What happens when people are missing important parts of the brain?

December 3, 2019

Written by: Claudia Lopez-Lloreda

 

Have you ever wondered how exactly we are able to perceive the world around us? How we see, hear, or smell? Our brains are equipped with specific areas and groups of cells that control our senses and actions. When we see an image, the signal travels all the way from our eyes through different groups of cells until it reaches an area called the visual cortex. This allows our brains to understand the exact image that we are observing. Neuroscientists have long studied the importance of these areas in allowing us to perceive the world through the different senses.

But what happens when scientists find people without these supposedly crucial brain areas? Sometimes, they turn out to have obvious impairments. For example, a woman born without the cerebellum, the part of your brain that controls posture and movement, was unable to walk until age nine1. These types of findings point to the importance of specific brain areas. But remarkably, sometimes it seems like missing an important part of the brain has no effect on a person. At all.

This is what researchers at the Weizmann Institute of Science in Israel discovered recently. While imaging brains of people for another study, they found a woman that seemed to lack olfactory bulbs. Olfactory bulbs are the two small areas of the brain that control the sense of smell (Figure 1). Air from the environment arrives into the nasal cavity which fills up with odorants, airborne odor molecules that make up a smell. The molecules then activate the endings of cells called olfactory sensory neurons that send a signal up to the olfactory bulbs. The olfactory bulbs integrate this information and send it out to other parts of the brain, like the cortex, to tell you what you are smelling.

Head_Olfactory_Nerve_Labeled
Figure 1. Odor molecules come into the nasal cavity and activate the nerves of the olfactory sensory neurons (olfactory nerves). Olfactory sensory neurons then send their axons to the olfactory bulb through small holes in the cribriform plate. There, the axons make connections with cells in the olfactory bulb, which then send the signal to other areas of the brain.

Damage to or lack of olfactory bulbs can cause anosmia, the inability to smell. However, the woman missing olfactory bulbs did not seem to have any apparent impairments in her ability to smell, which surprised the researchers.

The scientists sought to explore this one case of the missing olfactory bulbs, which they recently published in the science journal Neuron2. Surprisingly, when looking for normal-smelling controls for the study, they found a second woman who also seemed to lack olfactory bulbs. The researchers decided to study the two women and compare them to normal smelling women with intact olfactory bulbs.

First, they wondered whether the women seemingly lacking olfactory bulbs actually had them but were simply so small that normal imaging procedures would not be able to detect them. So, to make sure that they in fact did not have any olfactory bulbs, they used ultra-high resolution imaging. Again, no sign of the olfactory bulbs.

For this reason, they expected the women to be either unable to smell (anosmic) or have an altered sense of smell. However, the study showed that they were able to smell just fine! By using a standardized smell test, they compared the women to 88 normal smelling controls and identified that they actually had an intact sense of olfaction.

The next question on their minds was not if, but how they were smelling. Maybe they were able to detect smells, but not accurately identify different odors. They found that when asked to detail what exactly they were smelling with words, the women missing olfactory bulbs described the smells in the same way that 140 controls did. Not only they were able to smell, but they smelled similarly to women with intact olfactory bulbs.

Finally, they dug into publicly available data to assess just how common the absence of olfactory bulbs is. They found three more women who apparently lacked olfactory bulbs, but also had an intact sense of smell. In fact, they actually performed better than the average in olfactory tests. They did some math and estimated that approximately 0.6% of women and 4.25% of left-handed women potentially lack olfactory bulbs.

How could this be happening? Studies in rodents had suggested that even after lesioning the olfactory bulb, rats were able to retain their sense of smell3. It is believed that this could occur because the olfactory sensory neurons that sense the smell “bypass” the lesioned or the nonexistent olfactory bulb and send the signal directly to the cortex.

Another reason this could be happening is because we also use other senses to decipher the smell. The gustatory system, or the taste system, is very interconnected with the olfactory system. For example, sometimes we may not even be able to taste anything if we have a cold. Therefore, it is possible these women employ the use of other systems to be able to compensate for the loss of olfactory bulbs to accurately identify different smells. If neuroscientists are able to identify which of these mechanisms works to compensate for smell, they could try to promote them in people with anosmia to try to rescue their sense of smell.

It is still a mystery why exactly women who lack olfactory bulbs are able to smell so well. But no matter the reason, it does raise some doubt about olfactory bulbs and other brains areas—we may not even need them!

 

 

References

  1. Yu, F., Jiang, Q.-J., Sun, X.-Y., & Zhang, R.-W. (2014). A new case of complete primary cerebellar agenesis: clinical and imaging findings in a living patient. Brain, 138(6). doi: 10.1093/brain/awu239.
  2. Weiss, T., Soroka, T., Gorodisky, L., Shushan, S., Snitz, K., Weissgross, R., … Sobel, N. (2019). Human Olfaction without Apparent Olfactory Bulbs. Neuron. doi: 10.1016/j.neuron.2019.10.006.
  3. Slotnick, B., Cockerham, R., & Pickett, E. (2004). Olfaction in Olfactory Bulbectomized Rats. Journal of Neuroscience24(41), 9195–9200. doi: 10.1523/jneurosci.1936-04.2004.

 

Images

Cover image by Reigh LeBlanc via flickr, CC BY-NC 2.0.

Figure 1. From Patrick J. Lynch on Wikimedia commons, CC BY 2.5.

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