Dissecting the ocean’s unseen waves to learn where the heat, energy and nutrients go (Nature)

By Morgan Kelly, Office of Communications

Sonya Legg, Senior Research Oceanographer, Atmospheric and Oceanic Sciences at Princeton University, and a team of colleagues from other institutions created the first-ever model of the world’s most powerful internal ocean waves.

Sonya Legg, a senior research oceanographer in the Program in Atmospheric and Oceanic Sciences at Princeton University, and colleagues from collaborating institutions created the first “cradle to grave” model of the world’s most powerful internal ocean waves.

Beyond the pounding surf loved by novelists and beachgoers alike, the ocean contains rolling internal waves beneath the surface that displace massive amounts of water and push heat and vital nutrients up from the deep ocean.

Internal waves have long been recognized as essential components of the ocean’s nutrient cycle, and key to how oceans will store and distribute additional heat brought on by global warming. Yet, scientists have not until now had a thorough understanding of how internal waves start, move and dissipate.

Researchers from the Office of Naval Research’s multi-institutional Internal Waves In Straits Experiment (IWISE) have published in the journal Nature the first “cradle-to-grave” model of the world’s most powerful internal waves. Caused by the tide, the waves move through the Luzon Strait between southern Taiwan and the Philippine island of Luzon that connects the Pacific Ocean to the South China Sea.

Simulation of waves in Luzon Strait

The complexity of the Luzon Strait’s two-ridge system was not previously known. The Princeton researchers’ simulations showed that the two ridges of the Luzon Strait greatly amplify the size and energy of the wave, well beyond the sum of what the two ridges would generate separately. The simulation above of the tide moving over the second, or western, ridge shows that the tidally-driven flow reaches a high velocity (top) as it moves down the slope (left to right), creating a large wave in density (black lines) with concentrated turbulent energy dissipation (bottom). As the tide moves back over the ridge, the turbulence is swept away. For both the velocity and energy dissipation panels, the color scale indicates the greatest velocity or energy (red) to the least amount (blue). (Image by Maarten Buijsman, University of Southern Mississippi)

Combining computer models constructed largely by Princeton University researchers with on-ship observations, the researchers determined the movement and energy of the waves from their origin on a double-ridge between Taiwan and the Philippines to when they fade off the coast of China. Known to provide nutrients for whales and pose a hazard to shipping, the Luzon Strait internal waves move west at speeds as fast as 3 meters (18 feet) per second and can be as much as 500 meters (1,640 feet) from trough to crest, the researchers found.

The Luzon Strait internal waves provide an ideal archetype for understanding internal waves, explained co-author Sonya Legg, a Princeton senior research oceanographer in the Program in Atmospheric and Oceanic Sciences and a lecturer in geosciences. The distance from the Luzon Strait to China is relatively short — compared to perhaps the Hawaiian internal wave that crosses the Pacific to Oregon — and the South China Sea is relatively free of obstructions such as islands, crosscurrents and eddies, Legg said. Not only did these factors make the waves much more manageable to model and study in the field, but also resulted in a clearer understanding of wave dynamics that can be used to understand internal waves elsewhere in the ocean, she said.

Model of internal waves

Researchers from the Office of Naval Research’s multi-institutional Internal Waves In Straits Experiment (IWISE) — including from Princeton University — have published the first “cradle-to-grave” model of internal waves, which are subsurface ocean displacements recognized as essential to the distribution of nutrients and heat. The researchers modeled the internal waves that move through the Luzon Strait between southern Taiwan and the Philippine island of Luzon. Part of the Princeton researchers’ role was to simulate when and where the Luzon Strait’s internal waves are strongest as the tide moves westward from the Pacific Ocean into the South China Sea over a unique double-ridge formation in the strait. The above image shows the two underwater ridges — indicated in green, orange and red — between Taiwan (top) and island of Luzon (bottom). The color scale indicates elevation from lowest (blue) to highest (red). (Image by Maarten Buijsman, University of Southern Mississippi)

“We know there are these waves in other parts of the ocean, but they’re hard to look at because there are other things in the way,” Legg said. “The Luzon Strait waves are in a mini-basin, so instead of the whole Pacific to focus on, we had this small sea — it’s much more manageable. It’s a place you can think of as a laboratory in the ocean that’s much simpler than other parts of the ocean.”

Legg and co-author Maarten Buijsman, who worked on the project while a postdoctoral researcher at Princeton and is now an assistant professor of physical oceanography at the University of Southern Mississippi, created computer simulations of the Luzon Strait waves that the researchers in the South China Sea used to determine the best locations to gather data.

For instance, Legg and Buijsman used their models to pinpoint where and when the waves begin with the most energy as the ocean tide crosses westward over the strait’s two underwater ridges. Notably, their models showed that the two ridges greatly amplify the size and energy of the wave, well beyond the sum of what the two ridges would generate separately. The complexity of a two-ridge system was not previously known, Legg said.

The energy coming off the strait’s two ridges steepens as it moves toward China, evolving from a rolling wavelength to a steep “saw-tooth” pattern, Legg said. These are the kind of data the researchers sought to gather — where the energy behind internal waves goes and how it changes on its way. How an internal wave’s energy is dissipated determines the amount of heat and nutrients that are transferred from the cold depths of the lower ocean to the warm surface waters, or vice versa.

Models used to project conditions on an Earth warmed by climate change especially need to consider how the ocean will move excess heat around, Legg said. Heat that stays at the surface will ultimately result in greater sea-level rise as warmer water expands more readily as it heats up. The cold water of the deep, however, expands less for the same input of heat and has a greater capacity to store warm water. If heat goes to the deep ocean, that could greatly increase how much heat the oceans can absorb, Legg said.

As researchers learn more about internal waves such as those in the Luzon Strait, climate models can be tested against what becomes known about ocean mechanics to more accurately project conditions on a warmer Earth, she said.

“Ultimately, we want to know what effect the transportation and storage of heat has on the ocean. Internal waves are a significant piece in the puzzle in telling us where heat is stored,” Legg said. “We have in the Luzon Strait an oceanic laboratory where we can test our theoretical models and simulations to see them play out on a small scale.”

This work supported by the U.S. Office of Naval Research and the Taiwan National Science Council.

Read the abstract

Matthew H. Alford, et al. 2015. The formation and fate of internal waves in the South China Sea. Nature. Arti­cle pub­lished online in-advance-of-print May 7, 2015. DOI: 10.1038/nature14399



An improvement to the global software standard for analyzing fusion plasmas (Nuclear Fusion)

By Raphael Rosen, Princeton Plasma Physics Laboratory

The gold standard for analyzing the behavior of fusion plasmas may have just gotten better. Mario Podestà, a staff physicist at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL), has updated the worldwide computer program known as TRANSP to better simulate the interaction between energetic particles and instabilities – disturbances in plasma that can halt fusion reactions. The program’s updates, reported in the journal Nuclear Fusion, could lead to improved capability for predicting the effects of some types of instabilities in future facilities such as ITER, the international experiment under construction in France to demonstrate the feasibility of fusion power.

Podestà and co-authors saw a need for better modeling techniques when they noticed that while TRANSP could accurately simulate an entire plasma discharge, the code wasn’t able to represent properly the interaction between energetic particles and instabilities. The reason was that TRANSP, which PPPL developed and has regularly updated, treated all fast-moving particles within the plasma the same way. Those instabilities, however, can affect different parts of the plasma in different ways through so-called “resonant processes.”

The authors first figured out how to condense information from other codes that do model the interaction accurately – albeit over short time periods – so that TRANSP could incorporate that information into its simulations. Podestà then teamed up with TRANSP developer Marina Gorelenkova at PPPL to update a TRANSP module called NUBEAM to enable it to make sense of this condensed data. “Once validated, the updated module will provide a better and more accurate way to compute the transport of energetic particles,” said Podestà. “Having a more accurate description of the particle interactions with instabilities can improve the fidelity of the program’s simulations.”

Schematic of NSTX tokamak at PPPL with a cross-section showing perturbations of the plasma profiles caused by instabilities. Without instabilities, energetic particles would follow closed trajectories and stay confined inside the plasma (blue orbit). With instabilities, trajectories can be modified and some particles may eventually be pushed out of the plasma boundary and lost (red orbit). Credit: Mario Podestà

Schematic of NSTX tokamak at PPPL with a cross-section showing perturbations of the plasma profiles caused by instabilities. Without instabilities, energetic particles would follow closed trajectories and stay confined inside the plasma (blue orbit). With instabilities, trajectories can be modified and some particles may eventually be pushed out of the plasma boundary and lost (red orbit). Credit: Mario Podestà

Fast-moving particles, which result from neutral beam injection into tokamak plasmas, cause the instabilities that the updated code models. These particles begin their lives with a neutral charge but turn into negatively charged electrons and positively charged ions – or atomic nuclei – inside the plasma. This scheme is used to heat the plasma and to drive part of the electric current that completes the magnetic field confining the plasma.

The improved simulation tool may have applications for ITER, which will use fusion end-products called alpha particles to sustain high plasma temperatures. But just like the neutral beam particles in current-day-tokamaks, alpha particles could cause instabilities that degrade the yield of fusion reactions. “In present research devices, only very few, if any, alpha particles are generated,” said Podestà. “So we have to study and understand the effects of energetic ions from neutral beam injectors as a proxy for what will happen in future fusion reactors.”

PPPL, on Princeton University’s Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas — ultra-hot, charged gases — and to developing practical solutions for the creation of fusion energy. Results of PPPL research have ranged from a portable nuclear materials detector for anti-terrorist use to universally employed computer codes for analyzing and predicting the outcome of fusion experiments. The Laboratory is managed by the University for the U.S. Department of Energy’s Office of Science, which is the largest single supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, visit science.energy.gov.

Read the paper

Podestà, M. Gorelenkova, D.S. Darrow, E.D. Fredrickson, S.P. Gerhardt and R.B. White. Nucl. Fusion 55 053018

Decoding the Cell’s Genetic Filing System (Nature Chemistry)

By Tien Nguyen, Department of Chemistry

A fully extended strand of human DNA measures about five feet in length. Yet it occupies a space just one-tenth of a cell by wrapping itself around histones—spool-like proteins—to form a dense hub of information called chromatin.

Access to these meticulously packed genes is regulated by post-translational modifications, chemical changes to the structure of histones that act as on-off signals for gene transcription. Mistakes or mutations in histones can cause diseases such as glioblastoma, a devastating pediatric brain cancer.

Source: Nature Chemistry

Source: Nature Chemistry

Researchers at Princeton University have developed a facile method to introduce non-native chromatin into cells to interrogate these signaling pathways. Published on April 6 in the journal Nature Chemistry, this work is the latest chemical contribution from the Muir lab towards understanding nature’s remarkable information indexing system.

Tom Muir, the Van Zandt Williams, Jr. Class of ’65 Professor of Chemistry, began investigating transcriptional pathways in the so-called field of epigenetics almost a decade earlier. Deciphering such a complex and dynamic system posed a formidable challenge, but his research lab was undeterred. “It’s better to fail at something important than to succeed at something trivial,” he said.

Muir recognized the value of introducing chemical approaches to epigenetics to complement early contributions that came mainly from molecular biologists and geneticists. If epigenetics was like a play, he said, molecular biology and genetics could identify the characters but chemistry was needed to understand the subplots.

These subplots, or post-translational modifications of histones, of which there are more than 100, can occur cooperatively and simultaneously. Traditional methods to probe post-translational modifications involved synthesizing modified histones one at a time, which was a very slow process that required large amounts of biological material.

Last year, the Muir group introduced a method that would massively accelerate this process. The researchers generated a library of 54 nucleosomes—single units of chromatin, like pearls on a necklace—encoded with DNA-barcodes, unique genetic tags that can be easily identified. Published in the journal Nature Methods, the high throughput method required only microgram amounts of each nucleosome to run approximately 4,500 biochemical assays.

“The speed and sensitivity of the assay was shocking,” Muir said. Each biochemical assay involved treatment of the DNA-barcoded nucleosome with a writer, reader or nuclear extract, to reveal a particular binding preference of the histone. The products were then isolated using a technique called chromatin immunoprecipitation and characterized by DNA sequencing, essentially an ordered readout of the nucleotides.

“There have been incredible advances in genetic sequencing over the last 10 years that have made this work possible,” said Manuel Müller, a postdoctoral researcher in the Muir lab and co-author on the Nature Methods article.

Schematic of approach using split inteins

Schematic of approach using split inteins

With this method, researchers could systematically interrogate the signaling system to propose mechanistic pathways. But these mechanistic insights would remain hypotheses unless they could be validated in vivo, meaning inside the cellular environment.

The only method for modifying histones in vivo was extremely complicated and specific, said Yael David, a postdoctoral researcher in the Muir lab and lead author on the recent Nature Chemistry study that demonstrated a new and easily customizable approach.

The method relied on using ultra-fast split inteins, protein fragments that have a great affinity for one another. First, one intein fragment was attached to a modified histone, by encoding it into a cell. Then, the other intein fragment was synthetically fused to a label, which could be a small protein tag, fluorophore or even an entire protein like ubiquitin.

Within minutes of being introduced into the cell, the labeled intein fragment bound to the histone intein fragment. Then like efficient and courteous matchmakers, the inteins excised themselves and created a new bond between the label and modified histone. “It’s really a beautiful way to engineer proteins in a cell,” David said.

Regions of the histone may be loosely or tightly packed, depending on signals from the cell indicating whether or not to transcribe a gene. By gradually lowering the amount of labeled intein introduced, the researchers could learn about the structure of chromatin and tease out which areas were more accessible than others.

Future plans in the Muir lab will employ these methods to ask specific biological questions, such as whether disease outcomes can be altered by manipulating signaling pathway. “Ultimately, we’re developing methods at the service of biological questions,” Muir said.

This research was supported by the US National Institutes of Health (grants R37-GM086868 and R01 GM107047).

Read the articles:

Nguyen, U.T.T.; Bittova, L.; Müller, M.; Fierz, B.; David, Y.; Houck-Loomis, B.; Feng, V.; Dann, G.P.; Muir, T.W. “Accelerated chromatin biochemistry using DNA-barcoded nucleosome libraries.” Nature Methods, 2014, 11, 834.

David, Y.; Vila-Perelló, M; Verma, S.; Muir, T.W. “Chemical tagging and customizing of cellular chromatin states using ultrafast trans-splicing inteins.” Nature Chemistry, Advance online publication, April 6, 2015.

Frustrated magnets – new experiment reveals clues to their discontent (Science)

By Catherine Zandonella, Office of the Dean for Research

A crystal of frustrated magnet (Tb2Ti2O7). Image credit: Jason Krizan.

A crystal of frustrated magnet (Tb2Ti2O7). Image credit: Jason Krizan.

An experiment conducted by Princeton researchers has revealed an unlikely behavior in a class of materials called frustrated magnets, addressing a longdebated question about the nature of these discontented quantum materials.

The work represents a surprising discovery that down the road may suggest new research directions for advanced electronics. Published this week in the journal Science, the study also someday may help clarify the mechanism of high-temperature superconductivity, the frictionless transmission of electricity.

The researchers tested the frustrated magnets — so-named because they should be magnetic at low temperatures but aren’t — to see if they exhibit a behavior called the Hall Effect. When a magnetic field is applied to an electric current flowing in a conductor such as a copper ribbon, the current deflects to one side of the ribbon. This deflection, first observed in 1879 by E.H. Hall, is used today in sensors for devices such as computer printers and automobile anti-lock braking systems.

Because the Hall Effect happens in charge-carrying particles, most physicists thought it would be impossible to see such behavior in non-charged, or neutral, particles like those in frustrated magnets. “To talk about the Hall Effect for neutral particles is an oxymoron, a crazy idea,” said N. Phuan Ong, Princeton’s Eugene Higgins Professor of Physics.

Nevertheless, some theorists speculated that the neutral particles in frustrated magnets might bend to the Hall rule under extremely cold conditions, near absolute zero, where particles behave according to the laws of quantum mechanics rather than the classical physical laws we observe in our everyday world. Harnessing quantum behavior could enable game-changing innovations in computing and electronic devices.

Ong and colleague Robert Cava, Princeton’s Russell Wellman Moore Professor of Chemistry, and their graduate students Max Hirschberger and Jason Krizan decided to see if they could settle the debate and demonstrate conclusively that the Hall Effect exists for frustrated magnets.

To do so, the research team turned to a class of the magnets called pyrochlores. They contain magnetic moments that, at very low temperatures near absolute zero, should line up in an orderly manner so that all of their “spins,” a quantum-mechanical property, point in the same direction. Instead, experiments have found that the spins point in random directions. These frustrated materials are also referred to as “quantum spin ice.”

“These materials are very interesting because theorists think the tendency for spins to align is still there, but, due to a concept called geometric frustration, the spins are entangled but not ordered,” Ong said. Entanglement is a key property of quantum systems that researchers hope to harness for building a quantum computer, which could solve problems that today’s computers cannot handle.

A chance conversation in a hallway between Cava and Ong revealed that Cava had the know-how and experimental infrastructure to make such materials. He tasked chemistry graduate student Krizan with growing the crystals while Hirschberger, a graduate student in physics, set up the experiments needed to look for the Hall Effect.

Graduate student Max Hirschberger lowers the assembled experimental setup into a high-field magnet system, capable of creating fields as strong as 250,000 times the earth's magnetic field.  (Image credit: Jason Krizan.)

Graduate student Max Hirschberger lowers the assembled experimental setup into a high-field magnet system, capable of creating fields as strong as 250,000 times the earth’s magnetic field. (Image credit: Jason Krizan.)

“The main challenge was how to measure the Hall Effect at an extremely low temperature where the quantum nature of these materials comes out,” Hirschberger said. The experiments were performed at temperatures of 0.5 degrees Kelvin, and required Hirschberger to resolve temperature differences as small as a thousandth of a degree between opposite edges of a crystal.

To grow the crystals, Krizan first synthesized the material from terbium oxide and titanium oxide in a furnace similar to a kiln. After forming the pyrochlore powder into a cylinder suitable for feeding the crystal growth, Krizan suspended it in a chamber filled with pure oxygen and blasted it with enough focused light from four 1000-Watt halogen light bulbs to heat a small region to 1800 degrees Celsius. The final products were thin, flat transparent or orange slabs about the size of a sesame seed.

To test each crystal, Hirschberger attached tiny gold electrodes to either end of the slab, using microheaters to drive a heat current through the crystal. At such low temperatures, this heat current is analogous to the electric current in the ordinary Hall Effect experiment.

At the same time, he applied a magnetic field in the direction perpendicular to the heat current. To his surprise, he saw that the heat current was deflected to one side of the crystal. He had observed the Hall Effect in a non-magnetic material.

Surprised by the results, Ong suggested that Hirschberger repeat the experiment, this time by reversing the direction of the heat current. If Hirschberger was really seeing the Hall Effect, the current should deflect to the opposite side of the crystal. Reconfiguring the experiment at such low temperatures was not easy, but eventually he demonstrated that the signal did indeed reverse in a manner consistent with the Hall Effect.

“All of us were very surprised because we work and play in the classical, non-quantum world,” Ong said. “Quantum behavior can seem very strange, and this is one example where something that shouldn’t happen is really there. It really exists.”

The use of experiments to probe the quantum behavior of materials is essential for broadening our understanding of fundamental physical properties and the eventual exploitation of this understanding in new technologies, according to Cava. “Every technological advance has a basis in fundamental science through our curiosity about how the world works,” he said.

Further experiments on these materials may provide insights into how superconductivity occurs in certain copper-containing materials called cuprates, also known as “high-temperature” superconductors because they work well above the frigid temperatures required for today’s superconductors, such as those used in MRI machines.

One of the ideas for how high-temperature superconductivity could occur is based on the possible existence of a particle called the spinon. Theorists, including the Nobel laureate Philip Anderson, Princeton’s Joseph Henry Professor of Physics, Emeritus and a senior physicist, and others have speculated that spinons could be the carrier of a heat current in a quantum system such as the one explored in the present study.

Although the team does not claim to have observed the spinon, Ong said that the work could lead in such a direction in the future. “This work sets the stage for hunting the spinon,” Ong said. “We have seen its tracks, so to speak.”


The research was funded by the Army Research Office (ARO W911NF-11-1-0379, ARO W911NF-12-1-0461), the U.S. National Science Foundation (DMR 1420541), and the U.S. Department of Energy’s Division of Basic Energy Sciences, (DE-FG-02-08ER46544).


Max Hirschberger, Jason W. Krizan, R. J. Cava, N. P. Ong. Large thermal Hall conductivity of neutral spin excitations in a frustrated quantum magnet. Science. 10.1126/science.1257340

Revisiting the mechanics of the action potential (Nature Communications)

By Staff


The action potential (AP) and the accompanying action wave (AW) constitute an electromechanical pulse traveling along the axon.

The action potential is widely understood as an electrical phenomenon. However, a long experimental history has documented the existence of co-propagating mechanical signatures.

In a new paper in the journal Nature Communications, two Princeton University researchers have proposed a theoretical model to explain these mechanical signatures, which they term “action waves.” The research was conducted by Ahmed El Hady, a visiting postdoctoral research associate at the Princeton Neuroscience Institute and a postdoctoral fellow at the Howard Hughes Medical Institute, and Benjamin Machta, an associate research scholar at the Lewis-Sigler Institute for Integrative Genomics and a lecturer in physics and the Lewis-Sigler Institute for Integrative Genomics.

In the model, the co-propagating waves are driven by changes in charge separation across the axonal membrane, just as a speaker uses charge separation to drive sound waves through the air. The researchers argue that these forces drive surface waves involving both the axonal membrane and cytoskeleton as well as its surrounding fluid. Their model may help shed light on the functional role of the surprisingly structured axonal cytoskeleton that recent super-resolution techniques have uncovered, and suggests a wider role for mechanics in neuronal function.

Read the paper.

Ahmed El Hady & Benjamin B. Machta. Mechanical surface waves accompany action potential propagation. Nature Communications 6, No. 6697 doi:10.1038/ncomms7697

Do biofuel policies seek to cut emissions by cutting food? (Science)

By Catherine Zandonella, Office of the Dean for Research

2015_03_27_cornfieldA study published today in the journal Science found that government biofuel policies rely on reductions in food consumption to generate greenhouse gas savings.

Shrinking the amount of food that people and livestock eat decreases the amount of carbon dioxide that they breathe out or excrete as waste. The reduction in food available for consumption, rather than any inherent fuel efficiency, drives the decline in carbon dioxide emissions in government models, the researchers found.

“Without reduced food consumption, each of the models would estimate that biofuels generate more emissions than gasoline,” said Timothy Searchinger, first author on the paper and a research scholar at Princeton University’s Woodrow Wilson School of Public and International Affairs and the Program in Science, Technology, and Environmental Policy.

Searchinger’s co-authors were Robert Edwards and Declan Mulligan of the Joint Research Center at the European Commission; Ralph Heimlich of the consulting practice Agricultural Conservation Economics; and Richard Plevin of the University of California-Davis.

The study looked at three models used by U.S. and European agencies, and found that all three estimate that some of the crops diverted from food to biofuels are not replaced by planting crops elsewhere. About 20 percent to 50 percent of the net calories diverted to make ethanol are not replaced through the planting of additional crops, the study found.

The result is that less food is available, and, according to the study, these missing calories are not simply extras enjoyed in resource-rich countries. Instead, when less food is available, prices go up. “The impacts on food consumption result not from a tailored tax on excess consumption but from broad global price increases that will disproportionately affect some of the world’s poor,” Searchinger said.

The emissions reductions from switching from gasoline to ethanol have been debated for several years. Automobiles that run on ethanol emit less carbon dioxide, but this is offset by the fact that making ethanol from corn or wheat requires energy that is usually derived from traditional greenhouse gas-emitting sources, such as natural gas.

Both the models used by the U.S. Environmental Protection Agency and the California Air Resources Board indicate that ethanol made from corn and wheat generates modestly fewer emissions than gasoline. The fact that these lowered emissions come from reductions in food production is buried in the methodology and not explicitly stated, the study found.

The European Commission’s model found an even greater reduction in emissions. It includes reductions in both quantity and overall food quality due to the replacement of oils and vegetables by corn and wheat, which are of lesser nutritional value. “Without these reductions in food quantity and quality, the [European] model would estimate that wheat ethanol generates 46% higher emissions than gasoline and corn ethanol 68% higher emissions,” Searching said.

The paper recommends that modelers try to show their results more transparently so that policymakers can decide if they wish to seek greenhouse gas reductions from food reductions. “The key lesson is the trade-offs implicit in the models,” Searchinger said.

The research was supported by The David and Lucile Packard Foundation.

Read the abstract.

T. Searchinger, R. Edwards, D. Mulligan, R. Heimlich, and R. Plevin. Do biofuel policies seek to cut emissions by cutting food? Science 27 March 2015: 1420-1422. DOI: 10.1126/science.1261221.

When attention is a deficit: How the brain switches strategies to find better solutions (Neuron)

By Catherine Zandonella, Office of the Dean for Research

2015_03_26_JW_Schuck_NYC3Sometimes being too focused on a task is not a good thing.

During tasks that require our attention, we might become so engrossed in what we are doing that we fail to notice there is a better way to get the job done.

For example, let’s say you are coming out of a New York City subway one late afternoon and you want to find out which way is west. You might begin to scan street signs and then suddenly realize that you could just look for the setting sun.

A new study explored the question of how the brain switches from an ongoing strategy to a new and perhaps more efficient one. The study, conducted by researchers at Princeton University, Humboldt University of Berlin, the Bernstein Center for Computational Neuroscience in Berlin, and the University of Milan-Bicocca, found that activity in a region of the brain known as the medial prefrontal cortex was involved in monitoring what is happening outside one’s current focus of attention and shifting focus from a successful strategy to one that is even better. They published the finding in the journal Neuron.

“The human brain at any moment in time has to process quite a wealth of information,” said Nicolas Schuck, a postdoctoral research associate in the Princeton Neuroscience Institute and first author on the study. “The brain has evolved mechanisms that filter that information in a way that is useful for the task that you are doing. But the filter has a disadvantage: you might miss out on important information that is outside your current focus.”

Schuck and his colleagues wanted to study what happens at the moment when people realize there is a different and potentially better way of doing things. They asked volunteers to play a game while their brains were scanned with magnetic resonance imaging (MRI). The volunteers were instructed to press one of two buttons depending on the location of colored squares on a screen. However, the game contained a hidden pattern that the researchers did not tell the participants about, namely, that if the squares were green, they always appeared in one part of the screen and if the squares were red, they always appeared in another part. The researchers refrained from telling players that they could improve their performance by paying attention to the color instead of the location of the squares.

Volunteers played a game where they had to press one button or another depending on the location of squares on a screen. Participants that switched to a strategy based on the color of the square were able to improve their performance on the game. (Image source: Schuck, et al.)

Volunteers played a game where they had to press one button or another depending on the location of squares on a screen. Participants that switched to a strategy based on the color of the squares were able to improve their performance on the game. (Image source: Schuck, et al.)

Not all of the players figured out that there was a more efficient way to play the game. However, among those that did, their brain images revealed specific signals in the medial prefrontal cortex that corresponded to the color of the squares. These signals arose minutes before the participants switched their strategies. This signal was so reliable that the researchers could use it to predict spontaneous strategy shifts ahead of time, Schuck said.

“These findings are important to better understand the role of the medial prefrontal cortex in the cascade of processes leading to the final behavioral change, and more generally, to understand the role of the medial prefrontal cortex in human cognition,” said Carlo Reverberi, a researcher at the University of Milan-Bicocca and senior author on the study. “Our findings suggest that the medial prefrontal cortex is ‘simulating’ in the background an alternative strategy, while the overt behavior is still shaped by the old strategy.”

The study design – specifically, not telling the participants that there was a more effective strategy – enabled the researchers to show that the brain can monitor background information while focused on a task, and choose to act on that background information.

“What was quite special about the study was that the behavior was completely without instruction,” Schuck said. “When the behavior changed, this reflected a spontaneous internal process.”

Before this study, he said, most researchers had focused on the question of switching strategies because you made a mistake or you realized that your current approach isn’t working. “But what we were able to explore,” he said, “is what happens when people switch to a new way of doing things based on information from their surroundings.” In this way, the study sheds light on how learning and attention can interact, he said.

The study has relevance for the question of how the brain balances the need to maintain attention with the need to incorporate new information about the environment, and may eventually help our understanding of disorders that involve attention deficits.

Schuck designed and conducted the experiments while a graduate student at Humboldt University and the International Max Planck Research School on the Life Course (LIFE) together with the other authors, and conducted the analysis at Princeton University in the laboratory of Yael Niv, assistant professor of psychology and the Princeton Neuroscience Institute in close collaboration with Reverberi.

The research was supported through a grant from the U.S. National Institutes of Health, the International Max Planck Research School LIFE, the Italian Ministry of University, the German Federal Ministry of Education and Research, and the German Research Foundation.

Read the abstract.

Nicolas W. Schuck, Robert Gaschler, Dorit Wenke, Jakob Heinzle, Peter A. Frensch, John-Dylan Haynes, and Carlo Reverberi. Medial Prefrontal Cortex Predicts Internally Driven Strategy Shifts, Neuron (2015) http://dx.doi.org/10.1016/j.neuron.2015.03.015.