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ScienceWeek
SCIENCEWEEK
August 25, 2006
Vol. 10 - Number 34
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Back issues of ScienceWeek can be searched for subjects, names, terms, etc. at:
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I believe there is no philosophical high-road in science, with epistemological signposts. No, we are in a jungle and find our way by trial and error, building our road behind us as we proceed.
-- Max Born (1882-1970)
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Contents:
1. Astronomy: On Testing Star Formation Theory
Understanding how stars form is one of the outstanding challenges of modern astrophysics. It has become clear that stars form from dense interstellar clouds of gas and dust, called molecular clouds because gas in such clouds is predominantly in molecular rather than atomic form. However, despite substantial progress in recent years, there remain fundamental...
2. Geophysics: On "Supervolcano" Technology
In addition to depicting the ultimate volcano-eruption horror story, the recent Discovery Channel/BBC coproduction "Supervolcano" speculates about what technology will be available to the geophysicist in 2025 to monitor active volcanoes. The result is a fictional Virtual Geophysical Laboratory that, when fed the right data, predicts eruption scenarios...
3. Neuroscience: A New Dimension to Olfaction
In 1991, Linda Buck and Richard Axel reported the seminal discovery of the gene family that encodes odorant receptors in vertebrates. New work describes a second class of chemosensory receptor expressed by olfactory sensory neurons. Adding another dimension to a rapidly progressing field, this unexpected discovery provides new insights into...
4. Neuroscience: On the Recognition of Complex Objects
You look up from your desk and instantly recognize the person that has just walked into your office. Unlike the mathematical proof you were busy constructing, recognizing your colleague does not seem to require any mental effort at all. But the apparent ease of this task belies its computational difficulty, and the ability of the primate brain to recognize...
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Also Noted:
Rational Animals? Susan Hurley and Matthew Nudds, Eds. Oxford University Press, Oxford, 2006. Hardback: 579 pp., illus. $135. ISBN 0198528264. More information at:
http://www.amazon.com/exec/obidos/ASIN/0198528264/scienceweek
The Secret of Scent: Adventures in Perfume and the Science of Smell. Luca Turin Faber and Faber, London, 2006. Hardback: 217 pp., illus. £12.99. ISBN 0571215378. More information at:
http://www.amazon.com/exec/obidos/ASIN/0571215378/scienceweek
Sensuous Seas: Tales of a Marine Biologist. Eugene H. Kaplan Princeton University Press, Princeton, NJ, 2006. Hardback: 285 pp., illus. $24.95, £15.95. ISBN 0691125600. More information at:
http://www.amazon.com/exec/obidos/ASIN/0691125600/scienceweek
The Sperm Cell: Production, Maturation, Fertilization, Regeneration. Christopher J. De Jonge and Christopher L. R. Barratt, Eds. Cambridge University Press, Cambridge, 2006. Hardback: 371 pp., illus. $95. ISBN 0521853974. More information at:
http://www.amazon.com/exec/obidos/ASIN/0521853974/scienceweek
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1. ASTRONOMY: ON TESTING STAR FORMATION THEORY
The following points are made by Richard M. Crutcher (Science 2006 313:771):
1) Understanding how stars form is one of the outstanding challenges of modern astrophysics. It has become clear that stars form from dense interstellar clouds of gas and dust, called molecular clouds because gas in such clouds is predominantly in molecular rather than atomic form. However, despite substantial progress in recent years, there remain fundamental unanswered questions about the basic physics of star formation. In particular, it remains unclear whether molecular clouds undergo rapid gravitational collapse as soon as sufficient matter accumulates to make the clouds gravitationally bound, or whether there is some mechanism resisting collapse that delays the process and introduces new star formation scenarios. New work (1) provides new data regarding this important scientific question.
2) The "standard" model for the formation of low-mass stars such as our Sun has been that interstellar magnetic fields provide support against gravity in dense molecular clouds (2). In this picture, interstellar magnetic fields are "frozen" into interstellar matter by the small fraction of the gas and dust that is ionized. As material accumulates (due to the driving of flows by galactic spiral-arm shocks, supernovae explosions, the gravity of a galaxy, etc.), the magnetic field increases in strength as the gas density increases. After a molecular cloud accumulates sufficient mass to become self-gravitating, it will still not collapse and form stars because gravity is balanced by magnetic pressure.
3) If there were no other forces operating, molecular clouds would persist indefinitely and star formation would not occur. However, magnetic fields are frozen only into the ions of molecular clouds, not into the neutral gas and dust. The neutrals are therefore free to respond to gravity and collapse to form a much denser, gravitationally unstable core to the molecular cloud and eventually to form stars. However, as neutrals collapse through the ionized gas and dust, collisions with ions will occur. These collisions will greatly slow down the collapse rate, leading to molecular cloud lifetimes typically several orders of magnitude longer than the gravitational free-fall lifetime of a cloud.
4) In contrast to magnetically dominated star formation, the other extreme point of view is that magnetic fields are too weak to provide support against gravity. In this model, molecular clouds are intermittent phenomena, and the problem of cloud support for long time periods is irrelevant (3). Supersonic flows in the low-density turbulent interstellar medium produce regions of enhanced density. Star formation does not occur in every location where the gas is dense, but only in small volumes within clouds where sufficient mass accumulates to become self-gravitating. Collapse and star formation then proceed in that small fraction of the total cloud mass at a very rapid, free-fall rate.(4)
5) The new results strongly support the strong magnetic field model of star formation, at least in one region, and provide important new data to astrophysicists working to understand how our Sun and the other stars form.
References:
1. J. M. Girart, R. Rao, D. P. Marrone, Science 313, 812 (2006)
2. A discussion of the standard model is presented by T. C. Mouschovias, G. E. Ciolek, in The Origin of Stars and Planetary Systems, C. J. Lada, N. D. Kylafis, Eds. (Kluwer Academic, Dordrecht, Netherlands, 1999), pp. 305-339
3. Weak-field models are discussed by B. Elmegreen, Astrophys. J. 530, 277 (2000)
4. J. P. Vallee, Astron. J. 123, 382 (2002)
Science http://www.sciencemag.org
ScienceWeek http://scienceweek.com
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2. GEOPHYSICS: ON "SUPERVOLCANO" TECHNOLOGY
The following points are made by Gillian R. Foulger (Science 2006 313:768):
1) In addition to depicting the ultimate volcano-eruption horror story, the recent Discovery Channel/BBC coproduction "Supervolcano" speculates about what technology will be available to the geophysicist in 2025 to monitor active volcanoes. The result is a fictional Virtual Geophysical Laboratory that, when fed the right data, predicts eruption scenarios, thereby providing information to help guide civil emergency-response decisions. New work (1) reports a key step toward realizing such an advanced volcano-monitoring technology.
2) The authors have used time-dependent seismic tomography to study Mount Etna during its pre-eruptive and eruptive phases between August 2001 and January 2003. This method is analogous to CAT (computerized axial tomography) scanning in medical technology, except that earthquakes are used as energy sources and that regions of Earth are the target. In the present case, the region of interest is Mount Etna, a basaltic volcano in Sicily that is ~30 km in diameter and rises to ~3000 m above sea level.
3) The greatest challenge in this type of work is to obtain a sufficiently good earthquake data set. Patanč et al (1) combine data from multiple seismic networks to overcome this difficulty. They observe major changes in the ratio of seismic compressional to shear-wave speed (VP/VS) during the buildup to an eruption and during the eruption itself; these changes correlate closely with observed magma movements (2). Most notably, the authors map regions where VP/VS decreases, and attribute this decrease to the influx of magma that is rich in volatiles (SO2, CO2, and water vapor).
4) This is the first report of time-dependent seismic tomography applied to an erupting volcano. It builds on earlier work of the same kind done in geothermal areas in California and Iceland and the Long Valley Caldera, California. But the seminal example of major changes in VP/VS comes from The Geysers geothermal area in northern California. (3-5)
References (abridged):
1. D. Patanč, G. Barberi, O. Cocina, P. De Gori, C. Chiarabba, Science 313, 821 (2006)
2. The compressional and shear waves are the fastest and second-fastest waves to be radiated from an earthquake source, so they arrive first and second on seismograms. Their ratio provides information about pressure and about the presence of gas and liquid in the study volume. Thus, changes in their ratio can tell us about changes in pressure and gas/liquid, which are thought to accompany the buildup and occurrence of a volcanic eruption.
3. G. R. Foulger, C. C. Grant, A. Ross, B. R. Julian, Geophys. Res. Lett. 24, 135 (1997)
4. R. C. Gunasekera, G. R. Foulger, B. R. Julian, J. Geophys. Res. 108, 2134 (2003)
5. G. R. Foulger, B. R. Julian, Geotherm. Resour. Counc. Bull. 33, 120 (2004)
Science http://www.sciencemag.org
ScienceWeek http://scienceweek.com
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3. NEUROSCIENCE: A NEW DIMENSION TO OLFACTION
The following points are made by John Ngai (Nature 2006 442:637):
1) In 1991, Linda Buck and Richard Axel reported the seminal discovery of the gene family that encodes odorant receptors in vertebrates (1). New work (2) describes a second class of chemosensory receptor expressed by olfactory sensory neurons. Adding another dimension to a rapidly progressing field, this unexpected discovery provides new insights into the molecular and cellular mechanisms used to detect olfactory cues.
2) Since 1991, we have learned a great deal about how the vertebrate olfactory system receives and processes sensory information from myriad chemical cues. The initial sensors are receptors, known as G-protein-coupled receptors (GPCRs), which number more than 1000 in some mammalian species and are instrumental in the detection of tens, if not hundreds, of thousands of volatile odorants. In parallel, the vomeronasal organ -- an anatomical specialization of the nose in terrestrial vertebrates that is distinct from the main olfactory epithelium -- senses non-volatile chemical stimuli, including pheromones (3). Detection of chemosensory stimuli by the vomeronasal organ is thought to be mediated by two other families of GPCRs that function as pheromone receptors (4).
3) Liberles and Buck (2) set out to answer a simple yet important question: can the known family of olfactory receptors account for all the effects and behaviors that are mediated by olfaction? They used a large-scale screening approach, known as "quantitative reverse transcription-polymerase chain reaction" (Q-PCR), to survey mouse olfactory neurons for the expression of other GPCRs identified from the mouse genome. They found that members of a particular group of receptors, the trace amine-associated receptor (TAAR) family (5), are expressed in these neurons. TAARs were originally identified in a search for amine receptors in the brain and show sequence similiarities to receptors for the neurotransmitters serotonin and dopamine. Members of the TAAR family are activated by the trace amines found in the central nervous system (beta-phenylethylamine, tyramine, tryptamine and octopamine), and individual TAARs are expressed in small numbers of cells in the brain. Trace amines have long been suspected to be involved in psychiatric disorders, so TAARs have been postulated to play a role in depression and schizophrenia (5).
4) Liberles and Buck (2) now provide a different perspective on TAAR function. Using Q-PCR, they found that eight out of nine mouse TAAR subtypes are expressed in the olfactory epithelium, whereas none was detected in the brain. Although TAARs might be expressed in too few brain cells to be detected by this assay, at face value these results suggest that a major function of TAARs is the detection of olfactory cues by the nose. The authors also show that messenger RNAs for individual TAARs are expressed in a pattern in the olfactory epithelium that is reminiscent of that displayed by the canonical odorant receptors: individual receptors are sparsely expressed in discrete subdomains of the epithelium, and are not co-expressed with other TAARs. They are also probably not expressed with the odorant receptors. So, based on their expression patterns alone, TAARs seem to fit the bill as chemosensory receptors.
References (abridged):
1. Buck, L. & Axel, R. Cell 65, 175-187 (1991)
2. Liberles, S. D. & Buck, L. B. Nature (2006)
3. Dulac, C. & Torello, A. T. Nature Rev. Neurosci. 4, 551-562 (2003)
4. Mombaerts, P. Nature Rev. Neurosci. 5, 263-278 (2004)
5. Lindemann, L. & Hoener, M. C. Trends Pharmacol. Sci. 26, 274-281 (2005)
Nature http://www.nature.com/nature
ScienceWeek http://scienceweek.com
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4. NEUROSCIENCE: ON THE RECOGNITION OF COMPLEX OBJECTS
The following points are made by James J. DiCarlo (Nature 2006 442:644):
1) You look up from your desk and instantly recognize the person that has just walked into your office. Unlike the mathematical proof you were busy constructing, recognizing your colleague does not seem to require any mental effort at all. But the apparent ease of this task belies its computational difficulty, and the ability of the primate brain to recognize complex objects, such as faces, in complex environments, such as offices, is a pinnacle of evolutionary achievement. Investigating the brain mechanisms underlying object recognition is also very difficult. But new work (1) gives us new traction on the problem.
2) Although we still have only a rudimentary understanding of how the brain accomplishes visual object recognition, the work of many groups has uncovered a hierarchically ordered series of brain areas likely to support this remarkable feat (2). One of the most intriguing observations is that neurons at the highest levels of this series respond strongly when their "preferred" object is in view, often regardless of its exact position, size, contrast, pose, and even background clutter. Creating this neuronal response property is the computational crux of object recognition, as even small populations of these neurons can categorize objects with remarkable speed (3). We know that this neuronal activity is correlated with object perception by the brain (4), and even with imagination of objects (5). Yet there has been no direct evidence that these neurons actually cause subjects to perceive objects. Afraz et al (1) now provide such evidence.
3) In particular, the authors show that artificial activation of groups of high-level visual neurons that prefer one class of object -- faces -- causes subjects to tend to perceive faces, even if a face is not present. Working with rhesus monkeys, a well-studied animal model of human vision, the authors focused on the putative highest visual brain area in this species -- the inferior temporal cortex (IT) -- where neurons respond highly selectively to visually presented faces and other complex objects as outlined above. The researchers used fine microelectrodes to sample the activity of neurons at many locations across IT, and positioned the microelectrode at locations where many neurons responded preferentially to faces -- that is, responded with high activity only when images of faces were shown to the animal. To ask if IT neuronal activity somehow causes perception, the authors injected tiny amounts of electrical current through the microelectrode tip; this method is called microstimulation, and is known to activate neurons within several hundred micrometers.
4) The prevailing, but previously unsubstantiated view is that IT neurons that respond preferentially to an object are somehow responsible for allowing the subject to perceive and report the presence of that object. So artificial activation of face-preferring neurons by microstimulation might, in theory, lead to face perception. But how does one ask a monkey what it perceives? To do this, Afraz et al (1) drew on well-established behavioral methods. They first trained each monkey to look left if a face was presented and to look right if a non-face object was presented. Once the animals had mastered this face-detection task, the authors made the task more challenging by degrading the object images with varying amounts of visual noise. Averaged over many behavioral trials, this procedure provides a sensitive perceptual assay of the monkey's tendency to report "seeing" a face rather than a non-face. Using these methods, the authors showed that, in behavioral trials in which microstimulation was applied to face-preferring neurons, monkeys had a reliably greater tendency to report seeing a face, relative to trials in which no microstimulation was applied.
References (abridged):
1. Afraz, S. -R. , Kiani, R. & Esteky, H. Nature (2006)
2. Rolls, E. T. Neuron 27, 205-218 (2000)
3. Hung, C. P. , Kreiman, G. , Poggio, T. & DiCarlo, J. J. Science 310, 863-866 (2005)
4. Sheinberg, D. L. & Logothetis, N. K. Proc. Natl Acad. Sci. USA 94, 3408-3413 (1997)
5. Kreiman, G. , Koch, C. & Fried, I. Nature 408, 357-361 (2000)
Nature http://www.nature.com/nature
ScienceWeek http://scienceweek.com
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