Some women lack odour-detecting part of brain but still sense smells

The olfactory bulb, a structure at the very front of the brain, plays a vital role in our ability to smell. Or, at least, so we thought. A research team has now discovered a handful of women who have a perfectly normal sense of smell but who seem to lack olfactory bulbs – completely altering our long-held views about smell. 

The team, including Tali Weiss and Noam Sobel at the Weizmann Institute of Science in Israel, were looking for a connection between the ability to smell and reproduction. Their investigations suggested one of the female volunteers in their experiments didn’t possess olfactory bulbs. This is unusual but not remarkable: one in 10,000 people don’t have olfactory bulbs, and they can’t smell.

This woman was different. Sobel says the 29-year-old was adamant: I have a very good sense of smell, she claimed. Every test they threw at her suggested she did indeed have a good ability to distinguish odours, despite lacking the neurons typically believed necessary to do so. Then, as the researchers continued to investigate, they found another woman with the same curious condition. “We were blown away,” says Sobel. 

 Women only The researchers then turned to the Human Connectome Project, which gathers olfaction scores for all participants. They looked at information from about 600 women and 500 men, and found another three women who can smell without olfactory bulbs. None of the men had the condition. Using this data, they were able to estimate the odds of being a woman who can smell normally without these brain structures to be around 0.6 per cent. Mysteriously, the odds are raised dramatically in left-handed women. Among people who are left-handed, women who lack olfactory bulbs have a roughly four per cent chance of still being able to smell normally. Sobel says there are two suggested explanations for the findings. 

Perhaps this is just further proof as to how plastic, or adaptable, the brain is: it if lacks olfactory bulbs, it can reorganise itself so other brain regions can take on the task of odour perception. Alternatively, the discoveries might hint that we know less about our ability to smell than we thought. For now, it is impossible to distinguish between these two possibilities. Sobel says the next step is to image the brains of 500 left-handed women, with the aim of identifying about 20 with this condition. Using this larger sample size, Sobel and his colleagues want to probe the limits of the ability to smell in the absence of olfactory bulbs. Doing so could, they hope, help explain what we really need our olfactory bulbs for. Andreas Schaefer at the Francis Crick Institute in the UK says while the findings are unexpected, they make sense when you think about rodent studies and how rats can smell even after parts of the olfactory systems are lesioned. “This study is surprising and thrilling, but it makes sense,” he says. 

 Journal reference: Neuron, DOI: 10.1016/j.neuron.2019.10.006