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Wednesday, May 8th, 2019
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Wireless movement-tracking system could collect health and behavioral data We live in a world of wireless signals flowing around us and bouncing off our bodies. MIT researchers are now leveraging those signal reflections to provide scientists and caregivers with valuable insights into people’s behavior and health.
The system, called Marko, transmits a low-power radio-frequency (RF) signal into an environment. The signal will return to the system with certain changes if it has bounced off a moving human. Novel algorithms then analyze those changed reflections and associate them with specific individuals.
The system then traces each individual’s movement around a digital floor plan. Matching these movement patterns with other data can provide insights about how people interact with each other and the environment.
In a paper being presented at the Conference on Human Factors in Computing Systems this week, the researchers describe the system and its real-world use in six locations: two assisted living facilities, three apartments inhabited by couples, and one townhouse with four residents. The case studies demonstrated the system’s ability to distinguish individuals based solely on wireless signals — and revealed some useful behavioral patterns.
In one assisted living facility, with permission from the patient’s family and caregivers, the researchers monitored a patient with dementia who would often become agitated for unknown reasons. Over a month, they measured the patient’s increased pacing between areas of their unit — a known sign of agitation. By matching increased pacing with the visitor log, they determined the patient was agitated more during the days following family visits. This shows Marko can provide a new, passive way to track functional health profiles of patients at home, the researchers say.
“These are interesting bits we discovered through data,” says first author Chen-Yu Hsu, a PhD student in the Computer Science and Artificial Intelligence Laboratory (CSAIL). “We live in a sea of wireless signals, and the way we move and walk around changes these reflections. We developed the system that listens to those reflections … to better understand people’s behavior and health.”
The research is led by Dina Katabi, the Andrew and Erna Viterbi Professor of Electrical Engineering and Computer Science and director of the MIT Center for Wireless Networks and Mobile Computing (Wireless@MIT). Joining Katabi and Hsu on the paper are CSAIL graduate students Mingmin Zhao and Guang-He Lee and alumnus Rumen Hristov SM ’16.
Predicting “tracklets” and identities
When deployed in a home, Marko shoots out an RF signal. When the signal rebounds, it creates a type of heat map cut into vertical and horizontal “frames,” which indicates where people are in a three-dimensional space. People appear as bright blobs on the map. Vertical frames capture the person’s height and build, while horizontal frames determine their general location. As individuals walk, the system analyzes the RF frames — about 30 per second — to generate short trajectories, called tracklets.
A convolutional neural network — a machine-learning model commonly used for image processing — uses those tracklets to separate reflections by certain individuals. For each individual it senses, the system creates two “filtering masks,” which are small circles around the individual. These masks basically filter out all signals outside the circle, which locks in the individual’s trajectory and height as they move. Combining all this information — height, build, and movement — the network associates specific RF reflections with specific individuals.
But to tag identities to those anonymous blobs, the system must first be “trained.” For a few days, individuals wear low-powered accelerometer sensors, which can be used to label the reflected radio signals with their respective identities. When deployed in training, Marko first generates users’ tracklets, as it does in practice. Then, an algorithm correlates certain acceleration features with motion features. When users walk, for instance, the acceleration oscillates with steps, but becomes a flat line when they stop. The algorithm finds the best match between the acceleration data and tracklet, and labels that tracklet with the user’s identity. In doing so, Marko learns which reflected signals correlate to specific identities.
The sensors never have to be charged, and, after training, the individuals don’t need to wear them again. In home deployments, Marko was able to tag the identities of individuals in new homes with between 85 and 95 percent accuracy.
Striking a good (data-collection) balance
The researchers hope health care facilities will use Marko to passively monitor, say, how patients interact with family and caregivers, and whether patients receive medications on time. In an assisted living facility, for instance, the researchers noted specific times a nurse would walk to a medicine cabinet in a patient’s room and then to the patient’s bed. That indicated that the nurse had, at those specific times, administered the patient’s medication.
The system may also replace questionnaires and diaries currently used by psychologists or behavioral scientists to capture data on their study subjects’ family dynamics, daily schedules, or sleeping patterns, among other behaviors. Those traditional recording methods can be inaccurate, contain bias, and aren’t well-suited for long-term studies, where people may have to recall what they did days or weeks ago. Some researchers have started equipping people with wearable sensors to monitor movement and biometrics. But elderly patients, especially, often forget to wear or charge them. “The motivation here is to design better tools for researchers,” Hsu says.
Why not just install cameras? For starters, this would require someone watching and manually recording all necessary information. Marko, on the other hand, automatically tags behavioral patterns — such as motion, sleep, and interaction — to specific areas, days, and times.
Also, video is just more invasive, Hsu adds: “Most people aren’t that comfortable with being filmed all the time, especially in their own home. Using radio signals to do all this work strikes a good balance between getting some level of helpful information, but not making people feel uncomfortable.”
Katabi and her students also plan to combine Marko with their prior work on inferring breathing and heart rate from the surrounding radio signals. Marko will then be used to associate those biometrics with the corresponding individuals. It could also track people’s walking speeds, which is a good indicator of functional health in elderly patients.
“The potential here is immense,” says Cecilia Mascolo, a professor of mobile systems in the Department of Computer Science and Technology at Cambridge University. “With respect to imaging through cameras, it offers a less data-rich and more targeted model of collecting information, which is very welcome from the user privacy perspective. The data collected, however, is still very rich, and the paper evaluation shows accuracy which can enable a number of very useful applications, for example in elderly care, medical adherence monitoring, or even hospital care.”
“Yet, as a community, we need to aware of the privacy risks that this type of technology bring,” Mascolo adds. Certain computation techniques, she says, should be considered to ensure the data remains private. | | 12:00a |
Explosions of universe’s first stars spewed powerful jets Several hundred million years after the Big Bang, the very first stars flared into the universe as massively bright accumulations of hydrogen and helium gas. Within the cores of these first stars, extreme, thermonuclear reactions forged the first heavier elements, including carbon, iron, and zinc.
These first stars were likely immense, short-lived fireballs, and scientists have assumed that they exploded as similarly spherical supernovae.
But now astronomers at MIT and elsewhere have found that these first stars may have blown apart in a more powerful, asymmetric fashion, spewing forth jets that were violent enough to eject heavy elements into neighboring galaxies. These elements ultimately served as seeds for the second generation of stars, some of which can still be observed today.
In a paper published today in the Astrophysical Journal, the researchers report a strong abundance of zinc in HE 1327-2326, an ancient, surviving star that is among the universe’s second generation of stars. They believe the star could only have acquired such a large amount of zinc after an asymmetric explosion of one of the very first stars had enriched its birth gas cloud.
“When a star explodes, some proportion of that star gets sucked into a black hole like a vacuum cleaner,” says Anna Frebel, an associate professor of physics at MIT and a member of MIT’s Kavli Institute for Astrophysics and Space Research. “Only when you have some kind of mechanism, like a jet that can yank out material, can you observe that material later in a next-generation star. And we believe that’s exactly what could have happened here.”
“This is the first observational evidence that such an asymmetric supernova took place in the early universe,” adds MIT postdoc Rana Ezzeddine, the study’s lead author. “This changes our understanding of how the first stars exploded.”
“A sprinkle of elements”
HE 1327-2326 was discovered by Frebel in 2005. At the time, the star was the most metal-poor star ever observed, meaning that it had extremely low concentrations of elements heavier than hydrogen and helium — an indication that it formed as part of the second generation of stars, at a time when most of the universe’s heavy element content had yet to be forged.
“The first stars were so massive that they had to explode almost immediately,” Frebel says. “The smaller stars that formed as the second generation are still available today, and they preserve the early material left behind by these first stars. Our star has just a sprinkle of elements heavier than hydrogen and helium, so we know it must have formed as part of the second generation of stars.”
In May of 2016, the team was able to observe the star which orbits close to Earth, just 5,000 light years away. The researchers won time on NASA’s Hubble Space Telescope over two weeks, and recorded the starlight over multiple orbits. They used an instrument aboard the telescope, the Cosmic Origins Spectrograph, to measure the minute abundances of various elements within the star.
The spectrograph is designed with high precision to pick up faint ultraviolet light. Some of those wavelength are absorbed by certain elements, such as zinc. The researchers made a list of heavy elements that they suspected might be within such an ancient star, that they planned to look for in the UV data, including silicon, iron, phosophorous, and zinc.
“I remember getting the data, and seeing this zinc line pop out, and we couldn’t believe it, so we redid the analysis again and again,” Ezzeddine recalls. “We found that, no matter how we measured it, we got this really strong abundance of zinc.”
A star channel opens
Frebel and Ezzeddine then contacted their collaborators in Japan, who specialize in developing simulations of supernovae and the secondary stars that form in their aftermath. The researchers ran over 10,000 simulations of supernovae, each with different explosion energies, configurations, and other parameters. They found that while most of the spherical supernova simulations were able to produce a secondary star with the elemental compositions the researchers observed in HE 1327-2326, none of them reproduced the zinc signal.
As it turns out, the only simulation that could explain the star’s makeup, including its high abundance of zinc, was one of an aspherical, jet-ejecting supernova of a first star. Such a supernova would have been extremely explosive, with a power equivalent to about a nonillion times (that’s 10 with 30 zeroes after it) that of a hydrogen bomb.
“We found this first supernova was much more energetic than people have thought before, about five to 10 times more,” Ezzeddine says. “In fact, the previous idea of the existence of a dimmer supernova to explain the second-generation stars may soon need to be retired.”
The team’s results may shift scientists’ understanding of reionization, a pivotal period during which the gas in the universe morphed from being completely neutral, to ionized — a state that made it possible for galaxies to take shape.
“People thought from early observations that the first stars were not so bright or energetic, and so when they exploded, they wouldn’t participate much in reionizing the universe,” Frebel says. “We’re in some sense rectifying this picture and showing, maybe the first stars had enough oomph when they exploded, and maybe now they are strong contenders for contributing to reionization, and for wreaking havoc in their own little dwarf galaxies.”
These first supernovae could have also been powerful enough to shoot heavy elements into neighboring “virgin galaxies” that had yet to form any stars of their own.
“Once you have some heavy elements in a hydrogen and helium gas, you have a much easier time forming stars, especially little ones,” Frebel says. “The working hypothesis is, maybe second generation stars of this kind formed in these polluted virgin systems, and not in the same system as the supernova explosion itself, which is always what we had assumed, without thinking in any other way. So this is opening up a new channel for early star formation.”
This research was funded, in part, by the National Science Foundation. | | 12:00a |
The (evolving) art of war In 1969, the Soviet Union moved troops and military equipment to its border with China, escalating tensions between the communist Cold War powers. In response, China created a new military strategy of “active defense” to repel an invading force near the border. There was just one catch: China did not actually implement its new strategy until 1980.
Which raises a question: How could China have taken a full decade before shifting its military posture in the face of an apparent threat to its existence?
“It really comes down to the politics of the Cultural Revolution,” says Taylor Fravel, a professor of political science at MIT and an expert in Chinese foreign policy and military thinking. “China was consumed with internal political upheaval.”
That is, through the mid-1970s, leader Mao Zedong and his hardline allies sought to impose their own visions of politics and society on the country. Those internal divisions, and the extraordinary political strife accompanying them, kept China from addressing its external threats — even though it might sorely have needed a new strategy at the time.
Indeed, Fravel believes, every major change in Chinese military strategy since 1949 — and there have been a few — has occurred in the same set of circumstances. Each time, the Chinese have recognized that global changes in warfare have occurred, but they have required political unity in Beijing to implement those changes. To understand the military thinking of one of the world’s superpowers, then, we need to understand its domestic politics.
Fravel has synthesized these observations in a new book, “Active Defense: China’s Military Strategy since 1949,” published by Princeton University Press. The book offers a uniquely thorough history of modern Chinese military thinking, a subject that many observers have regarded as inscrutable.
“One way to understand how great powers think about the use of military force is to examine their [formal] military strategy,” Fravel notes. “In this respect, China has not been studied as thoroughly or systematically as the other great powers.”
Rethinking Mao
Fravel’s book examines military thinking during the entire length of the People’s Republic of China, dating to 1949, when Mao led the communist takeover of the country. China was not at that point regarded as a serious military power, although Fravel notes that the country’s leaders were giving the idea of becoming one serious thought back then.
“I think some people might be surprised to learn that China has been dedicated to building a modern military, and thus thinking about strategy, since the birth of the People’s Republic,” Fravel says.
As Fravel sees it, based on a significant amount of original archival research, there are nine times in modern China’s history when the government has issued comprehensive new military strategies. These formal strategic plans, he thinks, are critical to understanding what Chinese leaders have thought about military force and how to use it.
“It’s an articulation of principles that should guide subsequent activities,” Fravel says.
Of these nine strategies, Fravel finds three to be particularly significant: Those issued in 1956, 1980, and 1993. The first of these articulated a posture of forward defense meant to insulate the country from invasion by, principally, the U.S.
By the 1960s, however, the country had shifted toward a different military posture, one more in line with Mao’s own thinking, which featured an emphasis on guerilla-style retreat and concession of territory in the face of a potential invasion. The idea, deployed by Mao in China’s civil war in the 1930s, was to wear an enemy down over time while providing elusive targets for opponents.
The Soviet massing of military forces just outside China in the late 1960s raised concerns that it might be better to pursue a more “active defense” — and thus the title of Fravel’s book — in which China positioned its armed forces to contain enemies near the border. But given all the internal political conflict (and leadership purges) within China, this shift did not gain enough traction to be implemented in the 1970s. Moreover, as a distinct change from Mao’s ideas, the notion of active defense required considerable political unity to be implemented.
“In that sense, it was profoundly different, and perhaps challenging to pursue,” Fravel says. “They had to de-emphasize one of Mao’s core strategic principles.”
Still, the new strategy became official policy, and remained such for over a decade — until Chinese military leaders watched the 1991 Gulf War on television and recognized that the new era of precision aerial warfare demanded another shift in strategy for them as well.
“I think in many countries, the Gulf War catalyzed a complete rethinking of warfare in very short order,” Fravel says.
And yet, even as this was occurring, China was experiencing yet another moment of internal political division, following the Tianamen Square massacre of 1989. It took another year or two, and a new internal political consensus, before China could develop a new, contemporary strategy for fighting high-tech wars.
“What they wanted to do was really challenging,” Fravel says, noting that the new strategy requires complex coordination of different military domains — air, sea, and land — which had not previously been unified.
The nuclear exception
China’s 1993 statement of strategy remains a guidepost for its current military thinking. However, as Fravel notes, there is one area of military force — nuclear weapons — which is an “exception to the rule” he postulates about policy following unity. China has had nuclear weapons since the 1960s, while always considering them a deterrent to other countries, and not threatening first use of them.
“When you look at the nuclear domain, they’ve basically had the same strategic goal since testing their first device in 1964, which is to deter other countries from attacking China first with nuclear weapons,” Fravel says. “It’s also the one element of defense strategy never delegated by top party leaders. It was so important to them, they never let go of the authority to devise nuclear strategy.”
Other scholars regard “Active Defense” as a significant contribution to its field. Charles Glaser, a professor at George Washington University, states that Fravel “contributes significantly to our understanding of the evolution of China’s military strategy, and offers insightful theoretical arguments about civil-military relations.”
Avery Goldstein, a professor at the University of Pennsylvania, calls the book “an impressive achievement” and notes that Fravel “deftly draws on a wide range of literature about influences on military strategy” as well as “newly available sources of evidence” from historical archives.
For Fravel’s part, he says that identifying the strong pattern leading to changes in China’s military strategy will help as a guide to the future, as well.
“China is a country we know less about, in the study of international politics, than the other great powers,” Fravel says. “If there is a significant shift in the kinds of warfare in the international system, then China would be more likely to consider changing its military strategy.” |
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