This neural structure has so far been considered necessary for the existence of a sense of smell in mammals
Philosophers, since the days of Plato and Aristotle, distinguish between "necessary properties" and "accidental properties". A necessary property of something is a property without which it has no existence. A necessary feature of the sense of sight, for example, is the existence of eyes, while the color of the eyes or the length of the eyelashes are merely accidental features. Scientists of the Weizmann Institute of Science recently discovered that the sense of smell in humans can exist well even in the absence of an olfactory bulb - which until now was considered necessary for its existence. In a study published today in the scientific journal Neuron, the scientists showed that about 0.6% of women in general and more than 4% of left-handed women have a completely normal sense of smell, even though they lack olfactory bulbs. These findings may change the accepted theory on the functioning of the sense of smell in mammals.
The olfactory bulb is not only the first stop in the neural pathway from the nose to the brain, but it is also considered to have a decisive role in coding smells, and is therefore necessary for the act of smelling. Therans from birth, who cannot smell, have no olfactory bulb. How, then, does the system work according to the dominant theory? Our nose has about six million smell receptors divided into about 400 types. The large amount of information absorbed in them is transmitted via the olfactory nerve from the nose to the bulb, where all the data coming from a certain type of receptor converges at a specific point known as the glomerulus. This is how a unique smell map is created which is transferred from the tubercle to the olfactory center in the cerebral cortex. In 2004, Richard Axel and Linda Buck won the Nobel Prize for these findings. More than 20 years before the win, there were scientists who claimed that rodents' sense of smell remained active even after their olfactory bulbs were damaged, but the research methods they used and their findings were dismissed. And maybe they smelled something after all?
The new findings were discovered by chance, during an experiment in the laboratory of Prof. Noam Sobel in the Department of Neurobiology. Dr. Tali Weiss and Dr. Sagit Shoshan discovered during magnetic resonance brain scans (MRI) at the Azrieli National Institute for Imaging and Research of the Human Brain that one of the subjects who claimed to have a normal sense of smell, did not have an olfactory bulb. "We asked the subject if she was sure that her sense of smell was normal as she stated, and she was proud that it was not only normal, but excellent," recalls Prof. Sobel. "But not everyone who claims to have a normal sense of smell actually has one. In fact, even full-fledged tanners sometimes only discover this at a late age. But after conducting all the possible tests on her, we can state that she is right: her sense of smell is not only normal, it is Excellent. And yes, no tubercle was found in her - not even in particularly high-resolution brain scans."
The researchers, led by Dr. Weiss and research student Tamna Soroka, thought that the case they brought up was an exception that does not indicate the rule. They did not imagine that an even bigger surprise awaited them along the way. To understand how the sense of smell works in the brain of the subject, The researchers performed functional magnetic resonance imaging (fMRI) on her and compared the data to a control group, because the subject is left-handed, and there are functional differences Between lefties and righties, the researchers invited only left-handed women to the control group. "To our surprise, the ninth subject was found to have a normal sense of smell without a bulb. At this point all the alarm bells started ringing," says Dr. Weiss.
How is it possible that a finding that has never been documented was discovered by chance - twice - in Prof. Sobel's laboratory? "First, the tubercle is indeed a neural structure with a respectable volume of 58 cubic mm, but those who do not look directly may certainly miss its absence," says Prof. Sobel. "Secondly, in neuroscience studies that do not deal directly with hand preference, they will avoid "Most of the time, the subjects are left-handed - both men and women - as they add a source of variation to the findings and complicate them."
In fact, the researchers discovered that these findings were indeed recorded - without anyone noticing. They turned to an open database of brain scans, known as The Human Connectome Project and used by researchers around the world. The database stores brain scans of left-handed people according to their proportion in the population (about 10%) and contains brain scans of 1,300 people, men and women, many of them identical twins. Dr. Weiss and Soroka - with the help of Liav Tagnia, a high school student who is in the laboratory as part of an educational project - scanned all the data in the database, which also included a sense of smell score for each of the subjects. They did not find a single man with a normal sense of smell who did not have a tubercle, but they found Four women with a normal sense of smell, and even more so, and without bulbs. Of the four women found, three were left-handed. "Amazingly, three of the women on the left have identical twins with tubers whose data appears in the database. In all three cases, twins without tubercles have a better sense of smell than their twins," Soroka says.
How do the surprising findings align with the known concept of the sense of smell? One interpretation is that the sense of smell does work exactly as the dominant theory states, but thanks to extreme brain flexibility, the glomerulus array, which produces the smell map, is formed in these women elsewhere in the brain. The second possible interpretation is that the popular perception regarding the mechanism of the sense of smell is incorrect, and this is already an earthquake, but in order to decide on the matter, imaging technology with a higher resolution than that currently available in humans will be needed. "In my opinion, it is definitely a possibility that the sense of smell works in a simpler way than is currently believed. The concept of the olfactory map in the bulb sees the sense of smell as a complex multidimensional representation, but it is possible that this sense operates in a much lower and much simpler dimension. In any case, these are heretical reflections in our field , just like saying the world is flat (or round, depending on the point of view)," says Prof. Sobel. "Bottom line, these women smell the world properly, and we have no idea how they do it."
