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Some women who lack olfactory bulbs seem to be able to smell without a problem, according to a study published today (November 6) in Neuron. The finding is a surprise to neuroscientists, who have considered the brain structures to be the sites of scent processing.

When odors enter the nose and trigger chemical receptors there, electrical signals travel to the olfactory bulbs of the brain, where clusters of nerves called glomeruli process the smell’s signal. Without olfactory bulbs, a person should be unable to process odors, researchers had assumed.

In a study of healthy, left-handed women, neuroscientist Noam Sobel of the Weizmann Institute of Science in Rehovot, Israel, and colleagues happened to identify two women who appeared to lack olfactory bulbs. But when they conducted an odor test, the women showed no impairments to their sense of smell.

Turning to MRI scans of more than 1,000 people made public through the Human Connectome Project, which also includes participants’ scores on an odor identification test, Sobel’s team found three more women, one of whom was also left-handed, in the same boat—they had no detectable olfactory bulbs but performed normally on the smell exam. Extrapolating the findings to the general population, the scientists estimate that more than 0.5 percent of women, and more than 4 percent of left-handed women, can smell despite lacking typical olfactory bulbs.

“Replication within a publicly available dataset adds a lot of credibility to the finding,” Sobel says in a press release. “This is not just some oddity that shows up only in our hands.” (The researchers found no men in the dataset who lacked olfactory bulbs.)

Sobel says the findings suggest that the brain may be plastic enough to develop a sense of smell even in the absence of these seemingly critical brain structures. “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,” he says in the release. “Although such plasticity is amazing, it is not out of the realm of what we have seen in human development.”

MRI scans of a healthy control (A) and three women (B–D) thought to be missing their olfactory bulbs. Yellow arrows point to the location of missing olfactory bulbs. NEURON , DOI:10.1016/J.NEURON.2019.10.006, 2019

University of Pennsylvania neuroscientist Jay Gottfried isn’t so sure. “I am not convinced that the women are indeed missing their bulbs,” Gottfried, who was not involved in the study, tells Science News. They may simply be very small, and would therefore require antibodies to visualize.

Moreover, notes Rutgers University neuroscientist John McGann, the two women who underwent smell testing had trouble picking up low concentrations of a roselike scent. “It’s important to note that their [sense of] smell was not quite normal,” McGann, who was also not involved in the work, tells Science News. “But,” he adds, “there’s no question [they] could smell.”

Sobel says it’s unclear why the phenomenon appears to be more common in left-handed individuals, and he is now looking to test additional people to better understand what might be going on.