The sense of smell plays a significant role in the way we experience and enjoy the world. When a food or drink is particularly pleasant to our palate, this happens thanks to our nose as much as our tongue. Smells play a major role in remembering past experiences, positive as well as negative ones. Despite its importance, for years and up to the present time, the sense of smell has been the least understood of all the senses we use to perceive the world. Thus, in contrast to vision and hearing tests performed on babies shortly after birth, it’s not common practice to examine if their sense of smell is functional.
Since the 1990s, this sense has been receiving increasing scientific attention, culminating in the 2004 Nobel Prize in Medicine and Physiology, which was awarded to Linda Buck and Richard Axel for their studies which contributed to the understanding of the genetic and anatomical basis of the sense of smell. But now, a new study from the Weizmann Institute of Science just published in the journal Neuron, questions the basic assumptions made by Buck and Axel about the way this sense works in the brain. Their theory stipulated that two structures situated in the forebrain, the olfactory bulbs, are the first and essential station in the processing of information received by the brain from the nose.
Scientists began their in-depth studies of the olfactory bulbs in the 1990s. The 2004 Nobel Prize was awarded partly due to Buck and Axel’s success in showing how receptors in the nose translate a chemical message into a nerve signal that reaches the olfactory bulb, where different representations correspond to different smells. Nowadays, the prevailing conception is that the olfactory bulbs are a vital stage in the processing of olfactory information.
Therefore, says Prof. Noam Sobel, in whose lab at the neurobiology department at the Weizmann Institute the new study was carried out, he and his colleagues were surprised to find a female subject without olfactory bulbs. This was detected by Dr. Tali Weiss and Dr. Sagit Shushan during a brain scan of a woman who was part of a control group in another study that was carried out in their laboratory.
After this discovery, relates Prof. Sobel, the investigators asked the woman to take some tests of her sense of smell. These were standard tests that examine the ability to identify smells and distinguish between them. In addition, another test developed in Sobel’s lab produces an “olfactory fingerprint” which is unique to every person. The bulb-devoid woman passed all these tests with flying colors.
In order to study this effect in depth, the researchers performed an fMRI scan, which examines the activity of different brain areas while a subject performs different tasks, based on changes in regional blood flow. The scan looked at the way her brain processed smells. First, the researchers formed a control group that had identical characteristics. Since the woman was around 30 years old and left-handed, they gathered more than 20 left-handed females of a similar age. And then, says Sobel, “We were amazed to see that subject number nine also lacked olfactory bulbs.” This woman also passed the smelling tests, showing a perfectly normal sense of smell.
At this stage, they turned to an open global database of brain scans in order to further validate their discovery, in order to find out if this was a universal phenomenon. The database contained scans of a thousand subjects from across the world, as well as the results of similar tests of their sense of smell. Dr Weiss and her student Timna Soroka, with the help of high school student Liav Taganya who was in the lab as part of an educational project, managed to find four women (but no men) with no olfactory bulbs but with a normal sense of smell. Three out of the four were left-handed.
Curiously, although not statistically significant, says Prof. Sobel, all four had twin sisters who were part of the database. The twins had standard olfactory bulbs, but their sense of smell was inferior to that of their bulb-less twins.
In the final stage of the study, says Prof. Sobel, they performed fMRI tests on their olfactory-bulb-devoid subjects and found that their brain function while smelling was no different than that of women in the control group, who all had olfactory bulbs. Sobel notes, however, that fMRI tests of smelling are “noisy”, with limited resolution. One can thus only say that there was a general similarity in function.
Dr. Rafi Haddad from the Gonda Brain Research Center at Bar-Ilan University, who heads a research lab that studies the sense of smell, says that this study is very convincing. Haddad, who was Sobel’s student, says that it will be difficult to explain these results using current conventions.
Prof. Sobel nevertheless expects that not all the scientific community will be pleased with his discovery. “When I presented this at an annual conference in Florida a few months ago they wanted to kill me” he says. In fact, some studies in the ‘80s and ‘90s that showed that olfactory bulbs in rats and mice are not essential were met with responses that ranged from ignoring these studies to wrecking scientific careers. “I may experience this now” he says with amusement.
However, says Sobel, even though in the last 20-30 years there have been hundreds if not thousands of articles about olfactory bulbs, none managed to show a causal relationship between them and the brain’s ability to decipher smells. “I don’t dispute the fact that information received by receptors in the nose is mapped in the olfactory bulbs. But no article shows that brain actually reads this map in order to identify smells or distinguish between them” he says.
So how can one explain this finding that the bulbs are not essential for the sense of smell? Prof. Sobel says that one explanation could be that this is another example of the enormous plasticity of the human brain and of its capabilities of overcoming obstacles. He says that perhaps these women’s brains found ways of bypassing the absence of olfactory bulbs so that representational maps of information from the nose are formed elsewhere.
In order to see if this is the case, says Sobel, one has to show that cells representing smell develop in other parts of the brain. Although one can follow nerves going from the nose to the brain, current scanning methods don’t have the resolution required to identify clusters of such cells.
Another more exciting possibility, he says, is that olfactory bulbs aren’t essential for identifying smells or distinguishing between them. One way of strengthening this assumption is to show that the bulbs serve another function connected to the sense of smell. In most mammals, the sense of smell also serves to locate sources of smell in space, something humans don’t use much. If researchers could show that the women without olfactory bulbs cannot spatially identify a source of smell that a control group does successfully, they would have a new explanation for the role of these structures.
Another possible direction of research is finding a link between these bulbs and a social sense of smell. In recent years there is a growing understanding that humans communicate (usually unconsciously) also through smells. Many studies talk about the smell of fear and the way it’s contagious. Thus, says Sobel, his lab plans to investigate whether the absence of olfactory bulbs impairs these women’s ability to decipher social messages that are transmitted through smells, thereby better understanding the importance of these structures for our cognitive abilities.
And what’s the explanation for the fact that most of these women were left-handed? “We have no idea,” admits Prof. Sobel. “We have no theory to explain it.”
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