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How Does Scratching Relieve an Itch?

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If you have an itch, you scratch it. Scratch and itch; they go together like peas and carrots and everyone—humans, apes, dogs and cats—knows it. What we didn't understand for a very long time was the physiological connection between the two—why a good scratch relieves a bad itch.


A study by a group of neuroscientists at the University of Minnesota recently explained the itch-scratch link.* The group hypothesized that the relief mechanism doesn't take place along the nerves of itchy skin, as had been thought, but deep in the central nervous system, in the same area that the itches themselves are communicated. Previous studies showed that neurons in the spinothalamic tract (STT)—a sensory pathway originating in the spinal cord that transmits information about pain, temperature and touch to the thalamus—were activated with the application of itch-inducing chemicals, and these are the neurons that send itch sensations to the brain.

In the new study, the UM researchers implanted recording electrodes in the spinothalmic tracts (STT) of macaques monkeys (the STT is at the base of the spinal cord; most STT neurons respond to pain and some to both pain and itch). Then researchers injected itch-inducing histamines into the monkeys' legs and watched as the STT neurons fired. They then scratched the monkey's itchy legs with a device that mimicked the feel of monkey fingers, and the firing rate of the STT neurons dropped rapidly.

The sudden drop, the researchers said, is the neurological equivalent of the relief you feel after a good scratch, indicating that itching and relief sensations are both rooted in the spinal cord and relief from an itch comes from inhibiting—via scratching—the STT neurons. Scratching basically tells all those tattle-tale neurons who are whining to the brain about an itch to just shut up already.

scratchingOf course, the itch and the scratch still hold plenty of mystery. When the team scratched the monkey's legs without first inducing an itch, the STT neurons fired in a normal response to stimuli, but the scratching didn't slow the firing.


Scratching also had no effect on neurons' response to an application of capsaicin, the spicy component in hot peppers. The STT neurons, it appears, react differently to the sensation of a scratch depending on whether an itch exists, and the nerve-dampening effect of scratching only works when the neurons are firing because of an itch, not pain. Somehow, the neurons know the difference. Itching isn't all physiological, either; it can be caused by emotional and psychological factors and can even be picked up as a "contagious itch" (a study showed that itching can be induced purely by visual stimuli: watching other people scratch).

Once all that is sussed out, though, the UM team's discovery could lead to ways of duplicating the end results and benefits of scratching (quiet, polite STT neurons) without its drawbacks. That's great news for people with the sorts of chronic itching associated with AIDS, Hodgkin's disease and the side effects of some pain medications. Chronic itching, of course, leads to plenty of scratching, which can lead to skin damage, infections and worse (remember the New Yorker article with the woman who scratched right through to her brain?)

* Davidson et al. Relief of itch by scratching: state-dependent inhibition of primate spinothalamic tract neurons. Nature Neuroscience, 2009; 12 (5): 544

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iStock // Ekaterina Minaeva
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Man Buys Two Metric Tons of LEGO Bricks; Sorts Them Via Machine Learning
May 21, 2017
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iStock // Ekaterina Minaeva

Jacques Mattheij made a small, but awesome, mistake. He went on eBay one evening and bid on a bunch of bulk LEGO brick auctions, then went to sleep. Upon waking, he discovered that he was the high bidder on many, and was now the proud owner of two tons of LEGO bricks. (This is about 4400 pounds.) He wrote, "[L]esson 1: if you win almost all bids you are bidding too high."

Mattheij had noticed that bulk, unsorted bricks sell for something like €10/kilogram, whereas sets are roughly €40/kg and rare parts go for up to €100/kg. Much of the value of the bricks is in their sorting. If he could reduce the entropy of these bins of unsorted bricks, he could make a tidy profit. While many people do this work by hand, the problem is enormous—just the kind of challenge for a computer. Mattheij writes:

There are 38000+ shapes and there are 100+ possible shades of color (you can roughly tell how old someone is by asking them what lego colors they remember from their youth).

In the following months, Mattheij built a proof-of-concept sorting system using, of course, LEGO. He broke the problem down into a series of sub-problems (including "feeding LEGO reliably from a hopper is surprisingly hard," one of those facts of nature that will stymie even the best system design). After tinkering with the prototype at length, he expanded the system to a surprisingly complex system of conveyer belts (powered by a home treadmill), various pieces of cabinetry, and "copious quantities of crazy glue."

Here's a video showing the current system running at low speed:

The key part of the system was running the bricks past a camera paired with a computer running a neural net-based image classifier. That allows the computer (when sufficiently trained on brick images) to recognize bricks and thus categorize them by color, shape, or other parameters. Remember that as bricks pass by, they can be in any orientation, can be dirty, can even be stuck to other pieces. So having a flexible software system is key to recognizing—in a fraction of a second—what a given brick is, in order to sort it out. When a match is found, a jet of compressed air pops the piece off the conveyer belt and into a waiting bin.

After much experimentation, Mattheij rewrote the software (several times in fact) to accomplish a variety of basic tasks. At its core, the system takes images from a webcam and feeds them to a neural network to do the classification. Of course, the neural net needs to be "trained" by showing it lots of images, and telling it what those images represent. Mattheij's breakthrough was allowing the machine to effectively train itself, with guidance: Running pieces through allows the system to take its own photos, make a guess, and build on that guess. As long as Mattheij corrects the incorrect guesses, he ends up with a decent (and self-reinforcing) corpus of training data. As the machine continues running, it can rack up more training, allowing it to recognize a broad variety of pieces on the fly.

Here's another video, focusing on how the pieces move on conveyer belts (running at slow speed so puny humans can follow). You can also see the air jets in action:

In an email interview, Mattheij told Mental Floss that the system currently sorts LEGO bricks into more than 50 categories. It can also be run in a color-sorting mode to bin the parts across 12 color groups. (Thus at present you'd likely do a two-pass sort on the bricks: once for shape, then a separate pass for color.) He continues to refine the system, with a focus on making its recognition abilities faster. At some point down the line, he plans to make the software portion open source. You're on your own as far as building conveyer belts, bins, and so forth.

Check out Mattheij's writeup in two parts for more information. It starts with an overview of the story, followed up with a deep dive on the software. He's also tweeting about the project (among other things). And if you look around a bit, you'll find bulk LEGO brick auctions online—it's definitely a thing!

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Opening Ceremony
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These $425 Jeans Can Turn Into Jorts
May 19, 2017
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Opening Ceremony

Modular clothing used to consist of something simple, like a reversible jacket. Today, it’s a $425 pair of detachable jeans.

Apparel retailer Opening Ceremony recently debuted a pair of “2 in 1 Y/Project” trousers that look fairly peculiar. The legs are held to the crotch by a pair of loops, creating a disjointed C-3PO effect. Undo the loops and you can now remove the legs entirely, leaving a pair of jean shorts in their wake. The result goes from this:

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Opening Ceremony

To this:

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Opening Ceremony

The company also offers a slightly different cut with button tabs in black for $460. If these aren’t audacious enough for you, the Y/Project line includes jumpsuits with removable legs and garter-equipped jeans.

[h/t Mashable]

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