An Easy Way to Reverse Some Cognitive Decline

Researchers had volunteers smell a pleasant odor for two hours every night while they slept, a different odor every night. After six months, in tests of learning and memory, their cognitive abilities improved significantly. The people in the matched control group didn't improve. In fact, their cognitive abilities got a little worse.

Why would smelling something improve learning and memory? In the paper they wrote about the experiment, they explained why they did the study in the first place, as researchers often do. For a long time, medical scientists have recorded their observations that often, right before the loss of cognitive function or the impairment or decline of mental abilities, people lose olfactory abilities. In Parkinson's, Covid, dementia, schizophrenia, Alzheimer's — people often lose their sense of smell before they show signs of mental decline. In their paper, the researchers said they found about 70 neurological and psychiatric disorders that are often accompanied by olfactory loss.

So previous researchers figured maybe if you stimulated or challenged or exercised someone's sense of smell, it might prevent or reverse their mental decline. So they tried it on animals and then on humans, and it worked. But why?

It turns out the olfactory nerves and the olfactory bulb are intimately connected to parts of the brain responsible for memory. So when the olfactory bulb is stimulated, it also stimulates those connected parts of the brain too, and can restore function there. And the stronger the smell, the more people recover their cognitive abilities. The longer the exposure, the more recovery they make. And maybe most importantly, increasing the number of different smells (not all at once, but sequentially), caused more recovery.

One of the previous studies the researchers mentioned in their paper was of wine sommeliers going through a sommelier training. They did an MRI scan of the students' brains before and after their eighteen months of training, which constantly challenged them to make olfactory distinctions. At the end of it, the entorhinal cortex of their brains had become thicker (that's the part of the brain responsible for mediating information coming from, and going to, the hippocampus). Other studies have shown that olfactory stimulation increases gray matter in the hippocampus and the thalamus. Stimulation of the olfactory system increases the activity of brain regions related to memory.

In still other studies, olfactory enrichment improved subjects' ability to do Sudoku puzzles and verbal fluency tests, and decreased depression.

And just when you thought it couldn't get any better, previous studies have also shown that olfactory stimulation while someone sleeps makes their slow-wave sleep deeper. Slow-wave sleep has been found to be the most restful part of the sleep cycle. A different study found that people reported feeling more vigorous the next day after they had olfactory stimulation while they slept.

Given all that, the researchers decided on the protocol for this study. Volunteers used the infuser for two hours every night. The essential oils they used were: sweet orange, eucalyptus, lemon, rose, peppermint, rosemary, and lavender. The diffuser they used is from Diffuser World and they got the essential oils from The Essential Oil Company in Portland, Oregon.

They gave each person a diffuser and told them to start it when they went to bed and set the timer so it turns off after two hours. They used a different essential oil each night, rotating through the seven oils.

A control group also got a diffuser and went through the same ritual, but there was so little essential oil that it didn't smell like much. They tested everybody on learning and memory before they started, and then tested them again after six months. They also gave them fMRI scans before it started and again after six months. The results were very positive, especially for such an easy thing to do. This is how the researchers put it: "Minimal olfactory enrichment administered at night produces improvements in both cognitive and neural functioning."