How can you smell that?

By Nick Carne

Can humans smell the world around them even if they don’t have olfactory bulbs?

That’s not a question that’s often asked, but the answer – a surprising “yes” – appears to provide more evidence of the brain’s amazing flexibility.

And, say researchers from Israel and Australia, it also offers some hope that people born with congenital anosmia – an inability to smell – might be able to learn to do so. 

During olfaction, odour molecules activate sensory neurons in the nose. These send signals to the brain where each odour is mapped in patterns of small neural structures called glomeruli within the olfactory bulb. These glomeruli maps are considered essential to smell.

However, writing in the journal Neuron, researchers led by Noam Sobel from Israel’s Weizmann Institute of Science describe how, while reviewing MRIs of healthy, left-handed females for a different study, they found two with a sense of smell despite lacking apparent olfactory bulbs. 

Intrigued, they reviewed a further 1113 MRIs made public through the Human Connectome Project and found three more such women – one left-handed.  

“Replication within a publicly available dataset adds a lot of credibility to the finding. This is not just some oddity that shows up only in our hands,” says Sobel. 

Extensive tests of the first two women proved that they could indeed identify, detect and discriminate between odours as well as the average person. {%recommended 7528%}

Sobel and colleagues then ran a series of further tests – involving MRIs, diffusion MRIs, ultra-high definition 3D imaging, and whole-brain voxel-based morphometry – on these two women and a third who was known to have congenital anosmia.

These confirmed that all three had no visible olfactory bulbs but otherwise normal brain structures outside the olfactory system. So why could two of them smell?

“The simplest interpretation of our findings is that these women were born without an olfactory bulb, but thanks to the extreme plasticity of the developing brain, they developed an alternative glomeruli map somewhere else in the brain, not in the olfactory bulb,” says Sobel. 

“Although such plasticity is amazing, it is not out of the realm of what we have seen in human development.”

Amazing and potentially valuable.

As the researchers note in their paper, missing olfactory bulbs is probably an irreversible state, so not much effort has been directed at early detection of congenital anosmia. It is typically first diagnosed when people are in their teens.

However, if some humans can in fact develop olfaction without apparent olfactory bulbs, they add, “perhaps such compensatory mechanisms can be promoted early in life, when neural plasticity is at its highest”.

“Currently in the West, newborns are tested for vision, audition, and more, all within the first hours or days after birth,” they write. 

“It is perhaps time to start screening children, or perhaps even babies, using non-verbal measures of olfaction. Early identification of reduced olfaction could then perhaps be addressed within an odour enrichment program in the aim of triggering compensatory mechanisms…”