Of all the five senses, the sense of smell has been the least understood. Scientists and doctors have long believed that olfactory bulbs – located in the forebrain of vertebrates that receives neural input about odors detected by cells in the nasal cavity – are necessary to pick up smells.
The olfactory nerve is responsible for our sense of smell and partially responsible for our sense of taste. The function of this very short nerve, which bypasses the brain stem and connects directly to the brain, is to detect chemicals in the air and send information about them to the brain, where the data are interpreted.
Cells in the nasal cavity are lined with tiny hairs called cilia and connected to an axon, which is the main section of a nerve cell. Each axon passes through small holes in the skull called the cribriform plate. Just below the brain, they all pass into the olfactory bulb, where the signals from each axon come together and are sent on to the brain via the olfactory tract.
According to what scientists believed up to now, olfactory bulbs combining the information from the six million receptors in our noses, of some 400 different types, and encoding a unique “odor” signal to be passed on. Thus, unsurprisingly, some people who are congenitally anosmic ‒ that is, they never had a sense of smell ‒ indeed have no olfactory bulbs.
But now, researchers at the Weizmann Institute of Science in Rehovot have discovered an amazing thing – one doesn’t have to have olfactory bulbs to smell! They found that 0.6% of women, and more specifically, up to 4% of left-handed women, have excellent senses of smell, even though they lack olfactory bulbs.
Their findings, just published in the prestigious journal Neuron, seems likely to uproot some conventional theories that describe the workings of our sense of smell.
Although the centrality of the olfactory bulbs in olfactory perception is the “textbook” view, in the 1980s and 1990s, some researchers had removed the olfactory bulbs from the brains of rodents and found their sense of smell to remain functional. These findings, however, were not well received in the scientific community.
The new findings were unexpected: Drs. Tali Weiss and Sagit Shushan in the lab of Prof. Noam Sobel of the Weizmann Institute’s neurobiology department were conducting MRI scans of subject’s brains in the Azrieli National Institute for Human Brain Imaging and Research on campus.
One of the subjects, who had stated her sense of smell was normal, was found to be lacking olfactory bulbs in her brain. The woman insisted that not only was her sense of smell was normal – it was excellent. “We tested her smelling faculties in every way would could think of, and she was right,” recalled Sobel. “Her sense of smell was indeed above average. And she really doesn’t have olfactory bulbs. We conducted another scan with especially high-resolution imaging, and saw no signs of this structure.”
At first the researchers, led by Weiss and research student Timna Soroka, thought this might be the sort of exception that doesn’t disprove the rule. They took functional MRI scans of her brain and compared them with those of a control group. But they needed a unique control group. Since the subject was female and left-handed – both traits that can influence the organization of the brain – the researchers invited other left-handed women to have their brains scanned for comparison.
“When the ninth subject in the ‘control’ group also turned out to be lacking olfactory bulbs, alarm bells started ringing,” said Weiss.
The olfactory bulb, at around 58 cubic millimeters in volume, is visible to the naked eye in images, but, noted Sobel, if someone is not specifically looking for this structure, they are liable to miss it – or to miss its absence. And because the differences connected to handedness can complicate datasets, some researchers even stick to right-handers, assuming their findings on such things as the olfactory system will be relevant to left-handers as well.
And yet once they started looking for evidence for this phenomenon, the research team found it in an existing database: that of the Human Connectome Project. Out of the 1113 brain scans in this database, many of them of identical twins, some 10% are from left-handed individuals (around their incidence in the general population). And among the records contained in this open database are the results of smell tests given to the subjects.
Weiss and Soroka, with the help of Liav Tagania, a high school pupil working on an educational project in Sobel’s lab, searched through the information in the database. They did not find a single male who had both an intact sense of smell and a missing olfactory bulb, but they did find four women with both. Of those four women, two were left-handed. “Even more amazing was the fact that the two left-handed women were identical twins, and their twin sisters did have olfactory bulbs. Each of those with no olfactory bulb in their brains scored better on the smell test than their intact twin sisters,” said Soroka.
The Rehovot team suggest several possible explanations for the phenomenon – one is that the highly plastic brain creates a “smell map” in a different part of these women’s brains. But another possible explanation is that these exceptions do disprove the rule. “Current ideas posit the olfactory bulb as a ‘processing center’ for information that is complex and multi-dimensional, but it may be that our sense of smell works on a simpler principle, with fewer dimensions. It will take high resolution imaging – higher than that approved for use on humans today – to resolve that issue,” says Sobel. “But the fact remains that these women smell the world in the same way as the rest of us, and we don’t know how they achieve this.”