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The majority of us can probably agree that the sound of nails on a chalkboard is unpleasant, but why is that? Theories range from evolutionary survival mechanisms to the anatomy of the human ear. Find out more in this episode of SciShow!

Hosted by: Hank Green
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Sources:
http://www.jneurosci.org/content/32/41/14184.full
http://www.ncl.ac.uk/alumni/news/page.htm?nasty-noises-why-do-we-recoil-at-unpleasant-sounds-copy1
http://scitation.aip.org/content/asa/journal/jasa/124/6/10.1121/1.3006380
http://www.ncbi.nlm.nih.gov/pubmed/10848570
http://www.livescience.com/16967-fingernails-chalkboard-painful.html
http://www.smithsonianmag.com/ist/?next=/science-nature/what-is-the-most-annoying-sound-in-the-world-75317235/
http://www.theguardian.com/uk/the-northerner/2012/oct/10/newcastle-university-scientists-brain-unpleasant-sounds-reaction?newsfeed=true
https://www.psychologytoday.com/blog/i-got-mind-tell-you/201508/the-amygdala-is-not-the-brains-fear-center
.http://io9.com/5951439/what-is-the-most-cringe-inducing-sound-of-them-all?sidebar_promotions_icons=testingoff&utm_expid=66866090-67.e9PWeE2DSnKObFD7vNEoqg.1&utm_referrer=https%3A%2F%2Fwww.google.com%2F
http://phys.org/news/2006-10-fingernails-chalkboard-garner-psychologist-ig.html
http://www.sound101.org/badvibes/horriblesound.php?factoid=fingernails
http://philpapers.org/rec/MCDACI-4
http://thebeautifulbrain.com/2011/10/the-evolution-of-chalkboard-torture/
http://www.bu.edu/phpbin/news-cms/news/?dept=1127&id=41330

Image Sources:
https://commons.wikimedia.org/wiki/File:10.1371_journal.pbio.0030137.g001-L-B.jpg
https://commons.wikimedia.org/wiki/Category:Amygdala#/media/File:Amygdala_position.png
[SciShow intro plays]

Now, everybody’s got their own personal taste in music. Your driving tunes might sound to me like a couple of computers got in a fist fight, and my wind-down playlist might sound to you like you’re stuck on hold with customer service. But no matter how different our musical tastes, I bet there are at least some sounds we can agree on. Namely, the worst, most skin-crawling, ear-covering, cringe-inducing ones like fingernails on a chalkboard. Just saying it gives me the willies.

And it turns out that there’s a good scientific reason why certain sounds set most people’s teeth on edge: human ears are extra sensitive to a particular range of pitches, although where that sensitivity comes from is still up for debate. In 1986, a group of neuroscientists tried to get to the bottom of these intense reactions by doing what so many researchers do: unpleasant things to volunteers. Specifically, they asked 24 adults to rank a series of 16 sounds based on how unpleasant they thought those sounds were.

And although that isn’t a very big sample size, the noises that were consistently rated the nastiest -- like the sound of a fork scraping a plate or squealing brakes -- all had something in common: They share a frequency range between 2,000 and 5,000 hertz, where our ears happen to be especially sensitive. The researchers wondered if there might be an evolutionary explanation for this, and tried comparing those sound waves to the calls of seriously distressed macaque monkeys. They found that there were some similarities between those warning calls and the nasty scraping sounds, and suggested that our strong dislike for those noises might be leftover from an ancient reflex.

And that kind of does make sense -- I mean, it’s helpful to be especially alarmed by the sounds of your friends screaming. But it’s also kind of hard to prove. Plus, a 2004 study that subjected cottontop tamarins to those icky sounds and found that the animals reacted in similar ways to both the bad noises and just plain old white noise. So maybe our hatred for those sounds isn't a vestigial trait -- or at least, not one we share with cottontops.

It could be that we’re remembering the image of nails on chalkboard, or imagining the feeling of our own fingers scraping -- and that’s what triggers the willies. And it’s working for me... Then again, that’s just one study, and it’s possible that the results would have been different in, say, macaques, or other species. But whether or not our reactions have an evolutionary basis, humans are generally more sensitive to these kinds of noises. And there are a few different theories about how our brains turn a seemingly-innocent scrape into a painful screech.

One 2012 study out of Newcastle University tried to get a better picture of what these noises do to human brains by just looking directly at those brains. They subjected 16 people to 74 different sounds -- from running water to the scrape of knives on glass -- and then asked them to rank the sounds on a scale from pleasant to excruciating. Once again, the worst of the sounds fell in that bad-Hertz sweet spot, but more interesting than how the offensive sounds ranked was what they did to the people’s brains.

With the help of an MRI machine, researchers kept tabs on which brain parts lit up in response to the more irritating sounds. They found a strong connection between the auditory cortex, which processes sound, and the amygdala, which is involved in emotions like anger and fear. So it’s possible that when we hear these cringe-worthy noises, the amygdala influences the auditory cortex’s response, making it more sensitive and causing a negative emotional reaction.

Basically, that horrible screechy-scrape of nails-on-a-blackboard makes your amygdala take charge -- and suddenly, the auditory part of your brain processes that awful sound much more intensely than it would something more soothing, like flowing water or a soft applause. Other researchers think these noises might suck so bad because the anatomy of the human ear amplifies particular frequencies -- to the point that they trigger real, physical pain.

So, there are a lot of different possibilities, and we may never have a definitive answer for why we loathe these noises so much. But to all the teachers out there still using chalkboards: please, try to be careful.   Thank you for watching this episode of SciShow, which was brought to you by our patrons on Patreon. If you want to help support this show, you can go to patreon.com/scishow. And don’t forget to go to youtube.com/scishow and subscribe!