Friday, April 26, 2024

Answer: Consider the alternative?

 Most animals have a left- and a right-side...  

P/C. OpenAI [create a grid of diverse animal noses including human, dog, cat, gorilla, etc. ]

... nose... that is, we're (mostly) bilaterally symmetric, even to the level of the nasal passages.  I have two kidneys, two eyes, two nostrils, etc.  (Yes, I know--one heart, one stomach, one intestine... those are special cases.  But even your brain comes in two halves.)    

But when I noticed that I seem to switch from my left to my right nostrils, it caused me to wonder a bit about this bilaterality.  If I really DO switch nostrils during the day, what else might I switch from left to right?  Do my kidneys work differently at different times of the day?    

Let's turn this into a SearchResearch Challenge for this week: 

1. Is my nose just weird, or do people really have a slow change in breathing from side-to-side over the course of the day?  

I was really unsure about where to start, so I did the first search I thought of: 

     [ switch nostrils breathing during the day ] 

which led me to an article in Live Science ("Why don't we breathe equally out of both nostrils?")  that told me "..The dominant nostril switches throughout the day. This is called the nasal cycle.." 

That's an interesting phrase, nasal cycle.  Live Science tells us that people tend to breathe out of one side, then switch to the other.  That article points to a paper in PLOS One (a respected journal), "Measuring and Characterizing the Human Nasal Cycle."  That paper in turn tells us that "Nasal airflow is greater in one nostril than in the other because of transient asymmetric nasal passage obstruction by erectile tissue. The extent of obstruction alternates across nostrils with periodicity referred to as the nasal cycle."  

Fascinating.  Who knew that we have small patches of erectile tissue in our noses that alternate the side of breathing.  

A quick search in Google Scholar for [ nasal cycling ] leads to a bunch of papers, including this one in the journal Rhinology ("The nasal cycle: a comprehensive review") In that paper, we read "... It is based on the dilation/constriction of the venous cavernous tissue in the submucosa of the turbinates and septum, but also of the ethmoid sinuses. It is accepted that almost 70-80% of adults experience a regular
nasal cycle, but a true periodicity/reciprocity exists only in 21-39% of the population."  

Even more details--most people have nasal cycling, but a strong nasal cycling seems to be present in around a quarter of all people, although it's a little unclear why this happens.  It's a complicated mechanism--what's the evolutionary advantage?  My friend Rehan Khan sent me an article that he co-authored in Nature (1999).  That paper, "The world smells different to each nostril" suggests that the reason for switching the sides of your nose is that each nostril has a somewhat different set of receptors--so switching gives a better, higher-quality perception of what's going on in the environment. "..we show that this difference in airflow between the nostrils causes each nostril to be optimally sensitized to different odorants, so that each nostril conveys a slightly different olfactory image to the brain."  

In any case, this look like a real effect.  And now we picked up some terminology for the alternation--this should help us with the next Challenge.    

2. What other kinds of behaviors might happen for a while on one side of the body, and then switch to the other side of the body?  (Think outside the body: What about non-humans?  Do they have these odd behaviors?) 

I started with this query: 

     [ alternating cycles in humans  ] 

I didn't know if I'd find anything or if I'd just get some ideas to pursue.  

The results mostly did the latter, and suggested a couple of bilaterally symmetric systems to check--e.g., ovaries, kidneys, brain hemispheres, eyes, ears, etc.  But the results weren't incredibly specific.  So, I had to try each system in turn: 

     [ ovaries alternating cycles ] [ kidney alternating cycles ] etc... 

The first query (about ovaries) led me into a rabbit hole where search led to some results that indicated that humans DO ovulate alternatively on the left one month, and then on the right the next month.  This was apparently believed for quite a while (largely because it seems like common sense, and detecting the side of ovulation takes some pretty high resolution ultrasounds.  But once that visualization technology became available, it wasn't long until a decent sample size was reached, and it was discovered that NO.. in fact women ovulate or more less randomly (wrt left vs. right)!  I found this paper through my searches: "Side of ovulation and cycle characteristics in normally fertile women" published in the journal, Human Reproduction in April, 2000.  

On the other hand, searching for alternating phases of kidneys, testes, eyes, and ears all led nowhere.  I wasn't able to find ANYTHING to suggest that there are interesting alternations of perception or production.  

BUT, when I searched for :

     [ brain hemisphere alternating cycles ] 

I found (way down on page 2) a mention of hemispheres alternating in sleep patterns for marine mammals and certain birds!  

The paper "Unihemispheric sleep and asymmetrical sleep: behavioral, neurophysiological, and functional perspectives"  (in the journal, Nature and Science of Sleep) tells us that "... certain marine mammals and species of birds show a different sleep behavior, in which one cerebral hemisphere sleeps while the other is awake. In dolphins, eared seals, and manatees, unihemispheric sleep allows them to have the benefits of sleep, breathing, thermoregulation, and vigilance. In birds, antipredation vigilance is the main function of unihemispheric sleep.."  

Even more details:  ".... In some aquatic mammals, sleep and wakefulness periods alternate between the hemispheres, and it is the only way of sleeping. In other animals (seals and birds), unihemispheric sleep is a transitory sleep event intermingled with bilateral sleep." 

I checked multiple sources (naturally), and found lots of corroboration--those marine mammals listed DO sleep just one side at a time.  Remarkable.  I hope to see a semi-sleeping dolphin one day while out diving in the sea.  

SearchResearch Lessons 

1. Count on your friends.  Shortly after I wrote my SRS Challenge, Rehan wrote to say that he just happened to have published a paper on this topic.  It's a wonderful paper, well worth the read.  And it both confirmed that smelling alternates sides AND that there's a reason for the effect.  

2. Search for leads, not just answers.  It's often the case that when searching you'll learn something along the way that's useful in your later searching.  That's what happened when I did a search for [ alternating cycles in humans ] and found some suggestions that I was able to follow up and find gold.  Sometimes the path to an answer is another step away... and you need to find the terminological bridge to get there.  

3. Sometimes you just have to check each category.  When I did my search for alternating cycles of ovaries, kidneys, etc., I did so because I couldn't come up with a single search term what would capture the idea of "bilateral pairs of organs in human bodies."  It would have simplified my search if I had, but I couldn't find one.  So... I had to check each by hand, hoping that would work.  And it did!  

Hope you had as much fun as I did in searching for the answers.

Keep searching! 


1 comment:

  1. One more thing--I JUST read another fascinating paper about sleep asymmetry in LIZARDS. (I thought to search for lateralized sleep in lizards because they're not that distant from chickens..) To my surprise, I found this: "Brain Hemispheres Battle it Out During Sleep" -- with this astonishing quote: “Most animals (birds, reptiles, amphibians, fish, insects, mollusks, etc.) are, like humans, bilaterally symmetrical, and posess bilaterally symmetrical brains. .. during one phase of sleep, the two halves of the Pogona [lizard] brain compete with one another such that one side imposes its activity on the other, until the dominant hemisphere switches over to the other side, alternating back and forth throughout the night… As the night progresses, the fraction of REM sleep increases at each sleep cycle. In Pogona, the sleep cycle is much shorter (less than 2 minutes) and the two sleep states are equal in duration (45-60 seconds each) throughout the night. A dragon undergoes 250-350 such sleep cycles each night, alternating regularly between its versions of slow-wave and REM sleep” You can read the article at: Birds do it, dolphins do it, even little lizards do it...