Contrary to common belief, smell is a more encompassing sense than either sight or hearing.

Olfaction can be fine-tuned through exercising it.

Most of us would feel pressed if someone asked us to come up with a word or phrase to describe our favorite perfume or cologne. For one reason, most of us pay little attention to smell and therefore have a stunted vocabulary when it comes to describing an odorant (an inorganic compound that produces a distinct smell or aroma).

Our perception of our immediate surroundings is mostly determined by what we see, hear, and touch. Each of the receptors responsible for these sensations exists in specialized organs (retina at the back of the eye for vision; inner ear-cochlea for hearing; touch receptors on the boundaries of our skin). Vision is our dominant sensation thanks to its high resolution, vividness, and immersiveness. Sound follows as a close second. In most cases, we can determine by a form of echolocation where a sound is coming from.

Smell operates quite differently from sight, hearing, or touch. For one thing, we do not experience smell as closely confined to a discrete location. Rather, smells seem to diffuse throughout our environment. This non-locality is quite different from vision; when we look at something, photons travel in straight lines, striking the retina and generating electrical signals in the nerve bundles projecting eventually to the visual area of the occipital lobes (towards the rear of the brain). Sound also projects to specialized receptors from the inner ear to the cochlea and finally terminates in the peritemporal and temporal lobes. While odors also project to specialized receptors (the olfactory bulb), we cannot identify, secondary to its diffusion, exactly where a particular odorant is coming from.

For these reasons, it seems that comparing vision to olfaction is much like moving from objectivity to pure subjectivity; so many variables, so much variation in olfaction. But there is a problem discounting the importance of smell. Measurements of the human nose show that it can detect the presence of a particular odorant (smelly molecule) such as bell pepper at a concentration of 1 molecule per billion.

Experiments by Rockefeller University scientists have revealed that humans are not poor smellers. The human nose can discriminate at least 1 trillion olfactory stimuli. Compare this to the several million colors the eye can distinguish and the half a million distinguishable tones the ear can appreciate.

The human smell sense is not deficient at all, but a necessary filter. If we were aware of all of the odors emanating from places, people, things, and processes around us, we would be completely overwhelmed. Or as Ann Sophie Barwich, a cognitive scientist specializing in smells, puts it, “Most of our attention is directed to what we see and hear; sitting in the backseat of conscious awareness, we simply don’t pay much attention to what the nose knows,” Barwich writes in Smellosophy: What the Nose Tells the Mind.

Essentially, we are speaking here of a balance; our sense of smell remains sensitive to large numbers of changing and unpredictable background odors surrounding us, while, coincidentally, we don’t want to be consciously aware of all the odorants we process at any given moment. “If your mind was consciously processing all of the molecular information that your nose picks up, without any break in its conscious awareness, you’d soon agree to a lobotomy just to save your mind,” writes Ms. Barwich. Finally, there is one more important contributor to odorant appreciation: Naming.

To appreciate this, imagine yourself closing your eyes while I provide a chemical mixture for you to smell. I ask you to sniff it, hold that smell in your mind, and then open your eyes.

How will you react if I tell you the smell emanated from a small sample of vomit, which I had held close to your nose? No doubt the remembered smell will repel you (as well as arouse some degree of anger at my intrusive impertinence). But if I tell you the smell was Parmesan cheese, you would appreciate a quite different response, especially if you like Parmesan cheese. In fact, you’ll chide yourself for not recognizing it. Here is the point: The same chemicals—butyric, valeric, and isovaleric acids—are found in both vomit and Parmesan cheese. Thus, your interpretation of the smell depended upon what I told you about its identity. The majority of people given this test (over 80 percent) confirmed that the odorant identified corresponded to the label that had been suggested.

Thus, the subjective aspect of odorant identification is a semantic one: a vocabulary deficiency, if you will. When one cannot come up with a word to describe one's olfactory sensation, it’s all too easy to fall into the trap of accepting the terminology suggested by another person. And the same odorants can emanate from multiple sources and serve as constituents of many complex odor mixtures. Consequently, verbal descriptions of smells are always vague and difficult to convey, except for perfumers and others who have made deliberate efforts to enhance their olfactory receptors and verbal descriptive powers.

These findings form the basis for several olfactory strategies. It’s possible to improve one’s sense of olfaction by practice. Why would we do that? Because increasing one’s sensitivity to particular odorants enhances one’s ability to recognize those odorants and learn their names, thus increasing vocabulary and linking particular odorants with new words or the novel use of old words. Why might this be worth the effort?

Smell sensitivity ordinarily decreases with aging, as does general vocabulary. This is especially true for Alzheimer’s disease. In fact, loss of smell (anosmia) or decreased ability to smell is often one of the first objective signs of mild cognitive impairment, which in many affected people, specifically leads to Alzheimer’s disease. All of which contributes to the formation of an intriguing question.

Can the progress of Alzheimer’s disease be slowed down, or even stopped, by the use of olfactory training? No final answers yet, but research on the topic is currently taking place with results that are already sufficiently encouraging to justify efforts to enhance our odorant recognition ability, whatever our age. More about that in the next post.

Copyright Richard M. Restak, M.D.

© Psychology Today