TOP  |
GOOD STUFF  |
NEW STUFF  |
SEARCH








 
FINGERNAILS ON BLACKBOARD
Explained at last?
2003 W. Beaty

links

Fingernails on a blackboard, or cat claws scraped across an old chalky car hood! Or... a steel garden hoe hitting a smooth dry rock. Or tear a ball of cotton between your fingers. Write with a nearly-dry magic marker. Trying to saw through a brick with a steak knife. Rub a block styrofoam across cardboard. EEEEEEEURRRRRRRRT!!!

How can a simple sound make you feel so awful? Why do humans seem programmed to avoid it? We want to make it stop right freakin' now.

The answer came to me like a blinding flash. I was eating something at a picnic and I dropped it on the ground. I wiped it off and continued eating. (Oh, you DO SO do it too!)

As I was chewing, suddenly I heard SKKKKEEEEEERRRCH!!!!! ...as I bit down hard on a tiny stone. I think every single hair on my body stood on end, and my jaws froze instantly.

THAT'S IT! Fingernails-on-blackboard: it sounds exactly like the destruction of tooth enamel. It's not learned. Instead, we're instinctively programmed to respond instantly. Of course! It's so sensible and obvious. Every little kid knows it. I remember many incidents from my own childhood. Why didn't we adults ever realize? The scraping of fingers on a blackboard is the classic, high-frequency violin-like waveform of hard dry surfaces moving with chaotic stick/slip motion. And that could very well be why our instincts are programmed to repond to it so strongly, as strongly as a pain signal.

It's the sound of body damage; but it's a particular type of body damage for which there is no pain ...yet no healing.

We get no second chance with teeth. If we bite down on rocks, we wreck our enamel, and that could be why fingernails-on-blackboards makes everyone around us take drastic action to halt that noise. Why didn't anyone realize this origin? It's because we're too damned civilized, and we rarely have rocks in our food anymore. But whenever we bite down on something which is far harder than tooth-enamel, our inborn pain-avoidance instantly informs us about the problem in no uncertain terms. Sensible? Flesh can heal, but tooth surfaces do not.

I had rather hear a brazen canstick turn'd,
Or a dry wheel grate on the axle-tree;
And that would set my teeth nothing on edge...
- Shakespeare, Henry IV
But SOUND is not pain. Our skin is full of nerve endings, and pain normally teaches babies what not to do. The thick outer layer of our teeth lacks the pain sensors of other tissues. Won't animals need something besides pain to inform them that they're damaging themselves in a permanent way? Fingernails-on-blackboards could be entwined with evolution: animals who respond strongly to that particular noise will guard their teeth carefully, and they won't ever bite down on rocks if they can possibly avoid it. Animals who ignore that noise will die early from broken teeth. We're the product of a billion years of successful ancestors who can't freakin' stand the loud internal sound of our own teeth scraping on rocks. And hearing fingernails on chalkboard ...makes our teeth feel funny!

Is the above idea actually correct? Who knows. Well, I think mine's a much better theory than the recent paper about the Macaque warning calls. And I haven't seen any papers about teeth-on-edge written elsewhere. Perhaps scraping our fingernails on blackboards isn't making fellow primates stop and look for danger. Perhaps it's worse than that, since if the sound is a form of pseudo-pain, then it's forcibly informing all the reptillian brains of all the nearby organisms to stop biting that rock! NOW!!

Predictions

If I'm right about all of this, then it isn't just primates who will respond. Any delicate-toothed creature with a sense of hearing/vibration will hate the sound. It might be evolutionarily old; going back all the way to mollsuks or earlier, to the original appearance of hard bioapatite shells, teeth, and predation. But how can we test whether non-primate dogs and cats (and clams? snails?) can't stand fingernails-on-blackboards? Hmmmm, this might not apply to rodents (lab rats.) Rodent incisors grow continuously, so its less of a big deal when a rat finds itself nibbling on rusty steel or small rocks. Same with sharks and their conveyor-belt of replacement teeth. Or, perhaps it's tooth-damage plus mirror-neurons of animal groups with group-selection pressure, so dog packs might have the aversion, while lone pumas might not. With others, the aversion would be triggered by knawing on soft material, with bursts of the aversive sound added, in sync to jaw closing. (Animals who don't care when others bite rocks, *would* care when they themselves seemingly bite rocks.)

If I'm right, then the type of audio waveforms which trigger our revulsion will have major similarity to the audio waveforms heard via tissue/bone conduction when our teeth suffer damage by scraping across a hard object.

This might not necessarily be a particular sort of spectrum shape. If phase between spectral components can be sensed by brains, then time-domain features in the waveform itself might be more important for detecting tooth-damage than features in it's frequency spectrum. Or, perhaps the internal bone-conduction signal is the avoidance signature, and if sounds in air gets filtered so they resemble the tooth-damage signal inside tissues, that triggers our avoidance response.

Ooo! Idea! Maybe this auditory sensing of tooth-damage is why mammal/reptile/etc. ears evolved to be near their jawbones in the first place? If ears tended to move much farther away with time, then the bone-conduction sound wouldn't work very well, and that horrible noise of squealing tooth-enamel wouldn't be so... informative.

Going even farther: what if ears originally evolved to be tooth-damage sensors?

This would explain where the sense of hearing came from in the first place. Preventing constant bodily damage is an important advancement in the evolutionary track. The ability to detect the sounds of approaching underwater predators might have just been a fringe benefit, a spinoff from the vibrational tooth-damage detector in the jawbone. This leads to another prediction: the atmospheric sound channel wouldn't necessarily have the same evolutionary reinforcement as the through-flesh internal or 'bone conduction' channel. Someone should check to see if the frequency and phase detection of the internal signal channel has any vast differences from ears. Evolution would be expected to redesign ears to become excellent at the two very separate tasks (at least in toothed animals, not in all of them.) In particular, ultrasound is suppressed by air, so we might expect that the internal through-tissue channel might evolve to include the extreme shortwave ultrasound signals of enamel-destruction far about 20KHz, while the gas-vibration channel might have relatively little need for such wideband detection.

Subjectively to its victims, fingernails/blackboards seem very similar to a pain signal. Instant inbuilt avoidance! If you want to give yourself the willies, imagine biting down hard upon a dirty iron bar or on a smooth, dry pebble. Imagine grinding your teeth back and forth on the hard rough object so it squeaks... and CRUNCHES. (Almost as bad as imagining thorns under fingernails, or sharp branches poking towards your eyes.) The surface of the chewed pebble is just like chalkboard slate, but this time it's not your fingernails making the noise, it's your teeth. Yeesh. For me all this is like like visualizing stroking my fingertips along a knife edge, or watching Buñuel's sliced-eyeball film (Heh. Another "instinctive avoidance algorithm" programmed deeply. I call it by the name Triggered Creepout Effect: the everyday evidence of mammal instict remaining in humans.)

Now that I think about it, this might also explain the reason for the existence of "baby teeth." If pain doesn't work very well in stopping certain destructive behaviors, and if our instincts and our biology need to somehow *teach* us not to bite down hard on rocks... then it will take some time for us to get the hang of it. We could have the pseudo-pain avoidance response built in, but we don't instinctively know that bone is OK to knaw, but pebbles are not. But with teeth, we only get one chance. First, the baby arrives pre-programmed with chalkboard-noise sensitivity. Then baby bites on rocks a few times, experiences revulsion-sound, and learns not to do this any more. But by then it's too late, and Baby's teeth have suffered significant damage. No matter. Baby can shed teeth once. Finally no big need to shed teeth in adulthood, since the acoustic pseudo-pain already had taught Baby the lessons earlier... don't knaw rocks or mommy's socket set. Hmmm. In the same vein, I wonder if our sensitivity to fingernails/blackboard sound decreases in adulthood? As with childhood pain extrasensitivity, the pseudo-pain could wither away without major consequence. Baby animals would exhibit far more need to be sensitive to the signal. If we ever test whether non-primate animals are sensitive to the teeth-on-edge effect, we should also test across age, check out puppies or kittens for a human-like response childhood extreme response.

Suppose an animal ignores the tooth-damage sound. Why didn't we just evolve in such a way that teeth are rapidly replaced, like rats and sharks? But this constantly repeated teeth-shedding would cause problems for mammals. A fighting animal can't afford to be missing some teeth unnecessarily. Over tens of thousands of years, the few humans who have ongoing "multiple dentitions" might get into trouble during the times their old teeth are falling out and the new ones haven't yet grown back. Unless their teeth were almost certainly going to get ruined over time, there would be no good reason for new teeth to grow. Better just learn to avoid biting stone adzes and flint scrapers and obsidian arrowpoints and socket sets, and instead evolve the system to where teeth aren't being replaced.

Yet as above, there might be one significant event where a new set of teeth would give an overall payback: a one-time repair for the months of trial and error during the babyhood knawing stage. Hey, nature gives us a cheap set of "training-teeth" to destroy, before the long-term serious ones appear.

All speculation, of course. But it's like a missing puzzle-piece evolutionarily. Man o man, look at the explanatory power.

If I'm right, then someday the trivia-gam experts will know that there's a clear connection between fingernails on blackboard, the position of human ears and jawbone, and the need for our "baby teeth."

And when people say that the sound of fingernails on blackboards always "sets their teeth on edge"... or if they call it "teeth-gnashingly annoying"... we should leap up and shout "eureka That's IT!!!"

PS
Fingernails-on-blackboard is pseudo-pain. If I'm right, then any toothed animal is programmed to respond to it. That means we can build pain guns! Pain guns that repel barracudas, and make mollusks crawl away from us as fast as they can. Or actually "pain" guns. If you're being attacked by a predator, just take out your patented Bill Beaty Tooth-Annoyatron&tm; with Evolutionary Optimized Waveform&tm; and press the button. The agonizing high-power recordings of garden hoes scraping across dusty shale sidewalks will send those sharks or cougars racing away... while shaking their heads and holding their jaws carefully apart!

:)

LINKS

 

SOME EMAIL...

From: "sh"
To:
Subject: re: FINGERNAILS ON BLACKBOARD
Date: Mon, 11 Aug 2003 08:43:54 -0400

Sorry, nice try but you're off the mark (although I particularly liked the tie in between the jawbone and the ear to shift it to a tactile pain analogy - it's an interesting proposition). There have been several studies which demonstrate that the aspect of the "fingernails on blackboard" (or metal rake on concrete, in my case) sound which causes so much distress is actually due to aperiodic repetition of sounds in the 8-13 kHz range - well outside any tactile or vibratory input [1] (Halpern et al, 1986). In fact, it seems to be related to high frequency sounds made by human (or other primate) infants (Lounsbury & Bates, 1982) - with the aperiodicity (which could be caused by extreme distress, interrupted breathing etc) increasing the irritation factor. You can actually synthesize sounds in the highest frequency range which induce a similar reaction that are barely audible to most adults, but induce the reaction anyway (e.g., exposure to the ultrasonic components of machinery or dental drills can induce fear or anger).

Incidentally, just to let you know I'm not just spitballing here, I'm an auditory neuroscientist who has worked on this type of thing for a long time, and have a company which specializes in using sound to evoke specific emotional states with applicaytions in film, music and software(NeuroPop). But I really love your website and have been coming to it for many years - keep up the good work, fun links and interesting questions

SH, PhD

Date: Tue, 12 Aug 2003 01:15:22 -0700 (PDT)
From: William Beaty <>
To: "sh"
Subject: re: FINGERNAILS ON BLACKBOARD

On Mon, 11 Aug 2003, sh@.com wrote:
> Sorry, nice try but you're off the mark (although I particularly liked the
> tie in between the jawbone and the ear to shift it to a tactile pain
> analogy - it's an interesting proposition).

I certainly admit that it's total speculation! :)

> There have been several
> studies which demonstrate that the aspect of the "fingernails on
> blackboard" (or metal rake on concrete, in my case) sound which causes so
> much distress is actually due to aperiodic repetition of sounds in the 8-13
> kHz range - well outside any tactile or vibratory input (Halpern et al,
> 1986).

Uh... why do you mention "well outside tactile/vibratory"? I don't understand what this means, or how this disproves the bitten-stone idea. Biting a stone generates the high-freq sound in question.

If "bitten-stone" is wrong, I'd like to know the details of why this is so. (Also, if you try to disprove the idea that a bitten stone makes people cringe, won't you also disprove the idea that the fingernails/ chalkboard sound makes people cringe? To me they sound almost identical.)

By "tactile" are you talking about skin sense as opposed to hearing? The noise of chalk on chalkboard is certainly SOUND, not tactile or skin vibration, and it's certainly broadband and has components well above the one-KHz range. It would be interesting to put a contact microphone on my molars and then examine the sound spectrum of stone-biting.

>
> In fact, it seems to be related to high frequency sounds made by
> human (or other primate) infants (Lounsbury & Bates, 1982)
>

I've NEVER heard any sounds made by my baby daughter, or by animals on TV nature-shows which can trigger my "fingernails/chalkboard" response. That's why I concluded that the primate-screech idea was a load of BS as soon as I heard it proposed. It's simple: those sounds don't make me cringe. It's a nice theory, but it doesn't work in the real world. I've also never heard anyone else ever complain that monkey screeches make them cringe in that way.

But a metal tool dragged across a rock DOES make me cringe. So does a tooth dragged across a dry pebble. My own "fingernails/chalkboard" response is all about mechanical oscillators which suffer an outbreak of chaos, and I've never felt that "cringe" response from any vocal-style noises.

If someone has a recording of animal sounds which reliably trigger the usual fingernails/blackboard "cringe" feeling, I'd like to know about it. (If they exist at all, I'd expect that they already would be as well known an irritation as dragging fingernails on blackboards!)

On the other hand, there are a wide variety of squealing noises which trigger the effect for me personally, and all of them involve Dynamical Chaos in mechanical oscillators. Stick-slip Chaos of damped/driven oscillators is usually called "bearing chatter." Dragging a piece of chalk backwards across a chalkboard is a classic example of a Chaos signal.

> - with the
> aperiodicity (which could be caused by extreme distress, interrupted
> breathing etc) increasing the irritation factor. You can actually
> synthesize sounds in the highest frequency range which induce a similar
> reaction that are barely audible to most adults, but induce the reaction
> anyway (e.g., exposure to the ultrasonic components of machinery or dental
> drills can induce fear or anger).

> Incidentally, just to let you know I'm not just spitballing here, I'm an
> auditory neuroscientist who has worked on this type of thing for a long
> time, and have a company which specializes in using sound to evoke specific
> emotional states with applicaytions in film, music and software(NeuroPop).

Cool!

Are you aware that the aperiodic "sounds of chaos" always have a particular spectrum which contains fractal features?

I've long wondered exactly what it is about the fingernails/ blackboard sounds which triggers my own response. Would a simple high frequency tone with random chopped modulation do it? If not, then maybe my brain is keying in on something in particular, such as a signal which contains a *fractal distribution* of frequency peaks, one-over-F pattern.

Might you know if fractal frequency distributions tend to trigger the fingernails/chalkboard response?

(((((((((((((((((( ( ( ( ( (O) ) ) ) ) )))))))))))))))))))
William J. Beaty SCIENCE HOBBYIST website
http://amasci.com
EE/programmer/sci-exhibits amateur science, hobby projects, sci fair
Seattle, WA 206-762-3818 unusual phenomena, tesla coils, weird sci

Date: Sun, 7 Mar 2004 20:01:41 -0800
From: Tracy Butler tab2006@.edu
To:
Subject: Comments from billb amform

--- comments ---
I came across your site while searching for blackboard paint (to disguise an ugly file cabinet plus make it useful), but leaving off the "t." Your explanation for the nails on chalkboard effect is perfect... and testable: functional MRI during the sound labeled as "nails on chalkboard" versus an identical sound labeled as something innocuous like "teakettle" should activate the teeth area of secondary somatosensory cortex. I'm a neurologist doing fMRI research, and I'll credit you if I ever end up doing this. There's actually quite a bit of funding for dental pain research. Thanks for a fantastic website.

[Hrm, it might not link to dental pain nerves. There obviously are two pathways, the direct dentin damage versus the acoustic sensor, so where do they combine? The acoustic signal isn't felt as normal pain, so I'd predict that the two channels are totally independent, perhaps even ending in two brain regions both of which trigger aversive response. Or in other words, the YEEEESH! is nothing like OWWWW! But if the region was discovered, amygdala, it would be interesting to find if the same location lights up from other adversive 'yeesh' experiences (such as bruxation obviously, but perhaps watching films of eyeballs stabbed with needles, fingernails with toothpicks stabbed under, etc. -billb 2015]


[1] FREQUENCY: I find that vibration frequencies conducted in solids go up into at least hundreds of MHz, not 13KHz. Also, there's a little- known phenomenon where humans can hear 70KHz just fine under tissue conduction, and the detector is the human otolith/utricle



 





http://amasci.com/amateur/screet.html
Created and maintained by Bill Beaty. Mail me at: .
View My Stats