New Clues to the Processing of Memories

PASADENA, Calif.- Quick! Memorize this sentence: The temporoammonic (TA) pathway is a entorhinal cortex (EC) input that consists of axons from layer III EC neurons that make synaptic contacts on the distal dendrites of CA1 neurons.

If by chance you can't memorize this, say two researchers from the California Institute of Technology, it may be due to this very TA pathway that is modulating what your brain remembers.

In another clue toward understanding the processing of memories, graduate student Miguel Remondes and Erin M. Schuman, an associate professor of biology at Caltech and an assistant investigator of the Howard Hughes Medical Institute, have now gleaned two possible roles for the TA pathway that until now were not known. The research is reported in the April 18 issue of the journal Nature.

Using rat hippocampal slices, they've found that this pathway may be part of the brain's decision-making process about whether to keep a particular input and form a memory, or reject it.

Input from the senses—an odor, a sight, or a sound, say, is first received by the brain's cortex. Then, via a specific pathway of nerve fibers long known to scientists, the signals are sent on to the hippocampus. That organ processes the signals, then sends them back to the cortex, probably for long-term storage.

Scientists have also known about the TA pathway, but not its function. Now Remondes and Schuman report that the TA pathway may serve as a memory gatekeeper that can either enhance or diminish the signals of the specific set of neurons that form a memory. Further, they've shown that this pathway may also provide the hippocampus with the information it needs to form so-called place-selective cells; that is, cells that help animals to know where they are in their environments.

The hippocampal formation comprises several structures in the brain and includes the seahorse-shaped hippocampus and a second organ called the dentate gyrus. The formation is involved in saving and retrieving long-term memories. Scientists divide the hippocampus into four divisions, from CA1 to CA4. CA1 and CA3 play major roles in processing memory.

In their quest to understand how communication between neurons contributes to memory, scientists have focused on the "trisynaptic circuit." When input from the senses reaches the cortex, it's sent on to the dentate gyrus, then on to the hippocampus. There, the signals are serially processed by synapses in areas CA3 and CA1 of the hippocampus (synapses are gaps between two neurons that function as the site of information transfer from one neuron to another). Finally, the hippocampus sends a signal back to the cortex. That's the trisynaptic circuit.

Remondes and Schuman found that the TA pathway also sends signals. But its input comes from a different part of the cortex and goes directly to the CA1 section of the hippocampus. The TA pathway reacts depending on how close in time the synaptic signals from the hippocampus are from the original signal sent by the trisynaptic circuit. If it is close, within 40 milliseconds, the TA pathway will act as a signal (and thus a memory) enhancer; that is, it will allocate a stronger synaptic signal from the hippocampus. If it is far, more than 400 milliseconds, it will inhibit the signal.

"So the brain sends the information to the hippocampus," says Remondes, "and instead of just collecting the result of its activity, the hippocampus may very well perform 'quality control' on the potential memory. And it may be doing this by using the direct cortical input from the TA pathway." Perhaps, then, this is a further clue to how memories are stored—or forgotten.

In addition, although the scientists have not done any specific spatial memory experiments, their work may have relevance to how the brain forms place-selective cells. Since other studies previously established that the trisynaptic circuit is not necessary for spatial memory, some of the important information entering the hippocampus may actually be provided by the TA pathway.

"The TA pathway has been briefly described in the past, but not really acknowledged as a 'player' in the memory debate," says Remondes. "Hopefully, these findings will bring new insight into how we form, or don't form, memories."

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Researchers find new clue why Martian wateris found on the north pole, not the south

When astronauts finally land on Mars, a safe bet is that they'll head for northern climes if they intend to spend much time there. That's because nearly all the available water is frozen as ice at the north pole. Planetary scientists have been aware of this for some time, but they now have a new clue why it is so.

In the March 21 issue of the journal Nature, California Institute of Technology researcher Mark Richardson and his colleague John Wilson of the National Oceanic and Atmospheric Administration reveal that the higher average elevation of the Red Planet's southern hemisphere ultimately tends to drive water northward.

Their evidence is based on a computer model the two have worked on for years (Wilson since 1992, Richardson since 1996), coupled with data returned by NASA's Mars Global Surveyor.

"We've found a mechanism in the Martian climate that introduces annual average hemispheric asymmetry," explains Richardson, an assistant professor of planetary science at Caltech. "The circulation systems of Mars and Earth are similar in certain ways, but Mars is different in that water is not available everywhere."

The key to understanding the phenomenon is a complicated computer modeling of the Hadley circulation, which extends about 40 degrees of latitude each side of the Martian equator. A topographical bias in circulation pretty much means there will be a bias in the net pole-to-pole transport of water, Richardson explains.

A plausible explanation is that water ice is found at the north pole and carbon dioxide ice is found at the south for reasons having to do with the way the sun heats the atmosphere. As the Martian orbit changes on time scales of 50,000 years and more, these effects tend to cancel, with no pole claiming the water ice cap over geological time. It has been suggested that topography determines where carbon dioxide forms, and hence, where water ice can form, but the processes controlling carbon dioxide ice caps are poorly understood.

However, the mechanism Richardson and Wilson describe is independent of this occasional realignment of the pole's precession and the planet's eccentric orbit. The mechanism means that, while there is never a time in the past when water ice can be discounted at the south pole, one is more likely to find it more frequently at the north pole.

The importance of the study is its furthering of our understanding of the Martian climate and Martian water cycle. A better understanding of how water is transported will be particularly important to determining whether life once existed on Mars, and what happened to it if it ever did.

The Web address for the journal Nature is http://www.nature.com.

Contact: Robert Tindol (626) 395-3631

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Caltech and Purdue scientists determinestructure of the Dengue fever virus

Scientists at the California Institute of Technology and Purdue University have determined the fine-detail structure of the virus that causes dengue fever. This advance could lead to newer and more focused strategies for devising a vaccine to protect the world against a viral illness that causes 20,000 deaths each year.

Reporting in the March 8 issue of the journal Cell, Caltech biology professor James H. Strauss, lead author Richard J. Kuhn of Purdue (a former postdoctoral scholar in Strauss's lab), and Michael G. Rossman and Timothy S. Baker, both of Purdue, describe the structure of the virus they obtained with a cryoelectron microscope. The detailed electron-density map shows the inner RNA core of the virus as well as the other spherical layers that cover it. At the surface is the glycoprotein scaffolding thought to allow the virus to interact with the receptor and invade a host cell.

This is the first time the structure of one of the flaviviruses has been described, Strauss says. The flaviviruses are a class of viruses that include the yellow fever, West Nile, tick-borne encephalitis, and Japanese encephalitis viruses. All are enclosed with a glycoprotein outer layer that includes minor projections out of the lipid layer due to the geometry of the scaffolding.

"Most viruses that cause serious illness are enveloped, including influenza, hantaviruses, West Nile virus, smallpox, and herpes—though not polio," Strauss says.

The surprise for the researchers was the unusual manner in which the glycoproteins are arranged. Details from the Caltech and Purdue computer-generated images show a highly variegated structure of glycoprotein molecules that are evenly dispersed, but with a surprisingly complicated pattern.

"It's symmetrical, but not with the obvious symmetry of most symmetric viruses," Strauss explains. "This was not an expected result."

Strauss says it's still unclear what the odd symmetry will ultimately mean for future research aimed at controlling the disease, because the precise function of the different structural domains of the glycoproteins are still not known. Those that have been false-colored blue in the image are the domain of glycoproteins thought to be involved in receptor binding—and thus responsible for the virus's entry into a cell. The glycoprotein structures coded yellow are an elongated domain thought to be responsible for holding the scaffolding together; and the ones coded red have a function that is not yet known.

But a more detailed view of these structures is the beginning of a more informed strategy for a focused medical or pharmaceutical attack, Strauss says. "You can think of the protease inhibitors for HIV. Those in large part came from knowing the structure of the HIV enzymes you were trying to interfere with."

Thus, the new work could lead to drugs that will bind to the virus to prevent it from entering the cell, or perhaps from reassembling once it is already inside the cell.

Dengue fever is a mosquito-spread disease that has been known for centuries, but was first isolated in the 1940s after it became a significant health concern for American forces in the Pacific theater. A worldwide problem, the disease is found throughout Latin America, the Caribbeans, Southeast Asia, and India, and is currently at epidemic levels in Hawaii.

Especially virulent is the closely related dengue hemorrhagic fever, which is responsible for most of the deaths. The disease is a leading cause of infant mortality in Thailand, where there is an especially vigorous program to find an effective vaccine.

More information can be found on the Center for Disease Control Web site at http://www.cdc.gov/ncidod/dvbid/dengue/index.htm.

Contact: Robert Tindol (626) 395-3631

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Caltech astronomer to search for "hot Jupiters"with off-the-shelf camera lens

In an age when nearly all astronomical work requires really big telescopes, David Charbonneau is something of an anomaly. The Caltech astronomer will soon begin a multiyear survey for extrasolar planets at Palomar Observatory—not with the 200-inch Hale telescope, but with a tiny desktop-sized device he and JPL researcher John Trauger assembled largely from parts bought at a camera shop.

Basing his instrument on a standard 300-millimeter telephoto lens for a 35-millimeter camera, Charbonneau will begin sweeping the skies this spring in hopes of catching a slew of "hot Jupiters" as their fast orbits take them in front of other stars. Admittedly, the charge-coupled device at the camera-end of the lens is a good bit more costly than the lens itself, but the total budget for the project—$100,000—is still a paltry sum when one considers that the next generation of earthbound telescopes will likely cost upwards of $400 million apiece.

Charbonneau, a recent import to the Caltech astronomy staff from the Harvard-Smithsonian Center for Astrophysics, is one of the world's leading authorities on the search for "transiting planets," or planets that should be detectable as they pass into the line of sight between their host star and Earth. In November, Charbonneau and his colleagues made international news when they discovered the first planetary atmosphere outside our own solar system. But that work was done with the Hubble Space Telescope. The yet-to-be-formally-named telescope at Palomar Observatory will certainly be more modest in cost, but every bit as ambitious a program for searching out other worlds.

"Basically, the philosophy of this project is that, if we can buy the stuff we need off the shelf, we'll buy it," the Canadian native said recently in his new campus office.

At the fore-end of the new instrument is a standard 300-millimeter camera lens. Charbonneau settled on a telephoto lens because he reasoned that the optics have been honed to a fine degree of precision over the years. Too, he assumed that the lens would be robust enough for the duration of the three-year project.

The charge-coupled device (CCD), a standard imaging tool in astronomy for the last couple of decades, is a $22,000 item that accounts for the largest part of the instrument budget. The CCD will be mounted in a specially constructed camera housing to fit at the back of the lens, and the entire device will be fitted onto an inexpensive equatorial mount—also available at many stores carrying amateur astronomical equipment.

Meanwhile, the Palomar staff has stored away a 20-inch telescope so Charbonneau will have a small dome for his new instrument, and are also doing other preparations to mechanize the actual observing so that a telescope operator will not have to be on site at all times.

Palomar Observatory engineer Hal Petrie says the mountain crew is currently busy linking the new telescope with an existing weather-monitoring system at the nearby 48-inch dome, where another automated telescope is located. The system monitors the atmospheric conditions to determine whether the dome should be opened.

"The new telescope is a very good use of space," says Petrie. "The potential for results is very exciting."

Charbonneau will be able to photograph a single square of sky, about 5 degrees by 5 degrees. That's a field of view in which about 100 full moons could fit. Or, if one prefers, a field of view in which an entire constellation can be seen at one time.

With special software Charbonneau helped develop during his time at Harvard-Smithsonian and at the National Center for Atmospheric Research, he will compare many pictures of the same patch of sky to see if any of the thousands of stars in each field have slightly changed. If the software turns up a star that has dimmed slightly, the reason could well be that a planet passed in front of the star between exposures.

Repeated measurements will allow Charbonneau to measure the orbital period and physical size of each planet, and further work with the 10-meter telescopes at the Keck Observatory will allow him and his colleagues to get spectrographic data, and thus, the mass and composition of each planet.

"Once you get the mass and size, you have the density," he says. Weather permitting, Charbonneau will be able to get up to 300 images during an ideal night. Assuming that he can have 20 good nights per month, he should have about 6,000 images each month show up in his computer.

The ideal time will be in the fall and winter, when the Milky Way is in view, and an extremely high number of stars can be squeezed into each photograph. This, too, is an anomaly in astrophysical research, particularly to cosmologists, for whom the Milky Way is pretty much a blocked view of the deep sky.

"It's estimated that about one in three stars in our field of view will be like the sun, and that one percent of sunlike stars will have a hot Jupiter, or a gas giant that is so close to the star that its orbit is about four or five days," he says.

"One-tenth of this 1 percent will be inclined in the right direction so that the planet will pass in front of its star, so that maybe one in 3,000 stars will have a planet we can detect," Charbonneau adds. "Or if you want to be conservative, about one in 6,000."

Compared to other research programs in astronomy, the search for hot Jupiters is fairly simple and straightforward to explain to the public, Charbonneau says.

"An amateur could do this, except maybe for the debugging of the software, which requires several people working 10 hours a day.

"But it's easy to understand what's going on, and cheap to build the equipment. That's why everyone thinks it's an ideal project—if it works."

The new Palomar telescope is the final instrument in a network of three. Of the other two, one is located in the Canary Islands and operated by the National Center for Atmospheric Research; the other is near Flagstaff, Arizona, and is operated by Lowell Observatory.

The large span in longitude of the three-instrument network will allow Charbonneau and his colleagues to observe a patch of sky with one telescope while the patch is above the horizon in the night sky, and then pass it off to the next westward telescope as the sun comes up.

Contact: Robert Tindol (626) 395-3631

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Caltech scientists demonstratecompact silica laser

A team of applied physicists at the California Institute of Technology have demonstrated an ultrasmall Raman laser that is 1,000 times more efficient than previous devices. The device could have significant applications for telecommunications and other areas where compact, highly efficient, and tunable lasers are desirable.

Reporting in the February 7 issue of the journal Nature, Caltech applied physics professor Kerry Vahala and graduate students Sean Spillane and Tobias Kippenberg describe their progress in making the tiny device, which incorporates a small spherical glass bead and a stretched fiber-optic wire. The laser is especially efficient because of the way it stores light inside the microsphere, or resonator, as well as the manner in which the stretched optical wire permits efficient coupling of light into the sphere.

According to Vahala, the light wraps around the sphere in a ring orbit and subsequently intensifies over hundreds of thousands of orbits, resulting in extreme concentration of optical power within the sphere. In this way, very weak signals applied to the sphere from the fiber-optic wire can build to enormous intensities within the sphere itself.

At these higher power levels, the physics within the sphere enters a nonlinear regime wherein conventional rules for light propagation break down. In the Caltech work, the molecules of the glass bead itself are distorted, resulting in a process called Raman emission and lasing. Because Raman lasers require enormous intensities to function, they are usually power-hungry devices. The Caltech team uses the physics of the sphere to reduce both power and size. Normal Raman lasers turn on "with a shout"—these new devices require "only a whisper."

Central to this breakthrough was the ability to couple directly to the ring orbits, or whispering gallery modes, of the sphere while preserving the exquisite perfection of the sphere in terms of its ability to store and concentrate light. The Caltech team uses stretched optical fiber in the form of a taper to achieve coupling efficiencies, in which loss is negligible, both to and from the sphere.

Because Raman lasers and amplifiers can operate over a very broad range of wavelengths, they are important devices that extend other lasers into new or previously inaccessible wavelength bands. For example, Raman amplifiers are now used widely in commercial long-distance fiber communications systems because of this wavelength flexibility.

Also, through a process called cascading, it is possible to cover even greater wavelength bands by using one Raman laser as the pump for another. In this way, a whole series of wavelengths can be generated in a kind of domino effect. More generally, it can be used to extend the wavelength range of other laser sources into difficult-to-access wavelength bands for sensing or other purposes.

The article is titled "Ultralow-threshold Raman laser using a spherical dielectric microcavity," and is available at www.nature.com.

In the photo, the sphere has been doped, which enables observation of the ring orbit as green luminescence. The photo is by M. Cai of Vahala group.

Further discussion of this and related work can be found at the Vahala Caltech group website: www.its.caltech.edu/~vahalagr.

CONTACT: Robert Tindol (626) 395-3631

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Caltech Scientists Block Effect of Huntington Disease Protein in Cultured Cells

PASADENA, Calif.— Huntington's disease is a cruel disorder, destroying nerve cells in the brain that, over time, rob an individual of the ability to walk, talk, and eat. As yet, there is no cure or effective treatment for this hereditary disorder. The end result, then, is death, caused by such complications as infection or heart failure.

Now scientists at the California Institute of Technology have come one step closer to understanding how Huntington's disease develops and how it can be stopped. In a paper to be published in the January 22 issue of the Proceedings of the National Academy of Sciences, Paul H. Patterson, a Caltech professor of biology, postdoctoral scholar Ali Khoshnan, and research assistant Jan Ko have blocked the effects of the disease in cultured cells using antibodies.

Huntington's disease (HD) is caused by a mutation in the protein huntingtin (htt), specifically by the expansion of a site on the protein called polyQ. Such sites induce the production of antibodies that bind with a particular site, normally to kill the antigen. Khoshnan and his colleagues made an antibody that binds to the polyQ site, along with another antibody that binds to a different site, called polyP. The idea was to block either of these sites and see whether the toxic effects of mutant htt, which kills nerve cells in the brain, could be blocked.

"We knew that the polyQ site was critical because when it is expanded by mutation it causes HD," says Patterson." "It was also known that the polyP site on htt might be important for interfering with the functions of other proteins." The investigators produced a modified version of the antibodies that would allow them to be produced inside cells that also carry the toxic mutant htt. They found a key result: when the antibody against the polyP site is produced by cells carrying the mutant htt, the cells are rescued. That is, they are unaffected by the toxic HD protein. In striking contrast, when cells carrying the toxic htt are induced to produce the antibody against the polyQ site, the toxicity of htt is enhanced and the cells die even faster.

Khoshnan and coworkers suggest that the surprising result with the polyQ antibody may be due to the antibody stabilizing a shape of the mutant htt protein in its most deadly form. Most important, though, says Patterson, is that the rescue of the cells that produce the polyP antibody may indicate this is the site of the toxic htt in which the actual killing of cells takes place, and that covering it up with an antibody saves the cell. "Or, an alternative interpretation is that the binding of the antibody preserves the protein in a non-toxic shape," he says.

The researchers have two goals in mind with their work: elucidating the mechanism of neuronal death caused by mutant htt, and devising molecular strategies for blocking its toxic effects.

To arrive at their results, the scientists first developed eight monoclonal antibodies (mAbs), finding the three that either inhibited or exacerbated the toxicity of the mutant Htt protein. They next cloned the antigen-binding "domains" of the three; that is, the portion of the mAbs that does the actual binding. Finally, they caused these domains to be produced inside cells that were also making the mutant htt.

"Potentially, this knowledge could be useful in designing a therapeutic drug, one that covers up that part of the mutant protein that kills healthy cells," says Patterson. "The next stage of the work will be to deliver this antibody into the brains of mice that carry the human mutant gene and that have developed motor symptoms that are related to the disease. We want to see if this antibody can rescue these mice, even after they show signs of the disease. These experiments are, however, just beginning."

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Caltech biologists invent newer, better methodfor making transgenic animals

Using specially prepared HIV-derived viruses stripped of their disease-causing potential, California Institute of Technology biologist David Baltimore and his team have invented a new method of introducing foreign DNA into animals that could have wide-ranging applications in biotechnology and experimental biology.

The Baltimore team reports, on today's Science Express Web site, on their study of single-cell mouse embryos that have been virally infected in a manner that leaves a new gene from a jellyfish permanently deposited into their genomes. The mice, after they have been carried to term, carry at least one copy of the gene in 80 percent of the cases, and 90 percent of these show high levels of the jellyfish protein. Further, the study shows that the offspring of the mice inherit the genes and make the new protein. Thus the method makes transgenic mice that have new genetic potential.

According to Baltimore, who is president of Caltech, the use of the HIV-like viruses could prove far superior to the current method of producing transgenic animals by pronuclear injection.

"It's surprising how well it works," says Baltimore, whose Nobel Prize-winning research on the genetic mechanisms of viruses 30 years ago is central to the new technique. "This technique is much easier and more efficient than the procedure now commonly in use, and the results suggest that it can be used to generate other transgenic animal species."

The technique exploits features of HIV-like viruses known as lentiviruses, which can infect both dividing and non-dividing cells, as gene delivery vehicles. Unlike HIV, the lentivirus is rendered incapable of causing AIDS. The lentivirus carries new genes into the cell's existing genome. In this case, newly fertilized mouse eggs were engineered to carry the green fluorescent protein (GFP) derived from jellyfish.

Baltimore and his team developed two ways of introducing the lentivirus into cells: microinjection of virus under the layer that protects recently fertilized eggs, or incubation of denuded fertilized eggs in a concentrated solution of the virus. The latter method is easier, although less efficient.

The transgenic mice, once they are born, carry a protein marker in all body tissues that make them glow green under a fluorescent light. This trait is genetic because the trait is a permanent feature of the animal's genome, and thus is carried throughout life and is inheritable by offspring. The term "transgene" refers to the fact that the new gene has been transferred.

Transgenics holds promise to biotechnology and experimental biology because the techniques can be used to "engineer" new, desirable traits in plants and animals, provided the trait can be identified and localized in another organism's genome. A transgenic cow, for example, might be engineered to produce milk containing therapeutic human proteins, or a transgenic chicken might produce eggs low in cholesterol.

In experimental biology, transgenics are valuable laboratory animals for fundamental research. A cat with an altered visual system, for example, might better accommodate fundamental studies of the nature of vision.

According to Baltimore, the procedure works on rats as well as mice. This is a huge advantage to experimentalists because of the number of laboratory applications in which rats are preferable, he says.

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Astronomers detect atmosphereof planet outside solar system

Astronomers using NASA's Hubble Space Telescope have made the first direct detection of the atmosphere of a planet orbiting a star outside our solar system and have obtained the first information about its chemical composition. Their unique observations demonstrate that it is possible with Hubble and other telescopes to measure the chemical makeup of extrasolar planet atmospheres and to potentially search for chemical markers of life beyond Earth.

The planet orbits a yellow, sunlike star called HD 209458, a seventh-magnitude star (visible through an amateur telescope), which lies 150 light-years away in the autumn constellation Pegasus. Its atmospheric composition was probed when the planet passed in front of its parent star, allowing astronomers for the first time ever to see light from the star filtered through the planet's atmosphere.

Lead investigator David Charbonneau of the California Institute of Technology and the Harvard-Smithsonian Center for Astrophysics, Timothy Brown of the National Center for Atmospheric Research, and colleagues used Hubble's spectrometer (the Space Telescope Imaging Spectrograph) to detect the presence of sodium in the planet's atmosphere.

"This opens up an exciting new phase of extrasolar planet exploration, where we can begin to compare and contrast the atmospheres of planets around other stars," says Charbonneau. The astronomers actually saw less sodium than predicted for the Jupiter-class planet, leading to one interpretation that high-altitude clouds in the alien atmosphere may have blocked some of the light. The findings will be published in the Astrophysical Journal.

The Hubble observation was not tuned to look for gases expected in a life-sustaining atmosphere (which is improbable for a planet as hot as the one observed). Nevertheless, this unique observing technique opens a new phase in the exploration of extrasolar planets, say astronomers.

Such observations could potentially provide the first direct evidence for life beyond Earth by measuring unusual abundances of atmospheric gases caused by the presence of living organisms. The planet orbiting HD 209458 was discovered in 1999 through its slight gravitational tug on the star. Based on that observation the planet is estimated to be 70 percent the mass of the giant planet Jupiter (or 220 times more massive than Earth).

Subsequently, astronomers discovered the planet passes in front of the star, causing the star to dim very slightly for the transit's duration. This means the planet's orbit happens to be tilted edge-on to our line-of-sight from Earth. It is the only example of a transit among all the extrasolar planets discovered to date.

The planet is an ideal target for repeat observations because it transits the star every 3.5 days—which is the extremely short amount of time it takes the planet to whirl around the star at a distance of merely 4 million miles from the star's searing surface. This precariously close proximity to the star heats the planet's atmosphere to a torrid 2,000 degrees Fahrenheit (1,100 degrees Celsius).

Previous transit observations by Hubble and ground-based telescopes confirmed that the planet is primarily gaseous, rather than liquid or solid, because it has a density less than that of water. (Earth, a rocky rather than a gaseous planet, has an average density five times that of water.) These earlier observations thus established that the planet is a gas giant, like Jupiter and Saturn.

The planet's swift orbit allowed for observations of four separate transits to be made by Hubble in search of direct evidence of an atmosphere. During each transit a small fraction of the star's light passed through the planet's atmosphere on its way to Earth. When the color of the light was analyzed by a spectrograph, the telltale "fingerprint" of sodium was detected. Though the star also has sodium in its outer layers, the STIS precisely measured the added influence of sodium in the planet's atmosphere.

The team—including Robert Noyes of the Harvard-Smithsonian Center for Astrophysics and Ronald Gilliland of the Space Telescope Science Institute in Baltimore, Maryland—next plans to look at HD 209458 again with Hubble, in other colors of the star's spectrum to see which are filtered by the planet's atmosphere. They hope eventually to detect methane, water vapor, potassium, and other chemicals in the planet's atmosphere. Once other transiting giants are found in the next few years, the team expects to characterize chemical differences among the atmospheres of these planets.

These anticipated findings would ultimately help astronomers better understand a bizarre class of extrasolar planets discovered in recent years that are dubbed "hot Jupiters." They are the size of Jupiter but orbit closer to their stars than the tiny innermost planet Mercury in our solar system. While Mercury is a scorched rock, these planets have enough gravity to hold onto their atmospheres, though some are hot enough to melt copper.

Conventional theory is that these giant planets could not have been born so close to their stars. Gravitational interactions with other planetary bodies or gravitational forces in a circumstellar disk must have carried these giants via spiraling orbits precariously close to their stars from their birthplace farther out, where they bulked up on gas and dust as they formed.

Proposed moderate-sized U.S. and European space telescopes could allow for the detection of many much smaller Earth-like planets by transit techniques within the next decade. The chances for detection will be more challenging, since detecting a planet orbiting at an Earth-like distance will mean a much tighter orbital alignment is needed for a transit. And the transits would be much less frequent for planets with an orbital period of a year, rather than days. Eventually, study of the atmosphere of these Earth-like planets will require meticulous measurements by future larger space telescopes.

The Space Telescope Science Institute (STScI) is operated by the Association of Universities for Research in Astronomy (AURA), for NASA, under contract with the Goddard Space Flight Center, Greenbelt, Maryland. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA). The National Center for Atmospheric Research's primary sponsor is the National Science Foundation.

Contact: Robert Tindol (626) 395-3631

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Sound alters the activity of visual areas in the human brain,Caltech research reveals

Scientists at the California Institute of Technology have discovered that hearing can significantly change visual perception, and that the influence of hearing on visual perception occurs at an early perceptual level rather than at a higher cognitive level.

Ladan Shams, a Caltech postdoctoral researcher, and Shinsuke Shimojo, a professor of computation and neural systems at Caltech report that visual signals are influenced significantly by sounds at early cortical levels that have been believed to be "vision specific."

The team's initial behavioral finding was that when an observer is shown one flash of light accompanied by two beeps, the visual system is tricked so that the observer sees two flashes instead of one. In the new study, 13 healthy volunteers were asked to observe the stimuli on a computer screen and judge the number of flashes they saw on the screen.

While the participants performed the task, their brains' electric potentials were recorded from three electrodes positioned in the back of the scalp, where the early visual areas are located.

The researchers found that when the participants perceived the illusion—in other words, when sound changed the visual perception—the activity in the visual areas was modified. Furthermore, the change in activity was similar to that induced by an additional physical flash.

This suggests that the second flash, which is nothing but an illusion and is not due to a visual stimulus but rather caused by sound, invokes activity in the visual areas very similar to that which would be caused by a physical second flash. In short, sound induces a similar effect in this area of the brain to a visual stimulus.

The goal of this study was to get an understanding of how this alteration of vision by sound occurs in the brain. More specifically, the researchers asked whether the change in visual perception is caused by a change in the higher-level areas of the brain that are known to combine information from multiple senses, or whether it is a change that directly affects the activity of the areas that are believed to be exclusively involved in processing visual information.

The main result of the study was that the early visual cortical responses were modulated by accompanied sounds under conditions where the observers experienced the double-flash illusion. This suggests that the activity of the "visual" areas in the brain is affected by sound.

These findings challenge two traditional perspectives on how the brain processes sensory information. The first assumption is that humans are visual animals; vision is the dominant modality and hence not malleable by information from other modalities. Another general belief is that the information from different modalities is processed in the brain in parallel and separate paths.

The findings show that the visual information is affected by the auditory signals while being processed in the "modality-specific" visual pathway. These findings, together with earlier results in other modalities, suggest a paradigm of sensory processing that is more intertwined than segregated.

"The findings have an important implication for the new studies of human perception," says Shimojo. The overwhelming majority of studies in the field of perception have concerned themselves with one modality alone (based on the assumption of modality segregation).

This study, together with other studies indicating vigorous early plasticity and interactions across sensory modalities, is also very encouraging for applications such as sensory aids for children suffering from blindness or dyslexia, for educational applications, for man-made interfaces, and for media and information technology.

A report on this study will appear in the December issue of the journal NeuroReport. Ladan Shams is lead author of the paper.

Contact: Robert Tindol (626) 395-3631

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First gamma burst detected by new NASA satellite is pinpointed at Palomar Observatory

Astrophysicists have combined the Palomar Mountain 200-inch Hale Telescope with the abilities of a new NASA satellite to detect and characterize a gamma-ray burst lying at a distance of only 5 billion light-years from Earth. This is the closest gamma-ray burst ever studied by optical telescopes.

The origin of cosmic gamma-ray bursts, spectacular flashes of high-energy radiation followed by slowly decaying optical and radio emission that can be seen from great distances, is still a puzzle to astronomers. Many scientists believe that the bursts result from explosions that signal the birth of black holes; however, all agree that more data are needed before we can really know black holes' origins and nature.

NASA's new High-Energy Transient Explorer (HETE) detected a gamma-ray burst on September 21. Data indicated that the event was located in the Lacerta constellation, and refined information from the Interplanetary Network (IPN), a series of satellites with gamma-ray detectors scattered about the solar system, reduced the region astronomers needed to search to find the fading embers of the explosion. Scientists at the California Institute of Technology's Palomar Observatory, using the historic Hale 200-inch reflector, were able to locate the visual afterglow the following day. This was the first burst from the HETE satellite to be pinpointed with an accuracy sufficient to study the remains.

On October 17 the Caltech team used the Hale Telescope to obtain a redshift for the burst. This allowed a distance to be inferred, implying that the burst happened some 5 billion years ago. This makes the burst one of the closest ever found, and thus easier to study in detail. Also on October 17 the team members, led by Dale Frail from the National Radio Astronomy Observatory, detected a twinkling radio counterpart of the burst using the Very Large Array in New Mexico.

According to Shri Kulkarni, who is the MacArthur Professor of Astronomy and Planetary Science at Caltech, the team was able to find the rare optical afterglow because of the quick detection and localization abilities of the HETE satellite and the rapid follow-up with the Palomar Mountain Hale Telescope.

HETE, the first satellite dedicated to the study of gamma-ray bursts, is on an extended mission until 2004. Launched on October 9, 2000, HETE was built by MIT as a mission of opportunity under the NASA Explorer Program. The HETE program is a collaboration between MIT; NASA; Los Alamos National Laboratory, New Mexico; France's Centre National d'Etudes Spatiales, Centre d'Etude Spatiale des Rayonnements, and Ecole Nationale Superieure del'Aeronautique et de l'Espace; and Japan's Institute of Physical and Chemical Research. The science team includes members from the University of California (Berkeley and Santa Cruz) and the University of Chicago, as well as from Brazil, India, and Italy.

"I'm very excited. I could not sleep for two nights after making the discovery," said Paul Price, the Caltech graduate student who first identified the optical afterglow from Palomar.

"With this first confirmed observation of a gamma-ray burst and its afterglow, we've really turned the corner," said George Ricker of the Massachusetts Institute of Technology, principal investigator for HETE. "As HETE locates more of these bursts and reports them quickly, we will begin to understand what causes them.

"The unique power of HETE is that it not only detects a large sample of these bursts, but it also relays the accurate location of each burst in real time to ground-based optical and radio observatories," Ricker said.

Because the enigmatic bursts disappear so quickly, scientists can best study the events by way of their afterglow. HETE detects these bursts as gamma rays or high-energy X rays, and then instantly relays the coordinates to a network of ground-based and orbiting telescopes for follow-up searches for such afterglows.

Additional observations of this event, made with the Italian BeppoSAX satellite and the Ulysses space probe, were coordinated by HETE team member Kevin Hurley at the University of California. The combination of the localization by the Interplanetary Network with the original HETE localization provided the refined information needed by ground-based observers to point their optical telescopes.

The opportunity to see the afterglow in optical light provides crucial information about what is triggering these mysterious bursts, which scientists speculate to be the explosion of massive stars, the merging of neutron stars and black holes, or possibly both. Follow-up observations of GRB 010921 using the Hubble Space Telescope and the telescopes on the ground should move us a few steps closer to the answer of this cosmic puzzle.

The team that identified the counterpart to GRB010921 includes—in addition to Caltech Professors Shri Kulkarni, Fiona Harrison, and S. George Djorgovski—postdoctoral fellows and scholars Re'em Sari, Titus Galama, Daniel Reichart, Derek Fox, and Ashish Mahabal, graduate students Joshua Bloom, Paul Price, Edo Berger, and Sara Yost, Dale Frail from the National Radio Astronomy Observatory, and many other collaborators.

More information on HETE can be found at: http://space.mit.edu/HETE Palomar Observatory: http://www.astro.caltech.edu/palomar/ Caltech Media Relations: http://pr.caltech.edu/media/.

Writer: 
Robert Tindol
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