Tuesday, August 19, 2014

The physics of falling icebergs


Click here if video does not appear on screen.

By Carol Clark

For thousands of years, the massive glaciers of Earth’s polar regions have remained relatively stable, the ice locked into mountainous shapes that ebbed in warmer months but gained back their bulk in winter. In recent decades, however, warmer temperatures have started rapidly thawing these frozen giants. It’s becoming more common for sheets of ice, one kilometer tall, to shift, crack and tumble into the sea, splitting from their mother glaciers in an explosive process known as calving.

“Imagine a sheer, vertical ice face three times as tall as the tallest building in Atlanta breaking off from a glacier and flipping 90 degrees,” says Emory physicist Justin Burton. “In my lab, we can calculate how much energy is released during one of these events, which can be equivalent to several nuclear bombs.”

Burton studies the geophysics of calving icebergs in order to better understand and predict effects of climate change, such as sea-level rise.

“Ice coverage is one of the most sensitive indicators of climate change,” he says. About half of the loss of ice from the polar ice sheets is occurring due to melting and half due to iceberg calving. While it’s more straightforward to estimate iceberg melt rates, their calving rates are much harder to pin down.

Greenland's Ilulissat glacier is believed to have spawned the iceberg that brought down the Titanic.

For the 2012 film “Chasing Ice,” videographers endured subzero temperatures and years of patience to record stunning time-lapse footage of ancient glaciers receding. Their efforts also yielded the largest calving event ever captured on film. The area involved was about the size of Manhattan. The filmmakers described it as like watching skyscrapers rolling around in an earthquake and an entire city breaking apart before their eyes.

Direct field observations of calving icebergs are as dangerous as they are rare. So Burton and his colleagues developed ways to model these events in a controlled, laboratory setting. “We can measure things that can’t be measured in the field,” he explains, “and it’s also way cheaper and safer.”

He and his colleagues built a cylindrical, Plexiglas water tank as a scaled-down version of a fjord, similar to the ice-walled channel at the end of the Ilulissat glacier, which drains the Greenland ice sheet into the ocean. This well-studied glacier, also known as Jakobshavn, is considered an important bellwether for climate change.

While it is normal for glaciers to both accumulate and shed ice, Jakobshavn provides a vivid snapshot of how the shedding process has speeded up. The glacier retreated 8 miles during the 100-year period between 1902 and 2001, but has retreated more than 10 miles during the past decade. Greenland’s ice sheet appears to be out of balance, losing more ice than it gains.



Burton’s lab creates experimental models to gain a more precise understanding of these glacial processes. Rectangular plastic blocks that have the same density as icebergs are tipped in the water tank and the resulting hydrodynamics are recorded.

One hypothesis that the lab is investigating is how the waves unleashed by capsizing icebergs may be causing earthquakes that can be detected thousands of miles away. “It’s counterintuitive,” Burton says, “because usually you think of earthquakes as causing large waves and not the other way around.”

The lab models, however, suggest that the violent rotation of massive icebergs generates waves that release the brunt of their energy onto the sheer vertical face of the glacier, instead of dispersing most of it into the ocean.

“If we can correlate the frequencies of earthquake signals with the frequencies of icebergs rocking back and forth in the water, then that could be a direct measurement of the size of the icebergs that have broken off,” Burton explains. “Large iceberg calving events could then be detected and measured using remote seismic monitoring.”

Climate change and its impacts is one of the top problems in science, Burton says. “We’re seeing huge changes occurring within a few years and we’ve got to get on it. I’d like to think that, a few decades from now, we were able to do something.”

Photo of Ilulissat glacier by iStockphoto.com

Related:
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Crystal-liquid interface visible for first time

Friday, August 15, 2014

The anatomy of fear and memory formation


Click here if video does not appear on screen.

“A huge number of genes and proteins are involved in new memory formation, and we’re trying to get at the basis of that,” says Emory psychiatrist Kerry Ressler. “One of the most powerful ways to study memory formation is through the process of fear-memory formation. And fear memories are also clinically very important because they underlie disorders like post-traumatic stress disorder, panic disorder and phobias.”

In the primitive brain region called the amygdala, fear looks much the same in a mouse as in a human, Kessler explains. “What the amygdala does, we know now through decades of work by many in the field, is it hard-wires neural connections to multiple subcortical and brain stem areas that lead to the hard-wired fear reflex of rapid breathing, sweating, increased heart rate and in some cases processes like a freezing response.”

Watch the above video to learn more.

Related:
Emory Medicine: The anatomy of fear 
Does lack of fear drive psychopaths?

Tuesday, August 12, 2014

Written in poo: The story of prehistoric life

The mighty T-rex may be long gone, but descendents of its lowly pooper-scooper, the dung beetle, are alive and well and still telling tales.

By Eddy Von Mueller, Emory Magazine

An iridescent beetle, bright as a bead, has caught a whiff of paradise. She makes a beeline for a pile of fresh manure, high as a hill to her, recently left behind by 
a Maiasaurus striding through the rookery on some motherly errand.

It’s a noisy place, this. The herd is a big one, and there are hundreds of nests here. Adult animals rumble, or possibly honk or hiss as they jostle each other. Some of the nests are already full of broken eggshells and little Maiasaurs bawling to be fed.

The beetle takes no more notice of the clamoring dinosaurs than they do of her. She has family matters of her own to attend to. She’s a dung beetle, and, the risk of getting stepped on notwithstanding, hanging around a bunch of nine-meter-long herbivores the size of SUVs means living very large. She burrows eagerly into the heap.

Later, burrows and their chiropteran hardihood will help her million-times-great-grandchildren survive the asteroid impact that will doom Maiasaura and most of her kind 
to extinction.

Later, her descendants will be digging 
into the dung of proto-elephants on the savannahs where an ape will stand, starting no end of trouble.

Later, pyramids will rise, and the civilization that erects them will fall, having ironically put the humble dung beetle, the scarab, at the very center of their cosmology.

For now, a “now” seventy-five million years ago, this beetle will lay her eggs inside the tunnel she’s made, and seal it snugly behind her when she leaves, ensuring that the larva will 
be secure in a chamber literally made of food. “It’s dinner and a nursery,” notes Anthony 
Martin, professor of practice in the Department of Environmental Studies in Emory 
College of Arts and Sciences and the author 
of a new book, Dinosaurs without Bones.

It is coprolites, or fossilized dinosaur dung, that allow us to reconstruct the heartwarming domestic scene described above. Martin's book collects and describes these and scores of other fascinating finds that he and his fellow trackers are using to glean surprisingly intimate insights into how dinosaurs and other prehistoric creatures moved, healed, hunted, ate and excreted.

Read the whole article in Emory Magazine.

Related:
Bringing to life Dinosaurs without Bones

Photo: iStockphoto.com

Thursday, August 7, 2014

A brief history of mental illness

Detail of "The Extraction of the Stone of Madness," a painting by Dutch artist Hieronymus Bosch (c 1494).

Sander Gilman, a professor of psychiatry at Emory, was among the experts interviewed by the Australian Broadcasting Corporation for a radio program on the history of mental illness.

Below is an excerpt from Gilman's remarks:

"From the earliest medical texts that we have, in the so-called Hippocratic Corpus which is 3,000 years old, mental illness is treated by physicians. However, physicians and priests are very, very closely aligned, and mental illness is understood oftentimes in terms of violations of taboos, things that might have a, can we say, moral overlay.

"Modern medicine, starting really in the 17th and 18th century, starts to think about mental illness as a disease of (to use an 18th-century word) the faculties, of mind rather than morals. And so what happens in the Enlightenment, first of all in Britain then in France, right around the time of the French revolution, then in Germany by the 1830s, is the notion that the therapies for mental illness are therapies to correct antisocial behaviors, but it means that it can be corrected.

"So the asylum, we always think about Bedlam Asylum in London, goes from some place where people are restrained, literally restrained with shackles, to places by the 1830s and 1840s where the asylum is seen as a big family, people in asylums grow their own food, there are dances on the weekends. The head of the asylum is seen as almost the father of the asylum and that term is used over and over again. And so that's a big shift from the idea of un-treatability to the idea of treatability, from moral failing to behavioral change."

Click here to listen to the broadcast and read the transcript for the program.

Related:
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Tuesday, August 5, 2014

Physicists eye neural fly data, find formula for Zipf's law

The Zipf's law mechanism was verified with neural data of blowflies reacting to changes in visual signals.

By Carol Clark

Physicists have identified a mechanism that may help explain Zipf’s law – a unique pattern of behavior found in disparate systems, including complex biological ones. The journal Physical Review Letters is publishing their mathematical models, which demonstrate how Zipf’s law naturally arises when a sufficient number of units react to a hidden variable in a system.

“We’ve discovered a method that produces Zipf’s law without fine-tuning and with very few assumptions,” says Ilya Nemenman, a biophysicist at Emory University and one of the authors of the research.

The paper’s co-authors include biophysicists David Schwab of Princeton and Pankaj Mehta of Boston University. “I don’t think any one of us would have made this insight alone,” Nemenman says. “We were trying to solve an unrelated problem when we hit upon it. It was serendipity and the combination of all our varied experience and knowledge.”

Their findings, verified with neural data of blowflies reacting to changes in visual signals, may have universal applications. “It’s a simple mechanism,” Nemenman says. “If a system has some hidden variable, and many units, such as 40 or 50 neurons, are adapted and responding to the variable, then Zipf’s law will kick in.”

That insight could aid in the understanding of how biological systems process stimuli. For instance, in order to pinpoint a malfunction in neural activity, it would be useful to know what data recorded from a normally functioning brain would be expected to look like. “If you observed a deviation from the Zipf’s law mechanism that we’ve identified, that would likely be a good place to investigate,” Nemenman says.

“Letters and words in language are sequences that encode a description of something that is changing over time, like the plot line in a story,” Nemenman says.

Zipf’s law is a mysterious mathematical principle that was noticed as far back as the 19th century, but was named for 20th-century linguist George Zipf. He found that if you rank words in a language in order of their popularity, a strange pattern emerges: The most popular word is used twice as often as the second most popular, and three times as much as the third-ranked word, and so on. This same rank vs. frequency rule was also found to apply to many other social systems, including income distribution among individuals and the size of cities, with a few exceptions.

More recently, laboratory experiments suggest that Zipf’s power-law structure also applies to a range of natural systems, from the protein sequences of immune receptors in cells to the intensity of solar flares from the sun.

“It’s interesting when you see the same phenomenon in systems that are so diverse. It makes you wonder,” Nemenman says.

Scientists have pondered the mystery of Zipf’s law for decades. Some studies have managed to reveal how a feature of a particular system makes it Zipfian, while others have come up with broad mechanisms that generate similar power laws but need some fine-tuning to generate the exact Zipf’s law.

“Our method is the only one that I know of that covers both of these areas,” Nemenman says. “It’s broad enough to cover many different systems and you don’t have to fine tune it: It doesn’t require you to set some parameters at exactly the right value.”

Neurons turn visual stimuli into units of information.

The blowfly data came from experiments led by biophysicist Rob de Ruyter that Nemenman worked on as a graduate student. Flies were turned on a rotor as they watched the world go by, hundreds of times. The moving scenes that the flies repeatedly experienced simulated their natural flight patterns. The researchers recorded when neurons associated with vision spiked, or fired. All sets of the data largely matched within a few hundred microseconds, showing that the flies’ neurons were not randomly spiking, but instead operating like precise coding machines.

If you think of a neuron firing as a “1” and a neuron not firing as a “0,” then the neural activity can be thought of as words, made up of 1s and 0s. When these “words,” or units, are strung together over time, they become “sentences.”

The neurons are turning visual stimuli into units of information, Nemenman explains. “The data is a way for us to read the sentences the fly’s vision neurons are conveying to the rest of the brain.”

Nemenman and his co-authors took a fresh look at this fly data for the new paper in Physical Review Letters. “We were trying to understand if there is a relationship between ideas of universality, or criticality, in physical systems and neural examples of how animals learn,” he says.

The physicists are now researching whether they can bring their work full circle, by showing that the mechanism they identified applies to Zipf’s law in language.

In order to navigate in flight, the flies’ visual neurons adapt to changes in the visual signal, such as velocity. When the world moves faster in front of a fly, these sensitive neurons adapt and rescale. These adaptions enable the flies to adjust to new environments, just as our own eyes adapt and rescale when we move from a darkened theater to a brightly lit room.

“We showed mathematically that the system becomes Zipfian when you’re recording the activity of many units, such as neurons, and all of the units are responding to the same variable,” Nemenman says. “The fact that Zipf’s law will occur in a system with just 40 or 50 such units shows that biological units are in some sense special – they must be adapted to the outside world.”

The researchers provide mathematical simulations to back up their theory. “Not only can we predict that Zipf’s law is going to emerge in any system which consists of many units responding to variable outside signals,” Nemenman says, “we can also tell you how many units you need to develop Zipf’s law, given how variable the response is of a single unit.”

They are now researching whether they can bring their work full circle, by showing that the mechanism they identified applies to Zipf’s law in language.

“Letters and words in language are sequences that encode a description of something that is changing over time, like the plot line in a story,” Nemenman says. “I expect to find a pattern similar to how vision neurons fire as a fly moves through the world and the scenery changes.”

Related:
Biology may not be so complex after all 

Photos: iStockphoto.com