IRA FLATOW: This is Science Friday. I’m Ira Flatow. Take a deep breath in through your nose– shortcut deep into your brain. Hmm. Notice anything different, other than maybe how the room smells? Recent studies suggest that breathing through your nose can be linked to improved memory, and that’s a note to all of you mouth-breathers out there.

Here to untangle how olfaction is linked to deeper parts of the brain controlling emotion and memory is Christina Zelano, assistant professor at Northwestern University Feinberg School of Medicine. Dr. Zelano, welcome to Science Friday.

CHRISTINA ZELANO: Hi. Thank you. I’m happy to be here.

IRA FLATOW: We’re happy to have you. So what is so special about breathing through your nose?

CHRISTINA ZELANO: Well, nose breathing is special because it’s how we smell, and smell is a fascinating and really unique sensory system. So evolutionarily, smell is our oldest sense. It’s also arguably the least understood of our sensory systems. And there are several key aspects of smell that make it particularly interesting and that make nose breathing really different from mouth breathing.

So for one thing, the smell neurons, which live way up at the very top of our nasal cavities– they actually touch the air as it flows in and out of our noses just during natural breathing. And this is really unusual because these olfactory neurons are the only part of our central nervous system that makes direct contact with our external world. And the way that this information gets from these neurons inside of our noses into the brain is different from all other sensory systems.

So all other sensory systems information relays through a structure called the thalamus before it reaches the cortex and higher order brain areas. But in the olfactory system, information flows straight from the nose into the cortex and higher order brain areas as well without going through the thalamus.

And what’s special is that these olfactory structures are located in a part of the brain called the limbic system, which is involved in emotion and fear, learning, and memory. So this gives olfaction a sort of privileged access to emotion and memory areas in the brain. And because nose breathing is smelling, nasal inhalation provides a sort of entry point by which breathing rhythms can modulate brain activity in these structures.

IRA FLATOW: So just the act of breathing in through your nose– does your nose also sense that air is coming in and alert this pathway and the endpoint in your brain to say, hey, something interesting is about to happen?

CHRISTINA ZELANO: Right. Yes, actually, it does. So the receptors inside of the nasal cavity are not only responsive to chemicals. So they’re not only monitoring the chemicals in our environment, but they also are the [INAUDIBLE] receptors. So they can detect the air flowing in and out of the nose, yes. And we found that these natural respiratory rhythms– just breathing, even in the absence of any smell– drives activity in olfactory areas.

But interestingly, these respiratory-driven brain waves– they don’t stop there. So we found that they also propagate to nearby limbic structures, the ones involved in emotion and memory. So what this means is that as we breathe, activity across the limbic system is rhythmically increasing and decreasing with inhales and exhales. And this pattern– what we found is that it’s only present when you’re breathing through your nose. So if you breathe through your mouth, that rhythmic limbic activity goes away.

IRA FLATOW: You know, a lot of us believe, maybe erroneously, that you can suddenly smell something that you remember from 30 years ago. Is that a real thing? Well, for me it’s real, but is that something special to that sensory organ?

CHRISTINA ZELANO: Yeah. You know, the evidence is not clear. The studies are sort of conflicting on that. A couple of studies have found that perhaps olfactory memories tend to be earlier, not necessarily stronger. But I would say there’s no clear consensus on whether or not that’s true.

IRA FLATOW: What about breathing through your mouth? Does that do anything for you? I mean, as far as tasting food, we’ve always been told that you smell the food through your nose and it helps your taste buds taste what you’re actually doing. Is there a connection there?

CHRISTINA ZELANO: Right. That is true. That’s a good point, and that’s what’s called retronasal olfaction. So when we have food inside of our mouths, the air can get from our oral cavity into our nasal cavity, but it flows in the backwards direction. So it’s going outward rather than inward, and it definitely impacts flavors and taste, yes.

IRA FLATOW: Let’s talk about your study. How were you able to study this? I understand that you were able to test this with an fMRI?

CHRISTINA ZELANO: Oh, actually, no, and that’s a very interesting point. We did not use fMRI because in the human brain, the olfactory structures are actually located very, very deep, almost in the center of the brain, making them very hard to reach non-invasively. So we can’t record from these areas from electrodes on the surface of the scalp. And functional neuroimaging– we cannot really measure brain oscillations with that technique.

So to do this study, we worked in collaboration with brain surgeons who implant electrode wires directly into these limbic areas for patients who are undergoing brain surgery for epilepsy, and this provides a really rare opportunity to record this type of data from human olfactory and limbic structures.

IRA FLATOW: If olfaction and memory are related, do you see people with memory problems also having issues with smell?

CHRISTINA ZELANO: Right. That’s a really good point. A lot of neurodegenerative diseases actually do present with early olfactory decline. And while I think these findings that we’re discussing may not have direct implications for neurodegenerative disease, I think the impact of breathing on limbic activity certainly suggests potential new avenues and directions of research on these diseases.

For example, it would be interesting to know if these mechanisms may be altered in neurodegenerative disease. And I also think that our data point to the possibility that neurodegenerative disease could result in altered nasal breathing patterns. Breathing is so easy to measure, and this is something that we’re looking into in the lab. So do patients with different neurological disorders– do they breathe differently? Do they have characteristic nasal breathing patterns?

IRA FLATOW: And so that’s something you’re going to be following up with.

CHRISTINA ZELANO: Right. Yes.

IRA FLATOW: And any neurodegenerative disease in particular?

CHRISTINA ZELANO: We’re currently collecting breathing data from patients with Alzheimer’s disease and patients with Parkinson’s disease to see if we can differentiate the two states just by analyzing the shapes of their breathing waveforms.

IRA FLATOW: Are we the only– I’m going to just throw this out because I’m really dumb.

[CHUCKLING]

Why do we need to breathe through our mouths if we have our noses? And are we the only animals that breathe through our mouths?

CHRISTINA ZELANO: Yeah, you know–

IRA FLATOW: I’m sorry for that question.

CHRISTINA ZELANO: I really think that’s a great question, and I’ve spent a lot of time thinking about that, actually. I think the answer probably goes far beyond just the sort of immediate thought, which is that if your nose is congested, you have this alternate breathing route so that you can still get air in and still live, right?

IRA FLATOW: My guess number one. That was my guess number one, yes.

CHRISTINA ZELANO: Breathing is a vital function. But we’re finding that it really drastically changes our brain activity. So I think that the full answer to that question really remains to be seen, and I think it’s a really interesting point.

I know that, well, most mammals that I can think of do pant, and panting is typically something that involves mouth breathing. So dogs pant not only when they’re hot, but I think also when they’re stressed. Cats as well. And I think there’s something interesting about that, because mouth breathing is altering activity in these emotion centers and fear centers of the brain. So there may be really something behind that.

IRA FLATOW: Wow. I actually asked an intelligent question.

[CHUCKLING]

You’ll come back when you have the answer to that, OK? Because this is really an interesting question.

CHRISTINA ZELANO: Sure.

IRA FLATOW: Christina Zelano, assistant professor at Northwestern University Feinberg School of Medicine. Thank you for taking time to be with us today.

CHRISTINA ZELANO: Thank you.

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