When Niels Hoven was 10 years old, he discovered something curious about himself: he could not smell certain odors. As he was walking through the woods at summer camp one day, other kids started complaining of skunk smell, but he just couldn't detect it.
This bizarre olfactory phenomenon has persisted throughout his life. And it is not like Hoven does not have a sense of smell — it is just that he can't detect certain odors, mostly unpleasant ones. "For example, I often can't smell farts," he writes on his blog. It's "debatable whether or not this is a blessing in disguise, but I will say it's a little unnerving to flatulate (is that a word?) and have no idea whether or not the people around me can smell it."
Hoven's lack of sensitivity to certain smells has a scientific name: specific anosmia. Like Hoven, people with the condition are unable to detect one or several odors, but their olfactory detection of most substances is unaffected. It is therefore different from general anosmia, which is a complete loss of smell, either due to illness like chronic meningitis or head trauma.
The first scientific records of specific anosmia date back to 1893, when researchers described people who were able to detect every smell but vanilla. Since then, scientists thought it was a pretty rare phenomenon, with a prevalence of less than 1 percent for skunk smell, for instance.
But a new study suggests that the condition may not be rare at all, as its prevalence can reach even 20 percent, depending on the type of smell.
For the study, published this month in Cortex, the researchers recruited 1600 volunteers without known olfactory impairment and tested their sensitivity to a couple of different odors out of 20 that were available. Of all the odors used in the study, seven were related to food, three were associated with flowers, four were musky odors, two smelled of sandalwood and the four remaining ones represented other types of smells.
About 34 percent of the people in the study were anosmic to at least one of the 20 odors, the researchers found.
"For 18 out of the 20 odors, there was at least one participant with specific anosmia," the researchers said. But it turned out that the prevalence of the condition varied widely, reaching up to 20 percent for one sandalwood-like type of smell.
The investigators also found that, the higher molecular weight a substance had, the more people seemed to be insensitive to its smell. This is probably because higher molecular weight may make smells less volatile, hindering their passage through the nasal mucosa, the scientists said.
But there was more to the findings. In another experiment in the study, the researchers picked 25 people with specific anosmia. Most of them (15) were anosmic to musk odor. The people then underwent an olfactory training — they were given bottles with the specific odors they could not normally detect and were asked to sniff them twice daily. The training was initially supposed to last at least four months, but some of the participants finished it after two months due to time constraints.
At the end of the experiment, all 25 people showed improvement in their ability to detect the smells they were initially insensitive to. It is possible that the increased exposure to the specific odors made those odors relevant to the people, possibly stimulating the expression of smell receptors that are necessary to perceive those odors, the researchers said.
"Daily, we are exposed to thousands of different odors," study author Thomas Hummel, of the Smell and Taste Clinic at TU Dresden in Germany, told Braindecoder. "But we don't perceive them all; we just perceive a small portion of them consciously."
Since the gazillions of different odors that exist are not relevant to us all the time, only a certain percentage of smell receptors are normally expressed. But we may begin to express the receptors that were not expressed before once an odor becomes relevant to us.
Sensory information that we receive from our environment is normally filtered in the thalamus, but this part of the brain is largely bypassed when it comes to receiving olfactory information. The new findings suggest that specific anosmia may therefore serve as a filter of olfactory information that can be altered by exposure to smells.