Subject: [SciPol 515] ScienceWeek April 6, 2007

From: "Dan Agin" <[email-masked]>
Date: Fri, 06 Apr 2007 09:51:01 -0500

SCIENCEWEEK

April 6, 2007

Vol. 11 - Number 13

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The particle scientist is more or less happy.
He has no home. All his ladders go straight down
and claim the nameless.

-- Anthony Piccione

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Contents (full text below):

1. Neuroscience: Rapid Consolidation

2. Physics: So Small Yet Still Giant

3. Science And Society: Framing Science

4. Neuroscience: Controlling neural circuits with light

5. Obituary: Frank Albert Cotton (1930-2007)

6. Evolution: Unity from conflict

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1.
Science 6 April 2007: Vol. 316. no. 5821, pp. 57 - 58 DOI:
10.1126/science.1141812

Neuroscience: Rapid Consolidation

Larry R. Squire

We learn and remember better when new material can be related to
what we already know. Professional athletes can remember details
of particular plays that occurred in a long match. Experienced
poker players can reconstruct the card distribution and betting
sequence that occurred in previous hands. This is possible
because these individuals have a rich background of relevant
experience and therefore can organize new material into
meaningful and orderly patterns. On page 76 of this issue, Tse et
al. (1) make use of these ideas to explain their new findings in
rats.

In his classic 1932 monograph on remembering (2), British
psychologist Frederic Bartlett developed the concept of "schemas"
to refer to preexisting knowledge structures into which newly
acquired information can be incorporated. Although the schema
concept is fundamental to the psychological science of human
memory, it has been difficult to bring the concept to biology and
especially to studies with experimental animals. Tse et al. show
in rats how the schema concept is relevant to the phenomenon of
memory consolidation. Memory consolidation refers to the gradual
process of reorganization by which new memories become remote
memories (3, 4). Initially, the learning of facts and events
(declarative memory) depends on the hippocampus, a structure deep
in the temporal lobe of the mammalian brain. As time passes after
learning, the importance of the hippocampus gradually diminishes
and a more permanent memory is established in distributed regions
of the neocortex. This process typically takes a few years in
humans and at least a month in rodents. According to one
influential model (5), the process is slow because if changes
were made rapidly, they would interfere with the preexisting
framework of structured knowledge that has been built up from
other experiences.

In the Tse et al. study, rats learned to associate six flavors
with six places in a familiar testing arena. A rat was first cued
with a particular flavor (a 0.5-g morsel of flavored food) in one
of four start boxes and then could receive more of the same food
by going to the correct location in the arena. Animals learned
the six flavor-place associations gradually across 6 weeks of
training; each flavor-place pair was presented once per session
for training and three sessions were scheduled each week. Not
surprisingly, animals with hippocampal lesions failed to learn
the associations.

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2.
Science 6 April 2007: Vol. 316. no. 5821, pp. 63 - 64 DOI:
10.1126/science.1141972

Physics: So Small Yet Still Giant

Igor V. Lerner

Although electronic devices keep shrinking toward the nanometer
scale of atoms, physicists still deal with many-particle systems
in which tracing the paths of individual particles is beyond the
reach of theory and experiment. Because of this, we have to rely
on a statistical approach. Conventional wisdom, inherited from
19th-century statistical physics, says that physical measurements
on a given sample are well described by averaging over an
ensemble of identical samples.

This notion became obsolete more than two decades ago, however,
with the prediction of reproducible conductance fluctuations
(i.e., variations from sample to sample) in "mesoscopic"
structures with dimensions intermediate between atoms and bulk
matter (1, 2). These fluctuations do not decrease with sample
size (as they should in classical physics) but still remain much
smaller than the average conductance. On page 99 of this issue,
Price et al. (3) report the observation of the Coulomb drag (4)
in a bilayer system at very low temperatures where the
reproducible fluctuations of the drag turn out to be much larger
than its average value. Thus, the authors have discovered
mesoscopic fluctuations that, in contrast to the conductance
fluctuations, fully govern the effect rather than give
corrections to it--a very unusual situation in statistical
physics. In carrying out this work, they have developed a new
tool for studying the wave-like behavior of electrons in solids.

Mesoscopic fluctuations exist because quantum mechanics reigns
not only at microscopic scales, as had always been expected, but
at the much larger mesoscopic scale, defined as the scale over
which the phase coherence of electron quantum waves is
maintained. This scale increases as the temperature T decreases,
reaching hundreds of nanometers at T ~ 1 K. For these
temperatures, the wave nature of electrons reveals itself in the
interference between waves going along different paths as a
result of scattering from impurities. This leads to an
irregularly oscillating but reproducible dependence of the sample
conductance on magnetic field or electron concentration.

In metallic materials, these conductance fluctuations are always
very small. Price et al. have made an experimental breakthrough
by measuring the Coulomb drag at a temperature so low that this
effect is suppressed on average and is governed by the
fluctuations. The dominant role of the fluctuations in the
Coulomb drag at very low temperatures was recently predicted
theoretically (5); however, the observed effect turns out to be
four orders of magnitude higher than the prediction. Thus, these
fluctuations can truly be called giant, although they are still
an order of magnitude smaller than the intralayer conductance
fluctuations.

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3.
Science 6 April 2007: Vol. 316. no. 5821, p. 56 DOI:
10.1126/science.1142030

Science And Society: Framing Science

Matthew C. Nisbet1 and Chris Mooney

Issues at the intersection of science and politics, such as
climate change, evolution, and embryonic stem cell research,
receive considerable public attention, which is likely to grow,
especially in the United States as the 2008 presidential election
heats up. Without misrepresenting scientific information on
highly contested issues, scientists must learn to actively
"frame" information to make it relevant to different audiences.
Some in the scientific community have been receptive to this
message (1). However, many scientists retain the well-intentioned
belief that, if laypeople better understood technical
complexities from news coverage, their viewpoints would be more
like scientists', and controversy would subside.

In reality, citizens do not use the news media as scientists
assume. Research shows that people are rarely well enough
informed or motivated to weigh competing ideas and arguments.
Faced with a daily torrent of news, citizens use their value
predispositions (such as political or religious beliefs) as
perceptual screens, selecting news outlets and Web sites whose
outlooks match their own (2). Such screening reduces the choices
of what to pay attention to and accept as valid (3).

Frames organize central ideas, defining a controversy to resonate
with core values and assumptions. Frames pare down complex issues
by giving some aspects greater emphasis. They allow citizens to
rapidly identify why an issue matters, who might be responsible,
and what should be done (4, 5).

Consider global climate change. With its successive assessment
reports summarizing the scientific literature, the United
Nations' Intergovernmental Panel on Climate Change has steadily
increased its confidence that human-induced greenhouse gas
emissions are causing global warming. So if science alone drove
public responses, we would expect increasing public confidence in
the validity of the science, and decreasing political gridlock.

Despite recent media attention, however, many surveys show major
partisan differences on the issue. A Pew survey conducted in
January found that 23% of college-educated Republicans think
global warming is attributable to human activity, compared with
75% of Democrats (6). Regardless of party affiliation, most
Americans rank global warming as less important than over a dozen
other issues (6). Much of this reflects the efforts of political
operatives and some Republican leaders who have emphasized the
frames of either "scientific uncertainty" or "unfair economic
burden" (7). In a counter-strategy, environmentalists and some
Democratic leaders have framed global warming as a "Pandora's
box" of catastrophe; this and news images of polar bears on
shrinking ice floes and hurricane devastation have evoked charges
of "alarmism" and further battles.

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4.
Nature 446, 617-619 (5 April 2007) | doi:10.1038/446617a;
Published online 4 April 2007

Neuroscience: Controlling neural circuits with light

Michael Häusser & Spencer L. Smith

Two light-sensitive proteins from unicellular organisms have been
harnessed to rapidly activate or silence neurons. This optical
remote control allows precise, millisecond control of neural
circuits.

The prospect of direct brain control has long been a dream of
philosophers, science-fiction writers and dictators.
Neuroscientists have also indulged in this dream, because
observation alone is insufficient to find out how the brain
works. Rather, it is necessary to manipulate specific sets of
neurons, stimulating or silencing them, to determine their role
in brain function. A powerful approach for controlling neural
circuits is now described by Zhang and colleagues on page 633 of
this issue1.

The ideal tool for manipulating the activity of neurons would
allow targeting of genes in specific neurons, be innocuous in the
absence of stimulation, be fully reversible, have a rapid onset
and offset, allow bidirectional control, and act at a distance.
Such a tool would allow the experimenter to effortlessly
manipulate and replay natural patterns of activity in neural
circuits.

The traditional approach for controlling neural circuits uses the
trusty stimulation electrode, wielded by generations of
distinguished neurophysiologists from Charles Sherrington to John
Eccles to Bernard Katz. This device generates a local electric
field that is highly effective at triggering action potentials in
neurons - the wave of electrical depolarization, or 'spike', that
ultimately activates the release of neurotransmitter at the
synapses between neurons. Although the timing of electrode
stimulation is very precise, its specificity and spatial control
are poor because all neurons within range are activated.
Furthermore, silencing neural activity is as necessary as
stimulating it when deciphering brain function, as it is useful
to functionally remove cells from a circuit, particularly those
that exhibit substantial spontaneous activity. However, silencing
is difficult to achieve using a stimulation electrode.

An alternative to electrical stimulation has been the use of
chemical activation or inactivation of neurons, by directly
applying excitatory and inhibitory neurotransmitters. Recently,
'photo-uncaging' of caged neurotransmitters has made this
approach more sophisticated because the use of localized,
patterned light yields higher spatial resolution. But the
temporal and spatial resolution remains limited by the properties
of the cage and the diffusion of neurotransmitter, and
specificity cannot be guaranteed. Refinements of this approach,
such as two-photon uncaging2 or restricting receptors for caged
neurotransmitters to specific neurons3, have provided
improvements. Furthermore, new genetic tools for silencing
neurons4, 5, 6 and specific neural connections7 have helped to
address the specificity problem, but at the expense of temporal
resolution.

A pair of light-activated membrane proteins present in nature
have now come to the rescue. Zhang et al.1 - a collaboration
between neuroengineers at Stanford University and membrane
biophysicists at the Max Planck Institute of Biophysics in
Frankfurt - have harnessed the power of these proteins to
genetically engineer neurons to be controlled by light. In
earlier work8 they used a light-activated ion-channel protein
called channelrhodopsin-2 (ChR2) from green algae (Box 1). When
neurons were genetically engineered to express ChR2, blue light
rapidly and selectively triggered action potentials in the
neurons (Fig. 1a). This at last gave neuroscientists a
genetically encodable, remote-control 'on' switch for neurons.

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5.
Nature 446, 626 (5 April 2007) | doi:10.1038/446626a; Published
online 4 April 2007

Obituary: Frank Albert Cotton (1930-2007)

Stephen J. Lippard

Among the greatest thrills for a scientist are those that occur
at the moment of discovery. For a chemist, discovery is often
synonymous with the creation of a new molecule in the laboratory.
But understanding can be an even higher calling: the ability to
recognize and explain properties of matter that others have
previously encountered but could not comprehend. From
understanding, one can derive new principles with which to
interpret, or even predict, future results. Few individuals have
contributed as much to inorganic chemistry through both discovery
and understanding as Frank Albert Cotton. His death on 20
February, at the age of 76, marks the end of an extraordinary and
prolific career.

The signature discovery to emerge from Cotton's laboratory at the
Massachusetts Institute of Technology (MIT) came in the early
1960s. This was a time of renaissance in coordination chemistry -
the study of metal-ligand complexes - and it was fuelled by the
rediscovery of a theoretical model known as crystal field theory.
Cotton and his colleagues applied this theory in their search for
transition-metal complexes with unusual spin states and magnetic
properties. They obtained rhenium compounds of relatively low
oxidation state in which the distances between adjacent metal
atoms were shorter than in the bulk metal itself.

The geometry of the simple [Re2Cl8]2- ion was especially
noteworthy. In this ion, an extremely short metal-metal bond of
just 2.27 Å linked two {ReCl4-} fragments, with no additional
bridging ligand. Moreover, the two halves of the dimer were in an
'eclipsed' configuration, with the chlorine atoms on either side
swept back to a separation of about 3.5 Å to avoid non-bonded
repulsions between them. For some reason, the unit adopted this
geometry rather than avoiding such a steric clash by rotating one
half of the dimer by 45°.

Cotton quickly understood the importance of this result. It meant
that there was a quadruple bond in the [ReRe2Cl8]2- ion, the
first to be identified. In addition to one sigma and two pi bonds
between the rhenium atoms, as occurs in the triple bond between
two nitrogen atoms in the Nequivalent toN molecule, there was an
additional linkage, known as a delta bond. Formation of this bond
required the chlorine atoms to adopt the eclipsed geometry.

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6.
Nature 446, 616 (5 April 2007) | doi:10.1038/446616a; Published
online 4 April 2007

Evolution: Unity from conflict

Paul B. Rainey

Could the evolution of multicellular life have been fuelled by
conflict among selective forces acting at different levels of
organization?

The transition from unicellular to multicellular life brought
with it division of labour, development and a panoply of animal
and plant forms. But despite its pivotal role in the history of
life, the evolution of multicellularity is poorly understood.
Watching the de novo emergence of primordial groups in bacterial
populations in the lab has led me to think about mechanisms by
which higher levels of complexity could emerge.

In multicellular organisms, lower-level entities (cells) have
relinquished their ability to reproduce as independent units and
instead replicate exclusively as part of the larger whole. The
challenge for biologists is to explain how a population of cells
becomes a single entity capable of self-reproduction; that is,
how an evolutionary transition that involves a shift in the level
at which selection operates, from individual entities to groups
of entities, can occur.

Existing theory predicts a central role for cooperation. This
prediction is borne out by experiments using bacterial
populations in which the evolution of simple undifferentiated
groups has been observed. Groups arise from mutant cells that
overproduce a cell-cell glue; the failure of cells to separate at
cell division leads to the formation of a surface-colonizing mat.
The emergence of these mats - which form at the air-liquid
interface, an environment that is replete in oxygen - is a
triumph of selection acting at the higher level. Glue production
is costly to individual cells, but the trait spreads because the
group reaps a reward (oxygen) that is denied to cells within the
broth.

Although the evolutionary emergence of groups is itself a
profound event, such groups are typically short-lived. The reason
is simple: selection continues to act at the level of individual
cells. In so doing, selection favours mutant types that cheat;
that is, cells that no longer produce the glue, but nonetheless
take advantage of the benefit that accrues from being part of the
group. In the absence of any mechanism to repress or punish them,
cheaters prosper - ultimately weakening the fabric of the mat to
the point where it collapses.

Mechanisms that repress cheaters are thus integral to the success
of multicellular organisms, but their evolution is a paradox.
Cheater control is unlikely to evolve in the absence of selection
acting strongly at the group level. The catch-22 is that
selection is powerless to act at the group level because newly
emerged groups are incapable of differential reproduction.
Although individual microbes within the group continue to divide,
groups as a whole are not able to produce offspring; they are,
like soma (the non-reproductive cells of a multicellular
organism), an evolutionary dead-end.

What, then, represents a plausible route to multicellularity?
Cooperation is a necessary innovation but it is not the only one.
Also required is group reproduction and a way to minimize the
disruptive effects of cheats. Moreover, these three major events
need to occur more or less simultaneously.

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