processing areas of the brain.

Most studies say that from the age of 55, humans begin to lose their sense of smell. They probably mean noticeably lose it, because the graph below shows how many correct answers people gave when asked to identify odors, and there is a small gap between age 40 and 50s where the incorrect answers increase.

Adapted from a study by Murphy, 1986

But it's not problems identifying all smells that increase with age. A Danish comparison of 251 people 60-98 years old with 92 people 20-39 years old showed that the older population's sense of smell for fried meat, onions, and mushrooms is weaker,  but they smell orange, raspberry and vanilla just as well as younger adults.

What causes these problems in the first place? Again, the answer is complex. 

• long-term experience with nasal diseases
• cumulative exposure to various environmental insults
• eventual drying out of the mucus
• receptors losing their selectivity to many odors 

Here's a link to a very short article by the Mayo Clinic explaining more.

It might be irritating to not be able to smell pleasant aromas as much, whether flowers or foods, but it can also be dangerous if you can't detect smoke.

But what about that part of our sense of smell that seems linked to memory? Professor Michael Leon and his colleagues in California ran an experiment. They asked healthy 42 male and female adults aged 60–85 to take part in the six-month study. Some were in a control group, and none seemed to have any diagnosis of cognitive impairment or dementia. All of them were asked to run an odorant diffuser every night for 2 hours each time with one of seven fragrances: rose, orange, eucalyptus, lemon, peppermint, rosemary, and lavender. (The controls had only a weak trace of each odorant.) 

One type of diffuser common in aromatherapy

People with exposure to the odorants increased their scores on a word recall test by 226%. They also showed improvement in a part of the brain (one of many) that is related to learning and memory. Normally, that part of the brain deteriorates with age (and especially with Alzheimer's disease), and it is involved in retrieving long-term memories.

These results are not surprising. Other researchers have shown the following results.

• Loss of olfactory ability can be restored in patients with head trauma, infections, Parkinson's disease, or simple aging.
• Multiple odorants (compared to single ones) stimulated memory in mice.
• Exposure to odorants increased some gray matter in the hippocampus, a part of the brain associated with long-term and short-term memory as well as spatial memory we use to navigate our way.
• Verbal fluency of patients with Parkinson’s disease improved with odorant treatment.
• Patients with moderate dementia showed positive results on memory, olfactory identification, depression symptoms, attention, verbal fluidity, and language skills after odorant exposure.

So, bottom line, it might be worthwhile to surround yourself with an environment rich in smells, whether to hold off memory problems or maybe even to improve a fading memory a little.

In 2014, a researcher developed what he called the "ophone", a device that can send actual smells along with pictures. So, you don't necessarily have to constantly buy aromatic oils with your infuser. He claimed it could reproduce up to 300,000 odors. This website contains links that described how a popcorn manufacturer and meat company had created a dongle to do the same thing when attached to your iPhone.

The ophone, Pop Secret dongle, and Oscar Mayer's bacon dongle

Here's a YouTube video by Demystifying Medicine explaining some of this blog article in just over 5 minutes.