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ScienceWeek
SCIENCE-WEEK
A Weekly Email Digest of the News of Science
A journal devoted to the improvement of communication
between the scientific disciplines, and between scientists,
science educators, and science policy makers.
December 10, 1999 -- Vol. 3 Number 50
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All biologists are elements in a philosophical
paradox: Life measuring itself.
-- Anonymous
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Contents of This Issue:
1. On the Discovery of Electromagnetic Waves
2. Geophysics: Geomagnetic Reversals and the Earth's Mantle
3. A Thermodynamic Model for Predicting Hurricane Intensity
4. Molecular Evolution: Do Proteins Predate DNA?
5. X-Ray Structure of a Calicivirus Capsid
6. Neurotrophic Factors and Long-Distance Axon Development
7. In Brief: Bidi Use Among US Urban Youth 1999
In Focus: On Thinking Machines
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1. ON THE DISCOVERY OF ELECTROMAGNETIC WAVES
Sometime around the mid 1880s, the noted physicist Hermann
Helmholtz (1821-1894) suggested to one of his accomplished former
students that the student compete for a prize being offered by
the Berlin Academy of Science for work in the field of
electromagnetics. James Clerk Maxwell (1831-1879), the chief
theoretical architect of electromagnetics, was no longer on the
scene, and the field was in the doldrums. The former student of
Helmholtz had some interest in Maxwell's theoretical equations,
but not much interest in the Berlin Academy prize. Nevertheless,
he went to work to compete for the prize, and in 1888 the former
student of Helmholtz (the student now Professor of Physics at
Karlsruhe), Heinrich Hertz (1857-1894), presented to the world
the first theoretical prediction and experimental demonstration
of what later came to be called "radio waves". Hertz's
experimental demonstration was essentially as follows: If an
oscillating electrical potential is produced in an appropriate
circuit located at point A, that oscillation is propagated
through space and can be detected by an appropriate independent
electrical circuit located at point B, the two circuits having no
direct electrical connection (e.g., no wires) between them. Hertz
was 31 years old at the time of his presentation; what else he
might have done in physics in a full life was never to be known,
for he died before his 37th birthday of a blood disease. Thus
came into being one of the most important technological advances
of modern times -- the physics underlying radio communications,
microwave radiation, radar, satellite telecommunications, and so
on [*Note #1].
... ... Dominique Pestre (Ecole des Hautes Etudes en Sciences
Sociales Paris, FR) presents a biographical essay on Heinrich
Hertz and the discovery of radio waves and the controversy
surrounding the discovery, the author making the following
points:
1) The European physics community of the 1880s was divided
into two camps concerning the physics of electromagnetics. In one
camp were the British "Maxwellians", followers of James Clerk
Maxwell, who supported the idea of electromagnetic radiation
through an ether; in the other camp were the Continental
physicists such as Ernst Mach (1838-1916) and Hermann Helmholtz,
who viewed electromagnetics as an action-at-a-distance
phenomenon. "The first to seize upon Hertz's publications were,
of course, the British Maxwellians, as they were already
convinced that Maxwell was right. They welcomed the news that the
physics professor at Karlsruhe had found ways to produce
electromagnetic waves, to have them interfere, and to measure
their speed of propagation in air, which he found to be the
predicted 300,000 kilometers per second. They announced the
result everywhere and began mounting public demonstrations of the
marvel of the sparks induced at a distance by a Hertzian
generator."
2) In the following months, many physicists throughout the
Western world examined the issues. Studies of the various
parameters of the phenomenon were made, detectors and generators
constructed all over Europe, and new setups and diverse
interpretations conceived and put forward to explain the
phenomenon and certain apparent problems in the measurement of
wave propagation velocity. Many physicists quickly performed
experiments inspired by Hertz, all were able to generate sparks,
and Hertz came to be considered a true genius. "On the other
hand, most people were finally convinced by their own
experiments, their own devices and calculations, their own way of
adjusting proofs and expectations..."
3) The author concludes by noting the accounts of the
phenomenon in textbooks published 1888 to 1890 by Hertz, J.J.
Thomson (1856-1940), and Henri Poincare (1854-1912), with each
author providing a different proof and interpretation of the
observed phenomenon. "Hertz had made a major discovery, no doubt,
but what he had proved, and who had decisively improved our
understanding of this complex phenomenon, remained a matter of
opinion."
-----------
Dominique Pestre: Spark ignites physicists.
(Nature 21 Oct 99 401:745)
QY: Dominique Pestre, Ecole des Hautes Etudes en Sciences
Sociales Paris, FR.
-----------
Text Notes:
... ... *Note #1: In 1896, the first patent for wireless
telegraphy, i.e., the transmission of messages without wires, was
granted to Gugliemo Marconi in the UK. By the following year, the
Wireless Telegraphy Company had been formed to exploit the
invention, and in 1899, the Marconi Wireless Company of America
was set up. American Marconi, as it was called, soon began the
manufacture of wireless equipment for commercial and military
markets. Although Marconi's original invention (based on Hertz's
radio waves) was designed for fixed (point-to-point) and ship-to-
shore message communication, the idea of wireless as a one-way
medium to transmit speech to many people (i.e., "broadcasting")
was quick to follow. On Christmas Eve 1906, 18 years after the
discovery of radio waves by Hertz, Reginald Fessenden made the
first documented broadcast of speech and music from Brant Rock,
Massachusetts. His transmitter was a 1 kilowatt 50 hertz
alternator built by the General Electric Company. The signal was
received clearly in many locations and even on ships at sea. Lee
de Forest made some experimental broadcasts from New York in 1907
and from Paris in 1910. but his 500 watt transmitters were
inherently noisy. However, in 1906 one of de Forest's associates,
Henry Dunwoody, patented a solid-state detector using the newly
invented material carborundum. These crystal detectors soon
became the heart of early radio receivers. Used with headphones
which drew very little power, crystal sets had the great
advantage of not requiring any external source of electricity. In
summary, within a few decades of the discovery by Hertz of radio
waves, practical applications that would dramatically change
society were in place -- all of it an early part of the ongoing
technological upheaval to be witnessed by the 20th century.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 10Dec99
2. GEOPHYSICS: GEOMAGNETIC REVERSALS AND THE EARTH'S MANTLE
The earliest demonstration that the geomagnetic field of the
Earth changed polarity in the past was provided by P. David and
R. Brunhes, who in 1904-1906 described the magnetic properties of
young lava flows in the Massif Central region of France. They
found that clays baked by the lava flows had the same direction
of *remanent magnetization as the lavas, and that when the
magnetization direction in the lava was opposite to that of the
present-day field, the same was the case in the baked clay. They
interpreted the opposite polarities as evidence that the
geomagnetic field can reverse its polarity.
M. Matuyama (1929) was the first to associate the polarity
of remanent magnetization in lavas with their age as determined
*stratigraphically. Matuyama reported finding young *Quaternary
lavas with magnetization directions close to the present-day
field direction, whereas the directions of older Quaternary and
*Pleistocene lavas were clustered about an *antipodal direction.
He also found that one of three samples of *Miocene *basalt was
magnetized oppositely to the other two. Matuyama's interpretation
was that geomagnetic polarity had changed several times during
the *Late Tertiary time-frame.
Although generally accepted today, the idea that Earth's
geomagnetic polarity could change was controversial in the early
part of this century, and for many years skeptics sought
alternative interpretations of the data. Alternative
explanations, however, have not been successful, and the
phenomenon is now considered real and is studied as a special
branch of geophysics.
Geomagnetic polarity reversal is an inversion of the
*geomagnetic dipole. It is a global event, experienced
simultaneously all over the Earth, and such reversals, apart from
their intrinsic interest, provide a convenient means of
stratigraphic correlations and stratigraphic dating. The
paleomagnetic record indicates that the dipolar part of the
Earth's magnetic field, which is the dominant structure of the
geomagnetic field outside the *core, has reversed its polarity
several hundred times during the past 160 million years. The
reversal durations (i.e., the periods during which the reversals
are accomplished) are relatively short (typically 1000 to 6000
years), compared with the constant polarity intervals between
reversals. Another feature of the reversal period is that the
intensity of the magnetic field apparently decreases
significantly during this time-frame.
Unlike the nearly constant periods of the solar magnetic
cycle, geomagnetic polarity intervals evidently vary from a few
tens of thousands of years to "superchrons" of the order of tens
of millions of years. The duration of a superchron is roughly the
timescale required for significant changes in the thermal
structure of the Earth's *mantle to occur as a result of
*subduction of *tectonic plates and mantle convection, and this
observation and some noted correlations between *plate tectonics,
geomagnetic field intensity, and reversal frequency have led to
speculations that structural changes in the mantle may be
influencing convection and magnetic field generation in the fluid
outer core (the "geodynamo"). In particular, it has been
suggested that changes in both the total heat flow and the
pattern of heat flux over the core-mantle boundary may affect the
geodynamo.
... ... G.A. Glatzmaier et al (4 authors at 3 installations, US)
now present the results of a 3-dimensional numerical computer
simulation of the geodynamo, the work designed to study the
effects of a nonuniform pattern of heat flux over the core-mantle
boundary. The authors used an improved version of a model
developed several years ago (the Glatzmaier-Roberts geodynamo
model), which produced the first dynamically self-consistent
computer simulation of the geodynamo. Essentially, the 3-
dimensional, time-dependent field equations for the
thermodynamics, velocity, and magnetic fields are solved
simultaneously as a system of differential equations, with each
field in effect constantly feeding back to the others. The
authors report the investigation of 8 different patterns of heat
flux from the core to the mantle. The authors suggest their
results indicate the existence of correlations among the
frequency of geomagnetic polarity reversals, the duration over
which the reversals occur, the magnetic-field intensity, and the
long-term geomagnetic variation {secular variation). They report
that of the examined flux patterns, the results of the case with
uniform heat flux at the core-mantle boundary appear most "Earth-
like", and that this suggests that variations in heat flux at the
core-mantle boundary of the Earth are smaller than previously
believed.
-----------
G.A. Glatzmaier et al: The role of the Earth's mantle in
controlling the frequency of geomagnetic reversals.
(Nature 28 Oct 99 401:885)
QY: Gary A. Glatzmaier [glatz@es.ucsc.edu]
-----------
Text Notes:
... ... *remanent magnetization: Remanent magnetism is that
component of a rock's magnetization whose direction is fixed
relative to the rock and which is independent of moderate applied
magnetic fields. The significance of remanent magnetization in
rocks is that the polarity fixation occurs during the cooling of
molten rock or the sedimentation of sedimentary rocks in response
to the magnetic field of the Earth at that particular geological
time. Thus, remanent magnetization of rocks provides a history of
Earth's magnetic field.
... ... *stratigraphically: The term "stratigraphy" refers to the
study of layered sedimentary or metamorphic rocks, especially
their relative ages and the correlations between different areas.
(In general, "sedimentary rock" is any rock formed by the
consolidation of sediment, and "metamorphic rock" is any rock
resulting from partial or complete recrystallization under
temperature and pressure conditions elevated with respect to the
Earth's surface.)
... ... *Quaternary: Approximately the last 2 million years
before the present.
... ... *Pleistocene: A geological epoch with the time-frame 2.5
million years ago to 11,000 years ago.
... ... *antipodal direction: In this context, "antipodes" are
diametrically opposite points on the Earth.
... ... *Miocene: The time-frame 24.6 to 5.1 million years ago.
... ... *basalt: Basalt is a dark gray to black igneous
rock of volcanic origin that cools rapidly. "Igneous rocks" are
rocks that have congealed from a molten mass.
... ... *Late Tertiary: The approximate time-frame 65 million to
2 million years ago.
... ... *geomagnetic dipole: In general, the best mathematical
fit to the observed geomagnetic field using a single dipole
approximation. It is usually taken as the geocentric dipole
field, which is axial and inclined at approximately 11.3 degrees
relative to the Earth's axis of rotation. The "non-dipole" field
is the difference (of the order of 5 percent) between the single-
dipole field approximation and the total planetary field.
... ... *core: Seismic studies indicate the interior of the Earth
consists of three parts: a metallic core, a dense rocky mantle,
and a thin low-density crust. The central part of the core is
solid, but the outer part of the core is evidently liquid. The
mantle, the layer of dense rock and metal oxides between the
molten part of the core and the surface, has plastic properties
(i.e., it is a solid capable of flow under pressure).
... ... *mantle: See previous note.
... ... *subduction: The term "subduction" refers to the
process of underthrusting of the edge of an oceanic *plate into
the mantle underlying an adjacent plate.
... ... *tectonic plates: The term "lithosphere" refers to the
outer layer of the Earth, comprising the crust and upper mantle,
and extending to a depth of 50 to 70 kilometers. The traditional
view of tectonics (changes in the structure of the Earth's crust)
is that the lithosphere consists of a strong brittle layer
overlying a weak ductile layer. "Plate tectonics" is the current
consensus theory that the Earth's lithosphere is broken into
fairly rigid plates, seven major plates and many smaller plates,
and that convection within the underlying less rigid
"asthenosphere" causes the plates (and the associated continents
and crust) to move relative to each other.
... ... *plate tectonics: See previous note.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 10Dec99
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
DATA CONCERNING THE LAST TWO GEOMAGNETIC POLARITY REVERSALS
The Earth's magnetic field has been dynamic throughout its
history, and paleomagnetics is the study of the direction and
intensity of the Earth's magnetic field throughout geological
time. An important phenomenon in this context is remanent
magnetization, that component of a rock's magnetization whose
direction is fixed relative to the rock and which is independent
of moderate applied magnetic fields. The significance of remanent
magnetization in rocks is that the polarity fixation occurs
during the cooling of molten rock or the sedimentation of
sedimentary rocks in response to the magnetic field of the Earth
at that particular geological time. Thus, remanent magnetization
of rocks provides us with a history of Earth's magnetic field --
provided we can gather enough data to make such a history
reasonable. One important aspect revealed by this research is
that the Earth's magnetic field has undergone many polarity
reversals in the past, the record of these in the remanent
magnetizations of rocks formed from cooled lava and rocks formed
from sedimentation. The term "virtual geomagnetic pole" is the
phrase used to describe the Earth's magnetic polarity during a
particular past geologic time frame. J. E. T. Channell and B.
Lehman (University of Florida, US; CNRS-CEA, FR) now report
geomagnetic polarity transition records from North Atlantic drift
deposits with high sedimentation rates, the data from two sites
of the Ocean Drilling Program. Virtual geomagnetic polar paths
(wanderings) plotted from the data are similar within and between
sites for the last two apparent geomagnetic reversals, which
occurred at 780,000 and about 1 million years ago, and confirm
the applicability of such sedimentation records to the study of
polarity reversals. The authors suggest their data may help
resolve the inconsistencies between sediment data and volcanic
data in polarity reversal analysis.
QY: J. Channell [jetc@nervm.nerdc.ufl.edu]
(Nature 16 Oct 97) (Science-Week 14 Nov 97)
3. A THERMODYNAMIC MODEL FOR PREDICTING HURRICANE INTENSITY
The term "hurricane" is used in the Western hemisphere for the
general class of tropical cyclones that includes western Pacific
typhoons and similar systems. The storms are called "cyclones" in
the southern Pacific and Indian ocean. These are all intense
atmospheric vortices that derive their energy from the warm
surface waters of the summertime tropical ocean. Their damaging
winds are concentrated within approximately 100 kilometers of
their centers, just outside the clear central "eye".
... ... H.E. Willoughby (National Oceanic and Atmospheric
Administration, US), in a commentary about a recently proposed
thermodynamic model of hurricanes, makes the following points:
1) The thermodynamics of a hurricane can be modelled as an
idealized *heat engine running between a warm heat reservoir --
the sea, at approximately 300 degrees kelvin -- and a cold
reservoir 15 to 18 kilometers up in the tropical *troposphere at
approximately 200 degrees kelvin. The maximum intensity of a
hurricane (i.e., the strongest wind speed or lowest sea-level
pressure) is proportional to the difference in absolute
temperature between these two reservoirs. But most real
hurricanes are weaker than predicted by models based on pre-
existent sea-surface temperatures.
2) In response to the near disaster caused by Hurricane Opal
in 1995 (a hurricane which suddenly and unexpectedly changed from
a moderate to a dangerous severe storm), a symposium on intensity
changes in tropical cyclones was held at the 1997 meeting of the
American Meteorological Society in Phoenix (US). A consensus
emerged that intensity changes in a hurricane are a response to 3
factors: a) forcing by the surrounding atmosphere; b) the oceanic
heat source; and c) the internal dynamics of the storm. The
difficulty of reliable predictions is attributed to the complex
interplay of these factors. The author suggests that the recent
model of Kerry A. Emanuel (see below), if it stands the test of
forecasting experience, will shift the consensus towards a
dominance of oceanic forcing, and hurricane intensity forecasting
will perhaps become much less difficult.
... ... In the same issue of the journal, Kerry A. Emanuel
(Massachusetts Institute of Technology, US) presents a model
describing thermodynamic control of hurricane intensity, the
author making the following points:
1) To establish useful warning systems for hurricanes, it is
necessary to accurately predict both the hurricane intensity and
the hurricane track. But although forecasting of hurricane tracks
has improved over the past 30 years, the factors that control the
intensity of hurricanes are still poorly understood, leading to
almost no reliability in forecasts of hurricane intensity
evolution. Efforts to improve intensity forecasts have focused
almost exclusively on characterizing the dynamical interactions
between hurricanes and their atmospheric environment.
2) The author reports the use of a simple numerical model to
demonstrate that in most cases the evolution of hurricane
intensity depends mainly on 3 factors: a) the initial intensity
of the storm; b) the thermodynamic state of the atmosphere
through which the storm moves; and c) the heat exchange with the
upper layer of the ocean under the core of the hurricane.
3) The author suggests that such a limited number of
controlling factors "offers hope that, given an accurate forecast
of a hurricane's track, its intensity can be reliably forecast
using very simple models."
-----------
H.E. Willoughby: Hurricane heat engines.
(Nature 14 Oct 99 401:649)
QY: H.E. Willoughby [Hugh.Willoughby@noaa.gov]
-----------
Kerry A. Emanuel: Thermodynamic control of hurricane intensity.
(Nature 14 Oct 99 401:665)
QY: Kerry A. Emanuel [emanuel@texmex.mit.edu]
... ... *heat engine: In general, a "heat engine" is a device
that takes in heat from a hot source and does work, waste heat
being discharged to a colder body (usually the atmosphere). In
most cases, the work done is mechanical work.
... ... *troposphere: The term "troposphere" refers to the lowest
10 to 20 kilometers of the atmosphere (with the lower boundary
the surface of the Earth).
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 10Dec99
4. MOLECULAR EVOLUTION: DO PROTEINS PREDATE DNA?
Reconstruction of the molecular evolution of life on Earth is a
continuing challenge for molecular biologists. The consensus view
is that the first "living" or "quasi-living" systems involved
polymers, in particular RNA, DNA, and proteins, but which
polymers came into existence first and how they evolved in
sequence are central questions still without definitive answers.
... ... S.J. Freeland et al (3 authors at Princeton University,
US) present a commentary on the field, the authors making the
following points:
1) The central dogma of molecular biology -- that DNA makes
RNA, which in turn makes protein -- provokes the question of the
order in which these three fundamental biopolymers arose. The
proposition that RNA came into existence first has achieved wide
popularity, but the concept of a primordial "*RNA world" does not
identify which polymer came next: Was it DNA (a more stable
information storage medium than RNA) or protein (a more versatile
catalyst than RNA)? Consideration of the diverse catalytic
potential of certain RNA oligomers ("*ribozymes") has led some
researchers to suggest that proteins came last, the final
addition to a nucleic acid world.
2) However, new findings are supporting the view of an early
RNA/protein environment with DNA evolving last. Establishing the
correct chronology is important because it defines the context of
genetic code evolution and allows predictions to be made about
the distribution of RNA within the fundamental machinery of life.
3) The authors suggest that parsimony favors the notion that
life first evolved via a single biological macromolecule that
both stored genetic information and catalyzed the reactions
required for self-replication. In contrast, in present biological
systems, proteins are responsible for catalysis and DNA stores
information, with RNA capable of both activities.
3) The early idea that RNA dominated some primordial
biosphere has become mainstream theory as a result of two avenues
of research. First, researchers have produced a diverse
laboratory array of ribozymes that catalyze fundamental metabolic
reactions and bind specific ligands. Second, identification of
apparent "*molecular fossils" in extant metabolic systems
suggests a model in which modern proteins have only incompletely
replaced earlier ribozyme equivalents. DNA probably arose as an
RNA derivative, because all organisms make deoxyribonucleotides
by reducing ribonucleotides, and make *thymine by methylating
*uracil. Protein-first models cannot explain the presence of
functional RNA in extant processes such as *translation.
4) If RNA came first, which came next -- proteins or DNA?
The mainstream view of an RNA/protein biosphere has until now
been based on little hard evidence. Indeed, the very findings
that support an RNA world -- molecular fossils and laboratory-
produced ribozymes -- have prompted speculation that proteins
evolved last, once both types of nucleic acid were present.
5) The authors suggest that the distribution of catalytic
RNA within extant metabolism, together with the known difficult
biochemistry of ribonucleotide reduction, implies that
sophisticated proteins probably predate DNA. Transcription
machinery lacks ribozyme relics simply because ribozymes never
performed this function. This restricts the latest point at which
the genetic code could have arisen, because proteins (i.e.,
translation) must have evolved before DNA. "Yet life probably did
not originate with RNA. Abiotic synthesis of both ribose and
bases is problematic, and linking the two into nucleotides is
more difficult still. Increasing acceptance of an RNA world has
actually stimulated research into plausible models for a pre-RNA
world."
-----------
S.J. Freeland et al: Do proteins predate DNA?
(Science 22 Oct 99 286:690)
QY: Stephen J. Freeland [freeland@rnaworld.princeton.edu]
-----------
Text Notes:
... ... *RNA world: In general, this refers to the concept that
RNA nucleotide sequences with catalytic and self-replicating
capabilities predated catalytic protein systems in prebiological
epochs.
... ... *ribozymes: First discovered in 1981, ribozymes (not to
be confused with riboSOMES) are a small group of RNA molecules
that act as enzymes. They are found in the ciliate protozoan
Tetrahymena, and they are intriguing because they defy the usual
rule that enzymes are proteins.
... ... *molecular fossils: The term "molecular fossils" refers
to RNA fragments that appear in cofactors used in various
metabolic reactions (e.g., coenzyme A, flavin adenine
dinucleotide (FAD), nicotinamide adenine dinucleotide (NAD)).
Since other chemicals could have assumed the functions of these
RNA fragments, their presence is a presumed remnant of the
earlier RNA world.
... ... *thymine: A pyrimidine base, a monomer of DNA but not of
RNA.
... ... *uracil: A pyrimidine base, an integral component of RNA
but not of DNA.
... ... *translation: In this context, translation is
protein synthesis, the process during which polypeptides are
synthesized in accordance with RNA code.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 10Dec99
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
RNA WORLD: RNA-PHOSPHOLIPID MEMBRANE INTERACTIONS
The term "RNA world hypothesis" refers to the concept that RNA
nucleotide sequences possessing catalytic and self-replicating
capabilities predated catalytic protein systems in prebiologic
times. Another assumption of this hypothesis is that RNA was
competent for varied essential functions such as regulation of
membrane permeability. Biological cells communicate with their
environments across phospholipid bilayer membranes that are
relatively permeable to water but virtually impermeable to polar
molecules, and even impermeable to small ions [*Note #1]. In
order for cells to remain viable, their membrane permeability
must be regulated: facilitated transport of certain molecules
through the membrane is required. The RNA world hypothesis
postulates ancestral cells in which RNA played many of the
membrane-regulating roles of modern proteins.
... ... A. Khvorova et al (5 authors at 2 installations, US) now
present a study of the interaction of RNA with phospholipid
membranes, the authors reporting the following:
1) The authors approached the question of RNA involvement in
membrane permeability by first using "selection-amplification" to
isolate RNAs that bind to pure phospholipid membranes. In this
technique, novel RNA activities are isolated by selecting
infrequent molecules from large pools of RNAs with randomized
sequences. Repetitive selection (i.e., repetitive purification)
is made possible by nucleic acid amplification applied to the
partially purified pools, so that large cumulative purifications
are possible after multiple cycles of selection-amplification.
Ultimately, the technique produces useful amounts of single pure
active RNAs for study.
2) Secondly, the authors reintroduced purified membrane-
binding RNAs from selection-amplification into several membrane
systems to measure their effects on permeability, a general
methodology already in use for membrane channel proteins.
3) The authors report their experiments yielded RNAs that
bound *phosphatidylcholine-cholesterol liposomes. At least 8
distinct RNA sequences of approximately 95 residues bound well to
the outside of the lipid bilayer, though randomized sequences had
no such activity. No distinct sequence motif for lipid binding
was found. However, truncation of one such RNA showed that a
smaller 44-nucleotide irregular RNA *hairpin is an active
membrane binding domain. Bound RNA increases the permeability of
liposomes to (sup22)Na(+). In addition, using *voltage clamp
techniques to measure ion currents, 4 individual RNAs increased
the ion permeability of the plasma membrane of cultured human
*epithelial kidney cells.
4) The authors suggest that the existence of multiple RNA
sequences that bind membranes and provoke membrane permeability
changes indicates that these may be elementary RNA functions that
could be selected in vivo. The authors conclude: "RNAs may
partially insert and perturb both leaflets of the membrane or,
alternatively, may interfere with membrane structure by
interacting with the proximal phospholipid leaflet. Whether these
RNAs have structure or function comparable to transmembrane
proteins or use of a mechanism unique to RNA is unknown and a
matter for further experimentation. Nevertheless, even the
present data suggest that RNA might have functioned as
rudimentary [membrane] channels and taken other membrane roles
during cellularization in an RNA world."
-----------
A. Khvorova et al: RNAs that bind and change the permeability of
phospholipid membranes.
(Proc. Natl. Acad. Sci. US 14 Sep 99 96:10649)
QY: Michael Yarus [yarus@stripe.colorado.edu]
-----------
Text Notes:
... ... *Note #1: For further information on cell membranes, see
SW Focus Reports Biological Cell Membranes 1 and 2 at
[www.scienceweek.com/swfr.htm].
... ... *phosphatidylcholine-cholesterol liposomes: The lipids
phosphatidylcholine and cholesterol are ubiquitous in animal cell
membranes. "Liposomes" are laboratory-created vesicles
(spherules) in which the lipid molecules are spontaneously
arranged into bilayers with hydrophilic groups exposed to water
molecules both outside the vesicle and in the core.
... ... *hairpin: In this context, the term "hairpin" refers to a
turning of a polymer chain, the turning having the shape of
hairpin.
... ... *voltage clamp techniques: In general, a "voltage clamp"
technique is a method, involving an electrical feedback circuit,
for holding the membrane potential across a cell membrane
constant at an arbitrary value while measuring responding trans-
membrane currents. The technique can be used to separate the
total current into various ion currents, and with certain
simplifying assumptions concerning the electrical properties of
the membrane, the changes in ion currents are taken as changes in
ion permeabilities.
... ... *epithelial kidney cells: In animals, "epithelial cells"
compose the cell layers that form the interface between a tissue
and the external environment, for example, the cells of the skin,
the lining of the intestinal tract, and the lung airway passages.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 15Oct99
-------------------
Related Background:
RNA-CATALYZED NUCLEOTIDE SYNTHESIS
In research concerning the origin of life on Earth, the "RNA
world" hypothesis proposes that early life developed by making
use of RNA molecules, rather than proteins, to catalyze the
synthesis of important biological molecules. It is believed,
however, that the nucleotides constituting RNA were scarce on
early Earth, so that RNA-based life must have acquired the
ability to synthesize RNA nucleotides from simpler and more
readily available precursors such as sugars and bases. Apparently
plausible prebiotic synthesis routes have been proposed for
sugars, sugar phosphates, and the 4 RNA bases, but the coupling
of these molecules into nucleotides, specifically pyrimidine
nucleotides, poses a challenge to the RNA world hypothesis.
... ... P.J. Unrau and D.P. Bartel report the application of in
vitro selection to isolate RNA molecules that catalyze the
synthesis of a pyrimidine nucleotide at their *3' terminus. The
authors suggest the finding that RNA can catalyze this type of
reaction, which is modeled after pyrimidine synthesis in
contemporary metabolism, supports the idea of an RNA world that
included nucleotide synthesis and other metabolic pathways
mediated by *ribozymes.
-----------
P.J. Unrau and D.P. Bartel (Massachusetts Institute of
Technology, US): RNA-catalyzed nucleotide synthesis.
(Nature 17 Sep 98 395:260)
QY: David P. Bartel, Mass. Inst. of Technology 617-253-1000.
-----------
Text Notes:
... ... *3' terminus: Both DNA and RNA are polymers whose
constituent nucleotides are linked by 3',5'-phosphodiester bonds,
and the polymers have polarity, with one end a 5'-end unattached
and the other end a 3'-end unattached. This asymmetry is mirrored
in the differing functional involvement of the 2 ends in various
biochemical events in the living cell.
... ... *ribozymes: First discovered in 1981, ribozymes (not to
be confused with riboSOMES) are a small group of RNA molecules
that act as enzymes. They are found in the ciliate protozoan
Tetrahymena, and they are intriguing because they defy the usual
rule that enzymes are proteins.
-------------------
Summary & Notes by SCIENCE-WEEK 9Oct98
[For more information: http://scienceweek.com/swfr/search.htm]
5. X=RAY STRUCTURE OF A CALICIVIRUS CAPSID
Excluding *prions, whose categorization is still uncertain,
viruses are the smallest infectious agents, ranging from
approximately 20 to 300 nanometers in diameter, and containing
only one kind of nucleic acid (RNA or DNA) as their genome. In
general, the nucleic acid is encased in a protein shell, which
may be surrounded by a lipid-containing membrane, and the entire
infectious unit is termed a "virion". Viruses are inert in the
extracellular environment, replicating only inside living cells,
and they are essentially genetic-level parasites: the viral
nucleic acid contains information necessary for programming the
infected host cell to synthesize various virus-specific
macromolecules required for the production of viral progeny.
During the replicative cycle of the virus inside the host cell,
numerous copies of viral nucleic acid and viral-coat proteins are
produced, and the coat proteins assemble to form the "capsid".
The capsid encases and stabilizes the viral nucleic acid against
the extracellular environment outside the host cell when the
virus is later released, and also facilitates the attachment to
and penetration into new host cells. In general, a virus
infection may have little or no effect on its host cell, or the
infection may result in cell damage or cell death.
The Norwalk virus is a human pathogenic virus, the cause of
epidemic acute gastroenteritis, and a member of the group of
"caliciviruses". These viruses range in size from 27 to 38
nanometers, the particles appearing with cup-shaped depressions
on the surface. The genome is single-stranded RNA, 7.4 to 7.7
kilobases in size. The Norwalk virus is incorporated within a
shell (capsid) consisting of multiple copies of a single protein
of 56.6 kilodaltons molecular weight, the virion without a lipid
envelope. The Norwalk virus is a difficult pathogen to study
because the virus has not been successfully cultivated in either
tissue culture systems or animals, and only an extremely low
amount of the virus is excreted from infected humans.
... ... B.V.V. Prasad et al (6 author 2 installations, US) now
report the first x-ray structure of the Norwalk virus capsid (and
the first x-ray structure of any calicivirus capsid), the
crystallographic resolution 3.2 to 3.4 angstroms. The authors
report the capsid consists of 180 copies of a single protein,
with a protruding protein domain connected by a flexible hinge to
a shell domain. An identified subdomain of the protruding domain,
located at the exterior of the capsid, apparently has the largest
sequence variation among Norwalk-like human caliciviruses, and
the authors suggest this subdomain is likely to contain the
determinants of viral strain specificity and host-cell binding.
-----------
B.V.V. Prasad et al: X-ray crystallographic structure of the
Norwalk virus capsid.
(Science 8 Oct 99 286:287)
QY: B.V. Venkataram Prasad [vprasad@bcm.tmc.edu]
-----------
Text Notes:
... ... *prions: Prions are a class of poorly understood proteins
implicated in a number of exotic human neurological diseases and
in some common animal diseases such as sheep scrapie and bovine
spongiform encephalopathy in cattle ("mad cow disease").
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 10Dec99
[For more information: http://scienceweek.com/search/search.htm]
6. NEUROTROPHIC FACTORS AND LONG-DISTANCE AXON DEVELOPMENT
One of the major essential challenges of the embryonic phase of
the development of the vertebrate nervous system is the
construction of accurate "hard-wiring" -- the formation of
anatomical connections between various nerve cell groups that
must physiologically work together. What are the mechanisms that
control the directed growth of neuron extensions (axons) to make
contact with target cells, both target cells that are close by
and target cells that are often at relatively enormous distances?
Classic studies in neurobiology have demonstrated that many
neuronal populations in vertebrates are initially generated in
excess numbers during embryonic development, and that excess
neurons are subsequently eliminated by naturally occurring
neuronal cell death. This massive death of excess neurons usually
occurs soon after the axons of these neurons reach their targets,
and appears to result from a dependence of the neurons on
"trophic substances" (*neurotrophic factors) secreted by their
target cells. The trophic substances are present in limited
amounts, and the incoming axons evidently compete for these
limited amounts. This competition is thought to provide a
mechanism for matching the size of the presynaptic neuronal
population to the size of the target population, and has also
been suggested to provide a mechanism for eliminating
misprojecting neurons (axons that have extended to the wrong
location), the misprojecting axons not having access to the
target-derived trophic support. With the identification of many
neurotrophic factors in recent years, evidence has started to
accumulate that neurons that have reached their targets may
receive trophic support not just from their target cells, but
also from other cellular sources such as those located near their
cell bodies or axons. There is also evidence that some
differentiated neurons may have trophic requirements before they
reach their target fields.
... ... H. Wang and M. Tessler-Lavigne (University of California
San Francisco, US) now report that rat spinal commissural
neurons, a group of long-projection neurons in the central
nervous system, in addition to trophic support from final
targets, are also dependent for their survival on trophic support
from one of their intermediate targets, the "*floor plate" of the
spinal cord. The authors report this dependence occurs during a
period of several days when the axons extend along the floor
plate, following which period they develop additional trophic
requirements. The authors suggest that a dependence of neuron
axon growth on trophic support derived "en passant" from their
intermediate targets provides a mechanism for rapidly eliminating
misprojecting neurons, which may help to prevent the formation of
aberrant neuronal circuits during the development of the nervous
system.
-----------
H. Wang and M. Tessler-Lavigne: En passant neurotrophic action of
an intermediate axonal target in the developing mammalian CNS.
(Nature 21 Oct 99 401:765)
QY: Marc Tessler-Lavigne [marctl@itsa.ucsf.edu]
-----------
Text Notes:
... ... *neurotrophic factors: (neurotrophins) These substances
are polypeptides of 200 to 300 amino acids, and a number of
different neurotrophins have been identified.
... ... *floor plate: A small well-defined area at the ventral
margin of the developing spinal cord. (The "ventral margin" is
the margin toward the abdomen.)
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 10Dec99
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
NEURON PATHFINDING AND GROWTH-CONE CALCIUM ION TRANSIENTS
After neurons have differentiated and migrated to their intended
specific destinations, they extend *axons that select connection
targets from an enormous number of possibilities, and eventually
these axons form *synapses with appropriate cells in the target
region. These events depend on a complex of cellular and
molecular signals that guide axons and facilitate correct
connections. The signals involve *cell adhesion molecules that
regulate the interactions between axons and the surfaces upon
which they grow, diffusible molecules that attract growing axons,
and a family of molecules called "*neurotrophins" that promotes
and maintains stable synapses between axons and their targets.
The circuitry of the developing nervous system is thus gradually
constructed by means of such intricate interactions. The local
dynamics of growing axons are now known to involve the properties
of the "growth cone", a specialized structure at the tip of the
extending axon. Growth cones are highly motile structures that
explore the extracellular environment and respond to local cues
by changing the speed or direction of growth. Experiments
indicate that in vitro the intracellular calcium ion
concentration of growth cones is correlated with their motility,
but the links between environmental cues and axon growth in vivo
are unknown. ... ... T.M. Gomez and N.C. Spitzer (University of
California San Diego, US) now report a study of the development
of axonal projections of various types of nerve cells in the
spinal cord of the embryo of the toad (Xenopus). The authors
report that axon growth cones generate transient elevations of
intracellular calcium ion concentration as they migrate within
the embryonic spinal cord, and that the rate of axon outgrowth is
inversely proportional to the frequency of transients.
Suppression of calcium ion transients by photorelease of a
calcium ion chelator accelerates axon extension, whereas
mimicking transients with photorelease of calcium ion slows
otherwise rapid axonal growth. The authors report that the
frequency of calcium ion transients is cell-type specific and
depends on the position of growth cones along their pathway. The
authors further report that growth-cone stalling and axon
retraction, which are two important aspects of pathfinding, are
associated with high frequencies of calcium ion transients. The
authors suggest their results indicate that environmentally
regulated growth-cone calcium transients control axon growth in
the developing spinal cord.
-----------
Editor's note: In addition to the background material below, see
the Neurobiology New Book List at URL:
http://scienceweek.com/bklist8.htm
A number of related SW Focus Reports can be found at URL:
http://scienceweek.com/swfr.htm
-----------
T.M. Gomez and N.C. Spitzer: In vivo regulation of axon extension
and pathfinding by growth-cone calcium transients.
(Nature 28 Jan 99 397:350)
QY: Timothy M. Gomez [tgomez@biomail.ucsd.edu]
-----------
Text Notes:
... ... *axons: In those animals that have nervous systems, one
task of embryological development is to ensure the proper
functional connections between nerve cells and other nerve cells,
and between nerve cells and muscle cells. The innervation must be
exact, in the sense that the growing nerve cell extension (the
axon), which will ultimately serve to propagate information, must
reach a specific and often distant target. In humans, for
example, there are nerve cells whose growing axons reach specific
targets as much as a meter distant from the cell body.
... ... *synapses: The junction between the terminal of a
neuron's axon and another neuron is called a "synapse". When
studying the synapse, the first neuron is called the
"presynaptic" neuron, and the second neuron is called the
"postsynaptic" neuron.
... ... *cell adhesion molecules: In general, cell adhesion is
simply the ability of cells to remain in association with each
other.
... ... *neurotrophins: In general, neurons in the central
nervous system apparently depend for their survival on a number
of secreted substances called neurotrophins (neurotrophic
factors). These substances are polypeptides of 200 to 300 amino
acids, and at least 4 different neurotrophins have been
identified.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 23Apr99
-------------------
Related Background:
ACTIVITY-DEPENDENT CORTICAL TARGET SELECTION BY GROWING AXONS
Many of the connections between nerve cells (the "wiring" of the
nervous system) are programmed in mammals during development.
This is particularly true of the pathway connections of the
sensory systems and the basic motor output systems. During the
wiring of the connections between the *thalamus and *cortex in
mammals, there is an intermediate step in which thalamic axons
grow and interact with a special population of neurons (so-called
subplate neurons) before they contact their ultimate target
neurons within the cerebral cortex (cortical plate). Such
connections in the developing nervous system are thought to be
formed initially by a process not dependent on activity, the axon
pathfinding process and target selection only afterward refined
by neural activity. ... ... Now Catalano and Shatz (University of
California Berkeley, US) report that blockade of *sodium action
potentials by *intracranial infusion of *tetrodotoxin in cats
during the early period when axons from the *lateral geniculate
nucleus were in the process of selecting visual cortex as their
target altered the pattern and precision of this thalamo-cortical
projection. The majority of these neurons, rather than projecting
to visual cortex, elaborated a significant projection within the
subplate of cortical areas normally bypassed. Those axons that
did project to their correct target were *topographically
disorganized. The authors suggest that neural activity is thus
required for initial targeting decisions made by thalamic axons
as they cross the subplate, and that whatever the mechanisms, the
formation of connections between thalamus and *neocortex in
mammals be a special exception to the general rule that target
selection by developing axons is independent of neural activity.
QY: Carla J. Shatz
(Science 24 Jul 98) (Science-Week 14 Aug 98)
-------------------
Related Background:
... ... *thalamus: The thalamus is a deep brain structure that
consists of groups of nerve cells that project to various other
regions of the brain. In general, these groups of nerve cells are
specific relay stations for sensory information (e.g., visual,
auditory, pain, temperature, etc.)
... ... *cortex: The term "cortex" is often used as the short
form for "cerebral cortex", but there are other anatomical
structures also called "cortex", so the meaning is context-
dependent.
... ... *sodium action potentials: In vertebrates, most action
potentials are "sodium action potentials", due to a transient
increase in sodium ion permeability that is propagated down the
axon to the axon terminal(s).
... ... *intracranial infusion: In general, the introduction of
any solution into the brain within the skull.
... ... *tetrodotoxin: A neurotoxin that specifically blocks the
change in sodium ion permeability necessary for the production of
an action potential. Tetrodotoxin acts on sodium ion channels in
the axon membrane.
... ... *lateral geniculate nucleus: A thalamic cell group that
acts as a relay station in the visual pathway from the retina to
the primary visual area of the cerebral cortex.
... ... *topographically disorganized: Ordinarily, the
topographical organization of visual (photon) input to the retina
is for the most part projected isomorphically to the primary
visual area of the brain for analysis. Any topographical
disorganization of information in the pathway or at its terminus
(a disruption of the "mapping") can thus have devastating effects
on the ability of the system to analyze visual inputs.
... ... *neocortex: The most recently evolved part of the
cerebral cortex.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 14Aug98
[For more information: http://scienceweek.com/search/search.htm]
7. IN BRIEF: BIDI USE AMONG US URBAN YOUTH 1999
"Bidis" are small brown hand-rolled cigarettes primarily made in
India and southeast Asian countries, and consisting of tobacco
wrapped in a tendu or temburni leaf (Diospyros melanoxylon). In
the US, bidis are purchased for $1.50 to $4.00 for one package of
20 and are available in different flavors (e.g., cherry,
chocolate, and mango). Bidi use was first observed in the US
during the mid-1990s and is apparently now widespread among
various minority youth groups. Adolescents report a preference
for the taste of bidis over cigarettes and a belief that bidis
are less expensive, easier to buy, and safer than cigarettes. The
US Centers for Disease Control and Prevention (CDC) recently
presented a report on bidi use, the report making the following
points:
1) Preliminary data collected from a sample of adolescents
surveyed during March and early April 1999 in Massachusetts (US)
on the prevalence of bidi use among urban youth indicate that of
642 youth surveyed, 40 percent had smoked bidis at least once
during their lifetimes and 16 percent were current bidi smokers.
2) When tested on a standard smoking machine, bidis produced
higher levels of carbon monoxide, nicotine, and tar than
cigarettes, with one study reporting that bidis produced
approximately 3 times the amount of carbon monoxide and nicotine
and approximately five times the amount of tar as cigarettes.
Because of low combustibility of the tendu leaf wrapper, bidi
smokers inhale more often and more deeply, breathing in greater
quantities of tar and other toxins than cigarette smokers. Like
all tobacco products, bidis are mutagenic and carcinogenic. Bidi
smokers risk coronary heart disease, cancers of the oral cavity,
pharynx, larynx, lung, esophagus, stomach, and liver. Mortality
of both the fetus and newborn infant is also associated with bidi
use during pregnancy.
3) The report states that this investigation is the first in
the US to estimate the prevalence of bidi smoking among students
in grades 7 through 12, and that preliminary findings from this
study support the need for additional research on bidis,
particularly on smoking prevalence among youth from differing
geographic, educational, and socioeconomic backgrounds.
-----------
US Centers for Disease Control and Prevention: Bidi use among
urban youth -- Massachusetts, March-April 1999.
(Morbidity and Mortality Weekly Report 1999 48:796)
(J. Amer. Med. Assoc. 20 Oct 99 282:1416)
-------------------
Summary by SCIENCE-WEEK [http://scienceweek.com] 10Dec99
[For more information: http://scienceweek.com/search/search.htm]
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
IN FOCUS: ON THINKING MACHINES
"Instead of trying to produce a program to simulate the adult
mind, why not rather try to produce one which simulates the
child's? If this were then subjected to an appropriate course of
education, one would obtain the adult brain. Presumably, the
child-brain is something like a notebook as one buys it from the
stationers. Rather little mechanism, and lots of blank sheets.
(Mechanism and writing are from our point of view almost
synonymous.) Our hope is that there is so little mechanism in the
child-brain that something like it can be easily programmed. The
amount of work in the education we can assume, as a first
approximation, to be much the same as for the human child... It
is probably wise to include a random element in a learning
machine. A random element is rather useful when we are searching
for a solution to some problem. Suppose for instance we wanted to
find a number between 50 and 200 which was equal to the square of
the sum of its digits, we might start at 51 then try 52 and go on
until we got a number that worked. Alternatively, we might choose
numbers at random until we got a good one. This method has the
advantage that it is unnecessary to keep track of the values that
have been tried, but the disadvantage that one may try the same
one twice, but this is not very important if there are several
solutions. The systematic method has the disadvantage that there
may be an enormous block without any solutions in the region,
which has to be investigated first. Now the learning process may
be regarded as a search for a form of behavior which will satisfy
the teacher (or some other criterion). Since there is probably a
very large number of satisfactory solutions, the random method
seems to be better than the systematic. It should be noticed that
it is used in the analogous process of evolution. But there the
systematic method is not possible. How could one keep track of
the different genetical combinations that had been tried, so as
to avoid trying them again?"
-----------
Alan Turing: "Can a Machine Think?"
(Mind 1950 vol.59)
-------------------
Related Background:
COMPUTER SCIENCE: ALAN TURING
In 1936, the mathematician Alan M. Turing (1912-1954) proposed
consideration of an abstract computer that subsequently came to
be known as a "Turing machine". Essentially, the simplest Turing
machine system consists of a movable input tape, a black box (the
Turing machine) that reads the tape according to an internal
algorithm, and an output tape that records the output of the
black box. The machine originally considered by Turing was a bit
more complex, with a single input/output tape demarcated into
discrete small sections, and the machine capable of being in any
one of a set of states (determined by the algorithm or "Rule
Set") according to which section of the tape the machine happened
to be reading, and the output capable of moving the tape backward
or forward. As a mathematician, Turing's interest was to
determine the universe of problems capable of being solved by
such a machine, and his ideas have been of considerable influence
in both mathematical and engineering theories of computing
machines. Turing was also interested in several biological
problems, and to a number of mathematically inclined
biologists the Turing machine in its simplest form has been for
many decades an intriguing model for certain computational
processes in the nervous system. More recently, in molecular
biology, the Turing machine has been recognized as an analog of
the behavior of nucleic acid polymerases such as DNA polymerase
and RNA polymerase (enzymes which catalyze the formation of
nucleic acid polymers), which sequentially synthesize an output
polymer (output tape) according to the sequential reading of the
individual units of an input polymer (input tape). Also of
interest to neural systems researchers has been a little-known
paper by Turing on learning behavior of artificial neural
networks not published until 14 years after his death.
... ... B.J. Copeland and D. Proudfoot (University of Canterbury,
NZ) present a biographical essay on Alan Turing, the authors
making the following points:
1) All current digital computers are essentially Turing
machines. Turing also pioneered the field of artificial
intelligence, proposing the widely debated "*Turing test" as a
method of determining whether a suitably programmed computer
exhibits "intelligence" (i.e., can "think"). During World War II,
as part of a British intelligence unit, Turing was instrumental
in breaking the German "Enigma" code, a feat that has been said
to have shortened the war by two years. At the end of his short
life, Turing was engaged in the earliest work on what would now
be called "artificial life", his research involving simulation of
the chemistry of biological growth.
2) Throughout his career, Turing had no great interest in
publicizing his ideas, and as a consequence important aspects of
his work have been neglected or forgotten over the years. In
particular, few people are familiar with Turing's anticipation of
"connectionism" or neuron-like computing. Also neglected are his
groundbreaking theoretical concepts in the area of
"hypercomputation", a field devoted to the computational solution
of apparently intractable problems.
-----------
B.J. Copeland and D. Proudfoot: Alan Turing's forgotten ideas in
computer science.
(Scientific American April 1999)
B.J. Copeland, Dept. of Philosophy, University of Canterbury, NZ.
-----------
Text Notes:
... ... *Turing test: The Turing test is essentially a protocol
for distinguishing between real (human) thought and simulated
(computer) thought. A classic statement of the Turing test is as
follows: One room contains a person and another room contains a
machine. An interrogator in a third room asks questions of both
in an attempt to identify them. When the interrogator cannot
distinguish between them by questioning, the machine can be said
to possess human-like intelligence. [Editor's note: There are
aspects of the test as thus stated which are ill-defined. For
example, there is no operational definition of "intelligence".
Secondly, in terms of procedure, the test is perhaps more useful
when amended as follows: If a determined questioner can find no
question that can enable absolute identification of the machine,
then it can be concluded that, in the frame of reference of the
questioner, the analytical processes of the machine are at least
qualitatively equivalent to the analytical processes of the
human.]
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 9Jul99
[For more information: http://scienceweek.com/search/search.htm]
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