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A bubble freezing on a block of ice inside of a freezer. Image Credit: Farzad Ahmadi and Christian Kingett

The physics of freezing soap bubbles is cooler than you’d think

<p>If there’s anything more mesmerizing than iridescent swirls dancing across a soap bubble, it’s the sight of icy stars whirling around that bubble as it freezes.</p><p>So when a <a href="https://www.youtube.com/watch?v=H7pqoCJQp2I">YouTube video</a> of a bubble freezing made its way to mechanical engineer Jonathan Boreyko and his team at Virginia Tech, they weren’t just awed. They wanted to understand the physics behind the stunning display of swirling ice crystals.</p><p>“A lot of things in research are more motivated by a desire for truth or beauty,” Boreyko says, “like just trying to understand a new and mysterious thing.”</p>
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<p>The team’s <a href="https://www.nature.com/articles/s41467-019-10021-6">study</a>, published last month in the journal <i>Nature</i> <i>Communications</i>, reveals the physics of the “snow globe effect,” a phenomenon that causes hundreds of tiny ice crystals to float across a bubble’s surface and freeze simultaneously in a whimsical display.</p>
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<p>What Boreyko and his team set out to determine was where the hundreds of small ice crystals in the snow globe effect come from. Typically, when a relatively small quantity of a liquid freezes—like when water turns into an ice cube—the freezing process begins from a single point of origin. That point creates an expanding boundary between frozen and unfrozen liquid called a freeze front. As the freeze front expands outward, it cools the remaining liquid, causing it to change phase and become ice.</p><p>But in bubbles, Boreyko and his team noticed, this process can be different. When the researchers froze some bubbles in a walk-in freezer and took photos and videos of them, they saw what looked like hundreds of freeze fronts floating simultaneously across the bubbles’ surfaces.</p>
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<p>To look more closely at the physics behind the multitude of freeze fronts, the team developed a mathematical model to simulate bubbles freezing. The modeling, Boreyko says, was the hardest part. “Once all the experiments were done, it took over a year to fully model and understand the physics behind the stuff we were seeing,” he says.</p><p>What the team discovered follows the fundamental process of freezing itself: The moment a liquid freezes, it gets slightly warmer. This is because as a liquid’s molecules rearrange and fuse into a solid, excess energy is released in the form of heat, called latent heat.</p><p>When a bubble is in a freezing environment and lands on a cold surface, the first thing to freeze is the bottom of the bubble. As this freeze front expands, it releases latent heat into the adjacent liquid surface of the bubble. This warms the water at the edge of the freeze front ever so slightly and causes it to flow toward the top of the bubble.</p><p>As more latent heat is released, the water flow grows stronger. This puts stress on the freeze front and causes extremely tiny ice crystals to break off and slide across the bubble’s surface. These crystals then begin to grow simultaneously, giving the impression that there are multiple freeze fronts forming at once.</p><p>“It actually is only one freeze front,” Boreyko says, adding that the cold surface provides a point of origin to start the growth of ice crystals.</p><p>“This paper was not about ice, really. It was more about liquid flow,” says Kenneth Libbrecht, a physicist at Caltech specializing in ice crystals and snowflake formation who was not involved in the study. The study’s unique finding, he says, is that freezing creates liquid flows on the surface of bubbles that create a feedback loop, which enhances the freezing process itself.</p><p>This new understanding of the snow globe effect could someday help experts engineer better ways to freeze things, including biological material like human tissue.</p>
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<figcaption><p>The progression of the "snow globe effect" in three images. Image Credit: Farzad Ahmadi and Christian Kingett</p>
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<p>Currently, the low-temperature freezing of human tissue, called cryonics, is mostly limited to freezing small tissue samples or single cells, says Steven Van Sciver, a professor studying cryogenics and cryonics at Florida State University who was not involved in the study. Larger samples, such as full organs, he says, become damaged during the freezing and thawing process.</p><p>“Most of what you see in Star Wars and Star Trek and Hollywood is things like freezing people and bringing them back to life,” Van Sciver says. “And most of it is a hoax.”</p><p>But, to researchers looking for better ways to freeze things, bubbles present a unique opportunity, he says. “Understanding how these crystals are formed in very thin bubbles and how they move around could be helpful in understanding biological freezing done with cryogenics.”</p><p>But Boreyko says his team didn’t have an end use in mind when they started this project, just a desire to understand what they were seeing.</p><p>“People are always thinking about applications,” Libbrecht says. “I think that’s the wrong way to think about it. In this case, the research is very inexpensive....It’s OK to spend a little bit of money trying to understand what’s going on around you.”</p>
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<p>Freezing soap bubbles look like snow globes. This whimsical effect could help us improve biological freezing techniques—and is incredibly fun to watch.</p>

A magician transforms one card into another. Is it magic or science? Image credit: Emily Zendt, WGBH

Poof! Science reveals how easily a magician can fool you

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<p>With a simple gesture, a magician can transform a playing card right in front of your eyes. But the illusion isn’t magic—it’s science.</p><p>A <a href="https://journals.sagepub.com/doi/abs/10.1177/0956797618822969">study published earlier this year</a> in the journal <i>Psychological Science</i> reveals that no matter how closely you watch a magic trick, you’ll probably miss the sleight of hand.</p><p>Imagine a magician waves a card back and forth, seemingly causing it to transform into another card. The small motion, swapping one playing card for another, is hidden by the larger motion happening at the same time, the wave of a hand (like in the video below). This illusion exploits a phenomenon called change blindness, which refers to a viewer not noticing a change in visual stimuli.</p>
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<p>The magician swaps the card at the moment their hand changes direction. That abrupt change in direction is all the misdirection they need to distract the audience from their sleight of hand, says Katherine Wood, an author of the recent study and PhD candidate in psychology at the University of Illinois.</p><p>This is possible because our brains don’t process everything our eyes see.</p><p>“The illusion that we’re always seeing everything all the time is such a powerful part of who we are,” says Stephen Goldinger, a psychologist at Arizona State University who was not involved in the study. Although we feel like we see everything, he says, we only see snapshots of the world around us. “Our brains are remarkably adept at filling in the background.”</p>
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<p>This concept of change blindness isn’t new. Studies like the famous <a href="https://www.youtube.com/watch?v=vJG698U2Mvo">selective attention test</a>, in which people watching one action or activity fail to notice a person in a gorilla suit walk through the background, have demonstrated change blindness in much more complex scenarios. But this is the first time change blindness has been studied in a scenario as simple as a single motion concealing a single change, says Richard Yao, lead author on the study.</p><p>In the study, the researchers had 21 undergraduate students <a href="https://osf.io/aucve/">watch a set of six dots</a> moving as a group across a screen. One of the dots would abruptly rotate 15 degrees while the other five stayed the same. The team asked the students to pick out the rotating dot.</p><p>Although it didn’t happen every time, the participants usually fell victim to change blindness and misidentified the rotating dot. The change blindness effect observed in this study was incredibly strong, says Anthony Barnhart, a psychologist at Carthage College who was not involved in the study.</p><p>Studies like this, Barnhart says, are important in helping understand how we’re able to perceive the world. “In many ways, magicians have a head start on psychologists when it comes to how the brain works,” he says.</p>
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<p>How “change blindness” prevents you from seeing this 10 of clubs turn into an ace of spades.</p>

The rounded shape of the Apidima 1 cranium is a unique feature of modern humans and contrasts sharply with Neanderthals and their ancestors. Image credit: Katerina Harvati, Eberhard Karls University of Tübingen.

Skull fragment shows humans may have been in Europe earlier than previously thought

<p>A skull fragment extracted from the roof of a cave in Greece has sparked controversy among researchers studying early human origins in Europe.</p><p><a href="https://www.nature.com/articles/s41586-019-1376-z">In a study</a> published this week in the journal <i>Nature,</i> researchers presented new analyses on what appears to be a 210,000-year-old <i>Homo sapiens</i> skull fragment. If the findings are confirmed, Apidima 1, as it’s been named, would be the oldest human fossil found anywhere outside of Africa.</p>
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<p>Named after the Apidima Cave in Greece, where it was discovered, Apidima 1 was one of two skulls researchers analyzed in the new study. Anthropologists found Apidima 1 and the second skull, named Apidima 2, encased in rock in 1978.</p><p>Years later, researchers extracted the fossils. Apidima 1 turned out to be half of the back of one skull, while Apidima 2 was 66 fragments of a nearly complete skull. In the new study, University of Tübingen paleoanthropologist Katerina Harvati and her team used a CT-scanner to create 3-dimensional models of the fossils, then digitally reassembled the pieces.</p><p>They found that Apidima 2 has a characteristic outward bulge at the back of the skull, called the occipital bun, which is found in Neanderthals skulls. “It looks like when you put your hair up in a bun,” Harvati told <a href="https://www.nytimes.com/2019/07/10/science/skull-neanderthal-human-europe-greece.html">Carl Zimmer at <i>the New York Times</i></a>. But the reconstructed Apidima 1, the team found, has features consistent with <i>Homo sapiens</i>.</p>
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<figcaption><p>The Apidima 1 partial cranium (top) and its reconstruction from posterior view (middle) and side view (left). Image credit: Katerina Harvati, Eberhard Karls University of Tübingen</p>
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<p>Apidima 1 has a much rounder skull shape than Apidima 2, which makes a compelling case that the skull belongs to an early modern human, Laura Buck, a University of California, Davis paleoanthropologist who was not involved in the study, told Zimmer. “That very round shape is something we tend not to see in other groups,” she told Zimmer.</p><p>Not everyone is convinced by the team’s findings, however. “I cannot see anything suggesting that the individual belongs to the sapiens lineage,” Juan Luis Arsuaga, a University of Madrid paleoanthropologist who was not involved in the study, told <a href="https://www.nationalgeographic.com/science/2019/07/enigmatic-skull-may-be-oldest-modern-human-out-of-africa/">Maya Wei-Haas at <i>National Geographic</i></a>. Arsuaga examined Apidima 1 in 2017 and concluded the skull was Neanderthal.</p>
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<p>Havarti and her team are careful to highlight the limitations of their analyses. Their paper notes that the skulls were distorted and incomplete, and that the site where they were found lacked any other signs of early modern humans, like stone tools.</p><p>“As with any challenging new find, the appropriate initial reaction should be a healthy skepticism, even when my own name is on the paper,” study author Chris Stringer, a physical anthropologist at London's Natural History Museum, told Wei-Haas. “We don’t have the frontal bone, brow ridge, face, teeth, or chin region, any of which could have been less ‘modern’ in form.”</p><p>Although the Apidima 1 skull fragment is just one data point, it’s the start of a much longer search for answers. “Of course it would be lovely to find more,” Harvati <a href="https://www.washingtonpost.com/science/2019/07/10/enigmatic-skull-suggests-our-human-species-reached-europe-years-ago/">told Joel Achenbach at <i>The Washington Post</i></a>. “We intend to try to look.”</p>
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<p>A new analysis of a skull found in Greece decades ago suggests that early humans may have been in Eurasia as early as 210,000 years ago.</p>

A ‘mock meteorite’ made up of a ball of clay. Researchers used this model to show how objects moving through fluids like water will naturally erode into cone shapes. Image Credit: NYU's Applied Mathematics Laboratory

Cone-shaped meteorites are ‘just right’ for plummeting to Earth

<p>If a meteorite streaking through Earth’s atmosphere is too thin, it’ll tumble about. Too wide and it will flutter wildly. But if it’s taken on just the right cone shape, it’ll plummet straight to the ground.</p><p>Roughly a quarter to a third of meteorites that make it to Earth converge on this “Goldilocks” cone shape and, until recently, scientists haven’t been able to explain why. New findings <a href="https://doi.org/10.1073/pnas.1815133116">published last month</a> in the journal <i>PNAS</i> show that the answer has to do with fluid mechanics, the way forces interact with liquids, gasses, and plasmas.</p>
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<p>The story starts back in 2012, with a team of researchers from the Courant Institute at New York University holding balls of clay in a water tunnel to study erosion. The team observed that “if you hold an object fixed and let it erode, it makes a cone,” says study author Leif Ristroph, a mathematician at the Courant Institute who was also part of the team that made the <a href="https://www.pnas.org/content/109/48/19606">initial observations</a>.</p><p>More specifically, he says, erosion always sculpted these cones so their tips, which pointed toward the oncoming water, formed a V with an angle of about 90 to 100 degrees. This tip angle happens to be very similar to those of cone-shaped meteorites found on Earth.</p><p>A ball of clay stuck in water may not seem to have much in common with a meteorite crashing to Earth, but the forces at work on their surfaces aren’t all that different, Ristroph says. Meteorites entering the Earth’s atmosphere encounter intense heat that melts and reshapes them in a process similar to water eroding clay.</p><p>With that in mind, the researches decided to use water to take a closer look to determine if this cone shape might be favorable for meteorite flight: They dropped solid aluminum cones into water and used a video camera to track their motion.</p>
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<p>This is where they witnessed the Goldilocks principle at work. Cones with tip angles that were too slim or too wide moved in unstable paths through the water. But cones that hit a sweet spot between 60 and 100 degrees oriented themselves so that their tips pointed down and fell straight—just as a similarly shaped meteorite would fall through air.</p><p>The reason for this stability is the way forces from the surrounding fluids—­water in the 2012 experiment, and air in the case of meteorites—act on the cones as they fall, Ristroph says. As fluid is displaced by the cones, it flows around them, only to come back together in the cones’ wakes, like the turbulent surf behind a fast-moving boat. While narrow or wide cones get pushed around by these fluid forces, Goldilocks cones are just the right shape to stay stable.</p><p>When to team looked back at their data, they realized the same angles kept popping up and theorized that these erosion and stability effects might be related, says study author Jun Zhang, a physicist and mathematician at the Courant Institute and NYU Shanghai.</p><p>Erosion leads to a cone shape, which leads to stability, which enables further erosion. “These two mechanisms reinforce each other,” says Zhang.</p><p>But there may yet be more to the story.</p><p>Meteorites aren’t just made of clay, Maitrayee Bose, an isotope cosmochemist at Arizona State University who was not involved in the study, said in an email. Meteorites can be made of harder materials, like iron and stone, which could respond very differently to erosion stresses when falling through the atmosphere, she says.</p>
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<figcaption><p>A cone-shaped meteorite found east of the Wiluna township in Western Australia. Image Credit: Jon Taylor, Flickr, Wikimedia Commons</p>
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<p>And it’s not just what meteorites are made of that needs to be considered.</p><p>At supersonic speeds—like those of a falling meteorite—flight dynamics get very complicated, and surface pressure and temperatures can skyrocket, Ed White, an aerospace engineer at Texas A&amp;M University who was not involved in the study, said in an email. A water tunnel experiment may not be enough to properly capture everything that’s going on, he says.</p><p>Ristroph acknowledges these limitations as well. “We’ve sort of solved two parts of the problem individually,” he says, “but how that all works out in the full problem is not yet solved.”</p><p>Given the complexities of meteorite flight, it may be difficult to replicate every aspect of it in a lab. “We have our work cut out for us,” Ristroph says.</p>
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<p>Researchers eroding clay in water may have uncovered secrets of meteorites’ aerodynamic stability.</p>

Asian tiger mosquitoes (Aedes Albopictus) is known to carry several mosquito-borne diseases. Image credit: iStock

Scientists use radiation and bacteria to slash mosquito populations on two Chinese islands

<p>Scientists have nearly wiped out the local populations of one of the world’s most invasive mosquito species on two islands in Guangzhou, China.</p><p>In a <a href="https://www.nature.com/articles/s41586-019-1407-9">study published</a> Wednesday in the journal <i>Nature,</i> an international research team combined two insect-control techniques to prevent Asian tiger mosquitoes (<i>Aedes Albopictus</i>) from reproducing. It’s the first time the techniques have been deployed in combination to control wild mosquito populations.</p><p>The first approach, called the sterile insect technique (SIT), involves releasing huge numbers of insects that have been sterilized using radiation. The objective is for them to mate with their wild counterparts but produce no offspring. The technique hasn’t worked well in mosquitoes, though, because irradiating males enough to sterilize them makes them less competitive as mates. And as a result, females give preference to wild males.</p>
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<p>Using SIT on female mosquitoes isn’t a good option either. Although females require a lower dose of radiation than males to be sterilized, releasing sterilized females doesn’t affect the overall population because males will continue mating with wild females.</p><p>Enter the incompatible insect technique (IIT). Asian tiger mosquitoes are naturally infected with some strains of a bacterium called <i>Wolbachia</i>, and their offspring are only viable if both parents carry the same strain or strains. Scientists can interfere with reproduction by infecting lab-grown male mosquitoes with strains of <i>Wolbachia</i> not present in the wild populations. Unlike irradiated males, the males carrying new <i>Wolbachia</i> strains maintain their competitiveness as mates. And when one of these lab-grown males mates with a wild female, their eggs won’t be viable and the pair will fail to reproduce.</p><p>While IIT is generally effective at reducing mosquito reproduction, it quickly becomes ineffective if even a few females are unintentionally infected with a compatible <i>Wolbachia</i> strain in the lab and released. Females lay hundreds of eggs in their lifetime, and a mother can pass on her strains ofthe bacteria<i>,</i> quickly producing a new generation of fertile offspring that are reproductively compatible with those grown in the lab.</p><p>Combining SIT and IIT, the researchers hoped, could take advantage of each method’s benefits: Batches of lab-grown male mosquitoes could be infected with a new strain of the bacterium, keeping them competitive as mates but incapable of successfully reproducing with wild females. And then exposing the whole group to low-dose radiation could sterilize any females that were accidentally infected along with the males, without compromising the male population.</p><p>So, a team led by Zhidong Xi from Sun Yat-sen University–Michigan State University Joint Centre of Vector Control for Tropical Diseases in China, mechanically sorted Asian tiger mosquito pupae into male and female groups. They infected the male group with a strain of <i>Wolbachia</i> that doesn’t exist in Guangzhou’s wild population, then exposed them to low levels of radiation to sterilize any stray females that may have ended up in the mix. Then the researchers released the mosquitoes weekly during peak breeding season for two consecutive years on both islands.</p>
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<p>This combined approach reduced the number of the local population’s hatched mosquito eggs by more than 94 percent and reduced human bite rates by over 88 percent at the two locations studied.</p><p>“That’s very impressive,” Stephen Dobson, a medical entomologist at the University of Kentucky, Lexington and the founder of MosquitoMate, a company that commercializes <i>Wolbachia</i> as a mosquito-control tool <a href="https://www.nature.com/articles/d41586-019-02160-z">told Giorgia Guglielmi at <i>Nature</i></a><i>.</i></p><p>Asian tiger mosquitoes carry harmful viruses such as dengue and chikungunya, and can pose a significant public health concern. Current methods for controlling them<i>,</i> which include pesticides and removing standing water, are ineffective, Dobson told Guglielmi. And female mosquitoes can live as long as a month, biting humans and continuing to transmit disease throughout their life.</p><p>Though the technique has potential as a public health strategy in regions where the Asian tiger mosquito is found, it’s only a preventive measure, Zhiyong Xi, a medical entomologist at Michigan State University and lead researcher on the study <a href="https://www.newsweek.com/china-scientists-released-millions-infected-sterilized-mosquitoes-wild-wipe-out-1449709">told Kashmira Gander at <i>Newsweek</i></a>. "In terms of disease control, this approach may not be suitable in emergency situations during disease outbreak," Xi told Gander.</p><p>But, the potential to eliminate these pests before they spread could help stop public health crises before they start. “A new tool like what’s being described in this paper is very much needed,” Dobson told Guglielmi.</p>
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<p>Combining two insect-control techniques, researchers largely prevented reproduction in a mosquito species known to carry Zika, dengue, and yellow fever.</p>

Octopuses have problem-solving abilities similar to some vertebrates. But the way these eight-armed enigmas process information is completely different. Image Credit: AJ Fillo, WGBH

Thinking is for suckers, but if you’re an octopus, suckers are for thinking

<p>Like humans, octopuses are incredibly intelligent. But an octopus’ mind is about as alien to the human mind as the human mind is…well, to an alien’s.</p><p>“I like to [ask], ‘How are they intelligent?’ rather than ‘How intelligent are they?’” says Dominic Sivitilli, a behavioral neuroscientist and astrobiologist at the University of Washington who presented a new octopus cognition model at <a href="https://connect.agu.org/abscicon/home">the AbSciCon 2019 conference</a> this week.</p>
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<p>The model is designed to illustrate how octopuses process information, and Sivitilli believes it could eventually provide insight into how non-human intelligence evolved here—and possibly elsewhere in the universe.</p><p>Octopuses have problem-solving abilities similar to those observed in corvids (the group of birds that includes crows, ravens, and jays), parrots, and primates. But the way these eight-armed enigmas process information is completely different. Unlike humans and other vertebrates, which centralize mental processing in the brain, octopuses and other cephalopods use neurons throughout their suckers, arms, and brain to understand their world.</p><p>“What we’re looking at, more than what’s been looked at in the past, is how sensory information is being integrated in this network while the animal is making complicated decisions,” said David Gire, lead researcher on the project and neuroscientist at the University of Washington, in a statement.</p><p>The unique nature of octopus intelligence has sparked a rather peculiar debate recently: A group of researchers (not associated with Gire and Sivitilli’s study) has suggested that an octopus’ mind might seem so foreign because it may be alien. The hypothesis, <a href="https://www.sciencedirect.com/science/article/pii/S0079610718300798">published in 2018</a>, states that octopus evolution may have arisen, in part, because of a retrovirus (a type of RNA virus) delivered to Earth by an asteroid during <a href="https://phys.org/news/2019-06-plate-tectonics-driven-cambrian-explosion.html">the Cambrian explosion</a> about 541 million years ago.</p><p>But as alien as an octopus’ intelligence may seem, Sivitilli is careful to point out that they’re “definitely, based on the evidence in front of us, related to us at some point in our history.”</p><p>“I do not think that octopuses are aliens,” he says.</p>
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<figcaption><p>A Giant Pacific octopus at the New England Aquarium in Boston, Mass. Image credit: AJ Fillo, WGBH</p>
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<p>Because of octopuses’ distributed mental structure, scientists have long said that each octopus arm has a mind of its own. But it may be even more accurate to say that each <i>sucker</i> has its own brain, Sivitilli says. “There are tens of thousands of both chemical and mechanical receptors in each sucker,” he says. “To put that into perspective, each of your fingertips has a few hundred mechanical receptors.”</p><p>To track how their arms think, Gire and his team <a href="https://youtu.be/6laZ9ad-D54">filmed captive octopuses</a> as they explored new objects in their enclosures. With the footage in hand, the researchers used a computer algorithm to determine if the arms were working together, which would suggest central direction from the brain, or independently, which would suggest the arms were making decisions on their own.</p>
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<p>The results, which are preliminary, show octopuses’ arms working both with the brain and on their own. But they also suggest that individual suckers can process information independently—something scientists had previously only hypothesized.</p><p>The team found that if one of an octopus’ suckers finds something interesting, like food, it triggers a sucker next to it to double check. If that sucker is also interested in the potential treat, it triggers the next one. This creates a cascade of neurons down the arm’s ganglia (nerve cell clusters), encouraging the octopus’ arm to then wrap around the item of interest.</p><p>This entire process bypasses the animal’s brain—but the system isn’t perfect. Because the suckers can act independently of the brain there’s not always consensus, Sivitilli explains. “Sometimes, the arm plays tug of war with itself,” he says.</p><p>“This is really exciting work,” said Jennifer Mather, a behavioral psychologist specializing in cephalopods at the University of Lethbridge who was not involved in the study, in an email.</p><p>But, she says, “they haven’t shown that the arms ‘make decisions.’”</p><p>Scientists have known that octopuses can pass information from arm to arm, bypassing the brain, for years. What they don’t yet understand is how the arms are able to process that coordination.</p><p>Because this study is based only on video observations, Mather says, it doesn’t track the neural information being passed between and within the arms.</p><p>Still, “thinking” with an arm is an impressive feat, Sivitilli says. And, better understanding the way octopuses process information—whether with an arm or their actual brain—may help us better understand other forms of intelligence radically different from our own, including any that may exist elsewhere in the universe, he says.</p><p>Octopuses are clearly from Earth, but that doesn’t mean they can’t teach us something about the stars, Sivitilli believes. “Studying models of cognition on Earth, especial those along different evolutionary paths, can really contextualize the search for extraterrestrial intelligence."</p>
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<p>Octopuses “think” with neurons so distributed throughout their bodies that sometimes the left hand literally doesn’t know what the…left hand is doing.</p>

AJ Fillo