Question of the Week: How Do We Know That a Rock Found In The Ice In Antarctica Came From Mars?

Question of the Month Submitted by: Audra Martin, La Puente, Calif., and answered by: Bill Bottke, Postdoctoral Fellow, Division of Geological and Planetary Sciences, Caltech.

Meteorites are rocks that fall to Earth from space. Most are thought to be asteroid fragments that have survived fiery entry through Earth's atmosphere. Twelve of the thousand or so meteorites held in worldwide collections, however, are thought to come from Mars. Eleven of these meteorites, formed roughly 1.3 billion years ago, were named the SNC meteorites after the sites where they were found: Shergotty (India), Nakhla (Egypt), and Chassigny (France).

The 12th Martian meteorite is much older and different from the rest. It is called ALH 84001, named for the year it was discovered, 1984, and the location where it was found, Allen Hills, Antarctica. It was formed 4.5 billion years ago, when Mars had a much thicker atmosphere and liquid water on its surface. Recently, scientists have suggested that ALH 84001 might even contain fossil evidence of ancient Martian life.

How do we know that these meteorites are from Mars, when people have never been there and no rocks have been collected on its surface? In 1976, two NASA spacecraft named Viking 1 and 2 landed on Mars and analyzed its atmosphere and surface. These spacecraft examined soil and air samples using onboard instruments, making careful measurements and radioing their data back to scientists on Earth. After careful study, it was determined that the atmosphere of Mars was very different from Earth's atmosphere or any other combination of gases found in the solar system. Then, by analyzing small traces of gas trapped in the interior of these 12 meteorites, scientists were able to identify the characteristic "fingerprint" of the Martian atmosphere, proof that these rocks were blasted off the surface of Mars at some time in the past.

Even before gas was discovered in these meteorites, scientists were suspicious that they might have originated on Mars. The SNC meteorites have young formation ages, and all 12 Martian meteorites have complex chemical compositions that set them apart from other known meteorite classes. Moreover, the abundance of oxygen isotopes (different kinds of oxygen) in the meteorites are inconsistent with oxygen isotopes found in Earth rocks.

Question of the Month: We Hear of Humans Going to Distant Planets in the Future. But if Some Planets Are Light-years Away, How Could An Astronaut Live Long Enough To Get There?

Question of the Month Submitted by: Richard Scott, Whittier, Calif., and answered by: Teviet Creighton, Caltech doctoral student in theoretical physics.

At first, it might seem unlikely. After all, even the nearest star (after the Sun) is over four light-years away — that is, it would take more than four years for light, traveling at 300,000 kilometers per second, to make the trip. Most points of interest are much farther; the center of our galaxy, for instance, is roughly 30,000 light-years away. And as you may know, Einstein's theory of relativity tells us that nothing can accelerate past the speed of light. How could anyone hope to live through a flight lasting over 30,000 years?

Fortunately, the same theory which imposes this universal speed limit also provides a sneaky way of getting around it, at least for the people making the trip. In the theory of relativity, the time measured by a person moving very quickly (near the speed of light) is not the same as the time measured by someone standing still. In particular, if someone (call her Alice) accelerates to very near the speed of light and then decelerates to a stop, the time that she measures for the trip will be less than the time measured by her brother Bert, who was standing still. And the closer she comes to the speed of light (without, of course, ever going faster than light), the bigger the difference in times. This effect is called "time dilation."

Now let us imagine Alice taking a trip to a distant star. Her starship is designed to accelerate at "one gee," which means that the acceleration of the ship pushes her as hard as the force of gravity would on Earth. The farther she goes, the more speed she builds up along the way, and the more time she saves because of time dilation. In this way even the most distant galaxies can be reached within her lifetime! Here are her travel times to a few interesting destinations:

Nearest star (4.3 light-years away) ................... 3.6 years Center of our galaxy (30,000 light-years away) ......... 21 years Andromeda galaxy (2 million light-years away) .......... 29 years Edge of known universe (15 billion light-years away) ... 47 years

If she returns to Earth, the return trip will take an equal amount of time. However, people on Earth will have continued to age at the normal, undilated rate. So while Alice might be able to travel to Andromeda and back in only 60 years (as she sees it), the Earth and its people will have aged 4 million years!

Neural Research Shows That the Nose Needs Time To Smell

PASADENA— New research from the California Institute of Technology shows that it literally takes some time to smell the roses.

In the current issue of Nature, Caltech neuroscientists Michael Wehr and Gilles Laurent present work demonstrating that information about odors is contained in the temporal activity patterns of groups of neurons over an interval of time.

"Perfumers sometimes speak of 'top notes' and 'medium notes' in a bouquet," says Laurent, associate professor of biology and computation and neural systems. "These refer to early and late perceptions that unfold over time during long sniffs or successive sniffs. Our new research suggests that the brain is actually representing odors by making a neural melody of its own."

A helpful analogy Laurent offers for the research is the musical notes that make up a tune. A listener can perceive one note in an instant, but must listen for a time before he or she can recognize the tune. Therefore, the specific manner in which the notes follow one another is the very thing that gives a song its individual character.

"It is the order in which specific neurons are activated that appears to contain useful information about the identity of the odor," says Laurent, adding that different odors cause different neural "melodies."

Laurent and Wehr, a graduate student in computation and neural systems, did their research by analyzing the brain waves of locusts. When an odor was wafted by the olfactory organ of the locust, the collective response of neurons in the olfactory brain was such that specificity in the responses arose from considerations of their temporal characteristics. And because olfactory systems are very similar among most animals, the researchers think that these coding principles may be common to most, including humans.

What happens in the brain during the act of smelling is not well understood, but has been known for a long time to involve neural synchronization and oscillations of the EEG. The function of oscillations, which are observed also in all other sensory areas of the brain, remains totally speculative.

"If our hypothesis is right, oscillations are a kind of clock for the temporal codes we observe," Laurent says. In a parallel study by Laurent and Caltech behavioral biology graduate student Katrina MacLeod in the November 8 issue of Science, the authors described a method by which the neurons representing odors can be simply desynchronized, thereby eliminating the clock signal for the temporal codes. This result will now allow the researchers to directly test, in future experiments, whether these temporal codes are essential for odor perception.

The conclusions of the researchers is that animals as primitive as snails and as complex as humans do some mental "data crunching" each time they pick up a smell. This allows the neurons to separate a certain odor from the background, provided a window of time is available.

In a manner of speaking, the research shows that time is of the essence, and vice versa.

Writer: 
Robert Tindol
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Caltech Scientists Offer Theory of Ganymede's Oxygen and Ozone

Tucson, Arizona — When Galileo discovered Ganymede four centuries ago, little did he suspect that the third satellite from Jupiter might be glazed over with the very substance he was breathing.

It took modern astronomical instruments and chemical knowledge for scientists to detect the oxygen and ozone that coat Ganymede. Now, two planetary scientists affiliated with the California Institute of Technology have developed a theory to account for the presence of the substances, as well as the mechanism by which their concentrations are maintained.

Dr. Yuk Yung and Dr. Ming-Taun Leu, in a presentation at the annual Division for Planetary Science (DPS) meeting of the American Astronomical Society, today provide an explanation for the amount of oxygen and ozone detected by ground-based telescopes and the Hubble Space Telescope. According to Yung, the oxygen and ozone are remnants of the primordial water that became part of Ganymede when the solar system was formed 4.5 billion years ago.

Yung, professor of planetary science at Caltech, theorizes that the water ice on Ganymede has since time immemorial been attacked by two sources: ultraviolet light from the sun, and ions thrown off by the volcanic activity of the sister Jovian satellite Io. Both sources of disturbance have the effect of blasting the water molecules apart. Once the hydrogen and oxygen of the water are separated, the lighter, energetic hydrogen ions blast away from the light gravity of Ganymede into outer space, while the heavier oxygen molecules settle back onto the surface.

Yung also offers a theory to account for the relatively high concentration of molecular oxygen and ozone on Ganymede's surface. While each is present at about one-thousandth its concentration on Earth, both are far and away more prevalent than on any other body in the solar system except Mars, which by coincidence has roughly the same concentrations.

"So far, Earth, Mars, and Ganymede are the only bodies in the solar system with ozone," said Yung in an interview prior to the DPS conference.

Yung's theory of oxygen and ozone concentrations on Ganymede assumes the presence of a vast structure of tiny surface cracks in the ice where the frozen oxygen and ozone can reside. To test the hypothesis that such cracks indeed exist, Yung's colleague, Ming-Taun Leu of Caltech's Jet Propulsion Laboratory, devised an experiment in which the conditions of Ganymede could be simulated.

The results showed that such cracks could indeed occur in the ice matrix, and that even a very thin surface coating of ice could harbor the high concentrations of oxygen and ozone seen on Ganymede.

As for the destruction of the water molecules, Yung explains that the manner in which the molecules are blown apart can account for the ratio of oxygen to ozone, as well as the net amount of oxygen atoms on the surface. Light energy from the sun, for example, tends to split a molecule of oxygen (O2) into two oxygen atoms, and a molecule of ozone (O3) into a molecule of oxygen and a single oxygen atom.

An oxygen ion that finds its way from Io's volcanoes to Ganymede, however, tends to turn a molecule of oxygen it hits into a molecule of ozone. Or, if it hits an existing molecule of ozone, the particle from Io tends to combine with the three atoms to make two molecules of O2. By this scenario, the energy expended and consumed in these reactions accounts for the amount of oxygen as compared to the amount of ozone.

Yung says that his work is very basic in nature and has little to do with the present amount of oxygen on Earth, because our own planet is dependent on living processes for most of its oxygen. Nonetheless, he says that the study of Ganymede can perhaps lead to answers about how oxygen might have arisen on Earth and Mars before life began.

 

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Robert Tindol
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Caltech Seismo Lab Gets Location Data on October 3 Meteor

PASADENA— Caltech seismologist Kate Hutton has some data that shows where the October 3 meteor may have landed. She's providing the information publicly to help anyone and everyone who wants to try for the $5,000 reward UCLA is offering.

According to Hutton, any larger chunks from the meteor that lit up the Western skies on the night of October 3 may have landed in the Rose Valley area near Little Lake. Hutton figured this out by analyzing data from 31 of the seismic stations belonging to the Southern California Seismographic Network (operated by Caltech and the U.S. Geological Survey).

"As it fell, the atmospheric drag caused the meteoroid to explode in midair at least twice," Hutton says. "The explosions generated sound waves in the air similar to a sonic boom, which were detected by the seismographs.

"Using a procedure that is very similar to the one used to locate earthquakes underground, I used the arrival times of the sound waves at the various seismic stations to estimate where the explosions occurred."

Two of the explosions were well located, Hutton adds. Both were 20 to 30 miles above the Fivemile Canyon area in the eastern Sierra foothills. The explosions were separated by about 25 seconds, and the second was about five miles lower and about a mile further eastward than the first one.

Based on this data and on eyewitness accounts provided by John Wasson of UCLA and Caltech alumnus Mark Boslough of Sandia National Laboratory in New Mexico, Hutton thinks that any larger fragments that survived the fiery entry into Earth's atmosphere would have landed to the east-northeast of the explosions, perhaps in the Rose Valley area near Little Lake. Smaller fragments may have fallen more or less straight down from where the explosions occurred.

The Little Lake area would probably be the more seductive area to search, and for a very good reason. UCLA has offered a $5,000 reward for the first fragment that weighs at least four ounces.

Hutton says the seismographic instruments didn't pick up a meteorite impact on Earth, but this is not surprising, since a single fragment would probably have to weigh several tons in order for its impact to be detected.

The term "meteorite," by the way, refers to chunks of extraterrestrial debris that survive the entry into the atmosphere and end up on the ground. "Meteoroids" are chunks that travel through space, while "meteor" is the proper designation for the light show produced by a rock from outer space slowing down in Earth's atmosphere.

Any surviving meteorite fragments would probably have a fresh black matte crust. If the meteorite struck something on the ground, part of the crust might have chipped off to reveal a lighter interior. If anyone finds a meteorite fragment weighing at least four ounces, he or she should get in touch with Dr. John Wasson at UCLA. Wasson's e-mail address is wasson@igpp.ucla.edu.

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Robert Tindol
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Thundercloud Photos, Theory Suggest That Jupiter Is "Wet" After All

PASADENA— The Galileo probe that dropped into Jupiter's atmosphere last December detected a surprisingly small amount of water. But scientists at the California Institute of Technology have new thundercloud photographs and a theory to suggest that the solar system's largest planet may be "wet" after all.

According to Andrew Ingersoll, a professor of planetary science at Caltech, the probe didn't find much water because it dropped into a cloud-free area of the Jovian atmosphere. Because water would likely be manifested in clouds, Ingersoll thinks the probe might have detected the predicted amount of water had it fallen into another region. As it stands, the probe data suggest that water is about 10 times more scarce than most scientists once thought.

"This was the most cloud-free area on the planet," Ingersoll says, "so the probe went into an anomalous region. A low cloud abundance means that there's less of the condensable substances, including water."

At the 28th annual meeting of the Division for Planetary Sciences of the American Astronomical Society, Ingersoll and colleagues are offering photographic evidence and a theory to back up their assertion that water on Jupiter was merely hidden from the probe. The photographic evidence is a series of near-infrared images taken by the Galileo orbiter of the Great Red Spot. The images were released as part of a report just published in Science by the Galileo imaging team under the leadership of Dr. Michael Belton of Kitt Peak National Observatory near Tucson, Arizona.

The infrared images show several dozen thunderclouds to the northeast of the Great Red Spot. These round clouds jut about 20 miles above the cloud cover, and are of similar diameter.

The researchers are not sure yet whether the clouds are made of water. But they do know that they are high and that they evolve rapidly, like terrestrial thunderstorms. Ingersoll and Caltech graduate student Ashwin Vasavada documented how the features changed over a 70-minute interval.

"Water is the most likely cause of these explosive convective events," says Ingersoll. "Other gases present in Jupiter's atmosphere just don't have the energy."

The new theory, which Ingersoll developed jointly with graduate student Adam Showman, argues that the cloudless region the probe dropped into is part of a huge downdraft that extends deep into Jupiter's fluid interior.

Large-scale downdrafts, like those over Earth's deserts, are dry because the moisture falls out in the neighboring updraft regions, Ingersoll explains. "The trick here is to keep the downdraft going to the level where the Galileo probe failed, which is 40 miles below the cloud base. You have to keep the dry descending air from mixing with the moist air from Jupiter's interior.

"That can happen if the descending air is less dense (lighter) than the surroundings, but that violates another rule that says less dense air wants to rise.

"We are saying that the region where the probe went in is a heat engine running backward, like a refrigerator. It takes energy from the convection at neighboring latitudes, and uses it to stuff the less dense air downward. The energy released in the neighboring latitudes is not dissipated, so it has to go somewhere."

Thus, the researchers believe that the Galileo probe could have missed the water, and that Jupiter can still be viewed as a planet of violent thunderstorms, lightning, hurricanes, and rain.

"But if they get a good sample of different places on the planet and still fail to find water," Ingersoll says, "then I'll throw in the towel."

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Robert Tindol
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Caltech Biologists Identify Gene Thought to Initiate Neural Development

PASADENA— Biologists have identified a gene that determines whether a given cell in a human or animal embryo will become a neuron rather than some other kind of cell.

In an article appearing in today's journal Cell , California Institute of Technology Professor of Biology David Anderson and his colleagues announce that the gene encodes neurogenin, a member of the basic-helix-loop-helix (bHLH) family of proteins, which in turn control the activity of other genes. When neurogenin RNA appears in cells of the early embryo, the research shows, a genetic chain-reaction begins that turns the cell into a neuron.

According to Anderson, the discovery provides an important piece of information about how embryonic cells develop into cells with specific functions and locales within an organism. Up to a certain stage, all cells in an early embryo look alike in a microscope. But forces are at work that determine whether a specific cell will be a neuron, a muscle cell, a germ cell for sexual reproduction, or any of the other types of cell that make up an organism.

"It's been clear for decades that cells are different in an invisible way long before we are able to see them as being visibly different," says Anderson. "The idea has been that there must be specific genes that confer this invisible predisposition on particular cells."

To demonstrate that neurogenin indeed fulfills such a function, Anderson's coauthor Chris Kintner of the Salk Institute injected tiny amounts of neurogenin RNA in the embryo of a toad. Kintner performed the procedure on the left side of toad embryos at the two-cell stage, so that the effect of the injection could be traced as early in development as possible. The right side was left untouched so that it could serve as a "control."

As each embryo continued to grow by means of cell division, the side that had been flooded with neurogenin RNA became filled with neurons, while the right side developed in a normal manner. This indicated to the researchers that neurogenin is the substance that begins the cascade of genetic steps that turn an undifferentiated cell into a neuron.

Importantly, Anderson, Kintner and colleague Qiufu Ma (a research fellow in biology at Caltech) showed that, once cells make neurogenin, they inhibit their neighbors from becoming neurons by inhibiting their production of neurogenin. Thus, uncommitted embryonic cells are engaged in a winner-take-all competition to become neurons, the winner being decided by the cell that makes the highest level of neurogenin.

The research also showed that other genes suspected of being the initiators of neural development, such as neuroD, actually come into play later after the process has been started by neurogenin. Moreover, the fact that mouse neurogenin RNA can also be successfully used to artificially activate neurogenesis in frog embryos suggests that little difference exists in the gene from species to species.

"The neurogenin (used in the research) is probably about 80 percent identical in its 'business end' to that in humans," Anderson said.

The research builds on earlier work done by J. E. Lee and the late Harold Weintraub at the Fred Hutchinson Cancer Research Center in Seattle, Washington, as well as work done by others on the early development of nerve cells in fruit flies.

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Robert Tindol
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Question of the Month: How Do We Know That a Rock Found In the Ice In Antarctica Came From Mars?

Submitted by: Audra Martin, La Puente

Answered by: Bill Bottke, Postdoctoral Fellow, Division of Geological and Planetary Sciences

The short answer is that we do not know where the moon came from. It's difficult to know, because we have too few examples. Earth is the only terrestrial planet (that is, the only planet within the inner solar system, and made of rock as opposed to gas) that has a large satellite.How do we know that meteorites come from Mars?

Meteorites are rocks which fall to Earth from space. Most are thought to be fragments of asteroids which have survived fiery entry through Earth's atmosphere. Twelve of the thousand or so meteorites held in worldwide collections, however, are thought to come from Mars. Eleven of these meteorites, formed roughly 1.3 billion years ago, were named the SNC meteorites after the sites where they were found: "S"hergotty (India), "N"akhla (Egypt), and "C"hassigny (France).

The twelveth Martian meteorite is much older and different from the rest. It is called ALH 84001, named for the year it was discovered, 1984, and the location it was found, Allen Hills, Antarctica. It was formed 4.5 billion years ago, such that it was present when Mars had a much thicker atmosphere and liquid water on its surface. Recently, scientists have suggested that ALH 84001 might even contain fossil evidence for ancient Martian life.

How do we know that these meteorites are from Mars when people have never been there and no rocks have been collected on its surface? In 1976, two NASA spacecraft named Viking I and II landed on Mars and analyzed its atmosphere and surface. These spacecraft examined soil and air samples using on-board instruments, making careful measurements and radioing their data back to scientists on Earth. After careful study, it was determined that Mars' atmosphere was very different from Earth's atmosphere or any other combination of gases found in the solar system. Then, by analyzing small traces of gas trapped in the interior of these twelve meteorites, scientists were able to identify the characteristic "fingerprint" of the Martian atmosphere, proof that these rocks were blasted off Mars' surface at some time in the past.

Even before gas was discovered in these meteorites, scientists were suspicious that they might have originated on Mars. The SNC meteorites have young formation ages, and all twelve Martian meteorites have complex chemical compositions which set them apart from other known meteorites classes. Moreover, the abundance of oxygen isotopes (different kinds of oxygen) in the meteorites are inconsistent with oxygen isotopes found in Earth rocks.

However, the idea that makes most sense is that the moon arose from a giant impact during the formation of Earth about 4.5 billion years ago. While Earth was being formed, it was hit by very large objects, including bodies as large as Mars (about one-tenth the mass of Earth). If you had hit the growing Earth with such a body, material would have splashed out from the impact site, and a sizable amount of that would have gone into Earth orbit. This material may have come partly from the impacting body and partly from Earth itself. We think that this material could have then aggregated rather quickly into the moon as we know it.

According to this view of the origin of the moon, you would therefore have created a body that started out close to Earth (much, much closer than the present distance between Earth and the moon). This body would have been very hot, probably molten, because of the intense energy created in the impact, and it would have been a body with a composition similar to the outer parts of Earth, not Earth as a whole. The central part of Earth is an iron core; Earth as a whole has a lot of iron but the outer parts of Earth do not.

All of these characteristics are in agreement with the moon as we see it. The moon is moving away from Earth steadily and was therefore once much closer to Earth; it moves away because the tides that are raised by the moon in the oceans of Earth cause angular momentum to be transferred from the spin of Earth to the orbital motion of the moon.

In addition, we know from looking at lunar rock that the moon was once very hot, perhaps completely molten. When you look at the moon in the night sky or through a telescope, you see dark-colored regions and bright-colored regions; the bright-colored regions, called lunar highlands, are composed of very ancient rocks that arose through crystallization from molten rock, and this fits in with the idea of a very traumatic beginning.

The moon also has very little if any metallic iron; it does not have a significant core. This fits in with the idea that the moon is derived >from material that was in the outermost parts of the early Earth or of the projectile, most likely both. The absence of a core in the moon is otherwise very difficult to explain-- it's one of the main reasons we do not think that the moon formed somewhere else in the solar system and then got captured, since if it had formed somewhere else it would be very difficult to understand why it has this composition.

Perhaps the main reason why this idea of a giant impact is attractive is that the angular momentum of Earth and moon together is about what you would get from such an impact. It turns out that this amount of angular momentum, which was once in the spin of Earth and is now mostly in the orbital motion of the moon, is about what you would get if an object approximately the mass of Mars hit Earth (obliquely, not head on). So all of these things fit in together.

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Robert Tindol
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New Diagnostic Test Announced for Group of Brain Diseases

PASADENA— Scientists have developed a simple diagnostic test for transmissible spongiform encephalopathies (TSEs), a group of invariably fatal brain diseases that include "Mad Cow" disease in cattle and Creutzfeldt-Jakob disease (CJD) and kuru in humans.

According to Dr. Michael Harrington of the California Institute of Technology, he and his colleagues at the National Institute of Neurological Disorders and Stroke have developed the test by identifying a diagnostic protein found in the spinal fluid of infected humans and animals. Their research appears today in the New England Journal of Medicine.

Harrington, a scientist at Caltech's Beckman Institute, says that the test is an important contribution to public health because it can help prevent future transmissions of the diseases. "This should reduce the risk of accidental transmission, allow better patient management, and could even provide an objective measure for any future treatment," Harrington says.

Dr. Kelvin Lee, Harrington's colleague at Caltech, adds that the test should be of considerable interest in Great Britain. In the last year, a form of the disease known as bovine spongiform encephalopathy (also known as BSE, or "Mad Cow" disease) has been linked to a new strain of CJD, which has affected several individuals in Britain.

"This test potentially enables us to screen cattle and herds for BSE and thus reduce the possibility that BSE-contaminated beef could enter the food chain," Lee says. "Moreover, it will provide a means for selectively destroying BSE cattle as opposed to unaffected cattle, and make the international exchange of animals more safe."

TSEs are degenerative diseases of the nervous system characterized by rapidly progressive dementia and uncontrolled limb spasms in both humans and animals. The diseases are always fatal, and postmortem examinations reveal spongelike holes in the brain.

The causitive agent of these transmissible diseases is believed to be a prion, an infectious protein that accumulates in the brain and results in neuronal destruction. Prions have been shown to be particularly resistant to standard decontamination procedures.

Diagnosis of these disorders has relied on brain biopsy or postmortem examination for the presence of the prion. In work published by Harrington in 1986, he showed that a particular protein was present in the cerebrospinal fluid of patients affected with CJD and could serve as a useful molecular marker for this disease.

The new findings identify that protein as 14-3-3, a normal neuronal protein. "We hypothesize that 14-3-3, which is normally present in neurons, leaks into the spinal fluid as a result of the neuronal destruction that occurs in TSEs," says Harrington. The identification of the protein as 14-3-3 has enabled the development of a simpler test for the diseases.

Also involved in the study are Drs. Clarence J. Gibbs, Jr., Kimbra Kenney and Gary Hsich, all of the National Institutes of Health's National Institute of Neurological Disorders and Stroke.

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Robert Tindol
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New Research Shows How the Eyes Help the Body Navigate

PASADENA— Neuroscientists have new results on how our brains and eyes work together in getting our bodies from point A to point B without mishap. The research appears in today's issue of the journal Science.

According to Dr. Richard Andersen, James G. Boswell Professor of Neuroscience at the California Institute of Technology, the research shows how different parts of the brain work together to allow for navigation. When a driver is speeding down the interstate and looking at exit signs, for example, he can competently remain traveling in a straight line because of the way his brain is wired. The study in Science reveals more about the precise nature of this wiring.

Andersen's studies of neural impulses in the brain show that humans and animals see things literally in a straightforward way if they are moving and looking directly ahead. When the movement is simple, a fingernail-size area located within the cortex of each hemisphere of the brain an inch or two above each ear interprets the data from the eyes and allows the body to navigate forward. This region is known as the dorso-medial superior temporal area (MSTd), and is sufficient to compute the direction of self-motion as long as the eyes and body are both pointed directly ahead.

But when it becomes necessary or convenient to look around while the body is traveling forward, things get a bit more complicated in the brain. Andersen's research shows that the MSTd area still likes to interpret motion with the same visual neurons at work, but that additional information must come from another part of the brain to help process the more complicated information.

"When the eyes move, the images on the eyes shift as a result of the eye movements," says Andersen. "The brain knows that it's moving its eyes, so a signal about the eye movement is sent to MSTd."

The eye-movement information is then combined with the incoming visual signals to allow for reliable navigation to take place. The process is automatic, and presumably evolved so that humans and animals could walk forward while glancing to the side, Andersen adds.

"We know that the brain has no problem in doing this, just from the fact that people don't drive off the road when they look around," says Andersen.

Though the Science article reveals much about how the brain combines images and motor signals to allow for navigation, Andersen says that additional research should reveal more about the precise mechanism.

For example, the researchers are not yet sure whether motor areas of the brain that are responsible for moving the eyes are indeed projecting the signals back into this sensory area. Another possibility is that sensors in the eye muscles themselves tell the brain what the eyes are doing when they glance about. Also, the researchers would like to find out precisely how the mechanism works when the head rotates but the eyes do not move at all. Again, from everyday experience, humans are able to glance about either by moving the eyes or moving the entire head while moving in another direction, and one would suspect that head movements are also accounted for during navigation. Nonetheless, there are situations in which the eyes can play tricks on the brain, Andersen says. And this research could have several applications in the future for helping humans orient themselves. For example, the knowledge gained could help in the design of better flight instruments for pilots in cases where visual information is misleading.

Also, the research could lead to more realistic flight and driving simulations. And at a more basic level, the research is already providing new insights into the nature of vision and the brain. "The experiments will help us to understand how we perceive and act within the world around us," says Andersen.

The research was funded by the National Eye Institute, the Sloan Center for Theoretical Neurobiology at Caltech, the Office of Naval Research, and the Human Frontier's Scientific Program. Other authors of the paper were David C. Bradley, Marsha Maxwell, and Krishna V. Shenoy, all of Caltech; and Martin S. Banks of the University of California at Berkeley.

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Robert Tindol
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