Quirks and Quarks

Scientists surprised to discover women lacking olfactory bulbs can smell just fine

The olfactory bulb in the brain was thought to be an essential part of our sense of smell

The olfactory bulb in the brain was thought to be an essential part of our sense of smell

Researchers in Israel have found multiple cases of women who lack brain structures thought essential to the sense of smell, and yet who detect odours without any difficulty.

The olfactory bulbs in our brain are two small structures on the lower front part of the brain, above the nasal cavity. Sensory nerves from the nose bring signals to them that are then routed to other parts of the brain. The role of the olfactory bulb was thought to be in discriminating and filtering smells, and then acting as something of a map to appropriately interpret and route smell information. 

But an accidental discovery seems to suggest this may not be the case — at least not always.

Noam Sobel, a professor of neurobiology at the Weizmann Institute of Science in Rehovot, Israel, and his colleagues were doing MRI imaging of research subjects as part of a study on smell and reproduction.

Sobel's team found one woman who, to their great surprise, entirely lacked olfactory bulbs, despite the fact that she said she had a completely normal sense of smell.

How does she smell? Perfectly.

What followed was a long series of tests of her sense of smell and brain scans at higher resolution. Her sense of smell did, indeed seem to be normal. Her brain, on the other hand, wasn't

"We could never find evidence for an olfactory bulb in her brain," Sobel said in an interview with CBC Radio's Quirks & Quarks

Sobel figured they had a case study of just one person. Yet in the course of their study on left-handed individuals (handedness can have some influence on brain activity so they were focusing on them as a control in the experiment), they found another woman missing olfactory bulbs. 

In these MRI images, the brain of the woman on the left has olfactory bulbs, and the brain of the woman on the right is missing them. Both women have normal senses of smell. (Sobel et al.)

Two is beyond a coincidence, Sobel said. It suggested to the researchers that there's some association with sex and handedness and olfaction. 

A sinister smell connection

That's where another research project called the Human Connectome Project came in. The project includes brain scans and smell scores on 1,100 people. When Sobel's team reviewed the whole database, they found four individuals with no olfactory bulbs and a perfect sense of smell. All four were women. Two of the four were left-handed. 

The researchers extrapolate this to estimate that perhaps as many as four per cent of left-handed women have a normal sense of smell and no apparent olfactory bulbs. 

What does this all mean? Sobel said there's two main interpretations, one he calls boring and the other dramatic. 

The first is that the brain someone solves the lack of olfactory bulbs in some novel way. It would be an example of brain plasticity, where the brain takes over the task from the missing area.

"The more exciting version is … the current notion that we have on how the system works is not totally right. You in fact do not need that map in order to detect, discriminate or identify odorants."

Which possibility does Sobel favour? "At the risk of jeopardizing my career, the latter. I think that these maps of activity on the surface of the olfactory bulbs are not the way our brain decides that there is an odour or what the odour is."