In birds, certain parts of the brain continue to sprout new cells well into
adulthood. Experience can shape the fate of these neurons—chickadees that
don’t exercise their memories, for example, end up with brain regions that lack
their normal share of cells. To trace how a neuron’s fate is moulded by
experience, John Kirn and his colleagues at Wesleyan University in Connecticut
tracked the lives of a group of newly formed neurons in hearing and deaf zebra
finches.

“Most people use one snapshot of time to look at the number of neurons added
as a result of a given experience,” says Kirn. Instead, his team looked at the
time when the new cells arose and also examined the same cells once they had
been integrated into the brain.

The researchers deafened adult songbirds by surgically removing their
cochleas. Then they injected the birds with a radioactive marker that labels
dividing cells so they could trace the insertion of new neurons in a brain
region that controls singing, the hyperstriatum ventrale caudalis (HVC).

A month after the injections, the deafened birds had incorporated 70 per cent
fewer labelled neurons into the HVC than hearing birds. When the researchers
checked three months later, only a third of the labelled neurons in hearing
birds remained, suggesting that these neurons have very short lifespans. But in
the deafened birds, almost all the labelled neurons inserted into the HVC at one
month were still there.

Kirn argues that fewer new neurons int-egrate into the HVCs of deaf birds,
but that those that do live longer. This suggests that the birds suffer a lack
of turnover of neurons in their hearing centre as a result of their deafness. He
speculates that old neurons in the HVC make way for young ones as they die. If
the neurons of deaf birds live longer, their old neurons may bar young ones from
occupying positions in the HVC.

“This is a very novel and important result,” says Eliot Brenowitz, an expert
in the neurobiology of birdsong at the University of Washington. But he believes
Kirn has yet to establish that deafening prolongs the time that neurons survive.
Kirn is now planning to test his hypothesis by tracking neurons labelled before
the birds were deafened. Studies like Kirn’s may help develop new ways to repair
brain damage, as Kirn predicts that harnessing the adult human brain’s potential
for creating new neurons will greatly depend on understanding how neurons are
destined to live or to die.

Source:The Journal of Neuroscience(vol 19, p 10 554)

Almost a fifth of our utterances are “backchannels”—small words that
often serve as information receipts, acknowledging we have heard what another
person has said. Researchers have long suspected that speakers somehow prompt
listeners when to utter backchannels, but what these prompts are has been
unclear.

Researchers at the University of Tokyo, Japan, recorded informal
conversations in both English and Japanese. They then used statistical methods
to fish out patterns of speech that correlated with backchannelling. “We’re
interested in the pacing of conversation,” says the team leader, Nigel Ward. “We
looked for cues that tell one person to speak, or to shut up, or to say
łÜłó-łółÜłó.”

In both languages, the scientists found that listeners tend to backchannel
when speakers lower the pitch of their voice for 0.11 seconds or longer. Using
software built round this rule, the researchers can now talk to their computers
and hear them respond with seemingly natural yeahs and uh-huhs.

David Beaver, a linguistics expert at Stanford University in California, says
that Ward’s discovery of the correlation is an important step forward, but he
thinks it is premature to regard low pitch as a cue. Jean Fox Tree of the
University of California at Santa Cruz agrees. “It’s not just dropping in pitch
that elicits a backchannel,” she speculates. “It’s the meaning behind the drop
in pitch.” Ward, whose work will appear in the Journal of Pragmatics,
concedes he has yet to prove low pitch is itself a cue. But if he can, it could
help explain why we can respond to a speaker with a deceptively opportune
“uh-huh” even while reading the newspaper.

What goes on in one part of a plant often affects other parts. Sunlight
falling on a leaf, for example, can influence the flowering of a distant bud. In
the past, researchers have put these influences down to the movement of small
hormone molecules. But recent studies have hinted that important messages are
also carried by larger molecules—not only proteins but also messenger RNAs
(mRNAs), which carry genetic information.

To investigate this, Roberto Ruiz-Medrano and his team at the University of
California at Davis hunted for mRNAs in the vascular tubes called phloem of
pumpkin plants. They found mRNAs coding for proteins that regulate plant
development, such as one called CmNACP, and other mRNAs of unknown function.

To test whether the mRNAs were moving through the phloem, the scientists
grafted a cutting from a cucumber plant onto the stem of a pumpkin plant.
Cucumbers do not produce CmNACP RNA. So when the team picked up this RNA in the
cucumber cutting, they knew it was travelling in the phloem (
Development, vol 126, p 4405).

Further experiments showed that not all mRNAs move in the same way. Many
never venture into the phloem, others travel only short distances, and still
others seem to trek far into the growing tips of plants. This suggested to
William Lucas, who led the team, that plants use them to coordinate global
responses. Travelling RNAs might switch genes on and off, and choreograph the
activity of cells.

“I think we will probably find some pretty damn novel information-processing
molecules,” says Lucas. “One can’t even begin to imagine how much it may open up
the field of signalling.”

Robert Singer of the Albert Einstein College of Medicine in New York, an
expert in mRNA movement, points out that the real purpose of the mRNA transport
is not yet clear. “The question is: does it really have any significance or
physiological impact?” he asks. “At some point one has to find out what happens
if the RNAs don’t move,” adds plant physiologist Sarah Hake of the University of
California at Berkeley. Lucas and his colleagues are now setting up experiments
to test this.

Whether or not mRNA traffic is essential, the findings may spur biologists to
rethink their ways of studying plant genes. Genes are often assumed to be active
when mRNA is spotted nearby. If mRNAs can romp throughout the plant, this
assumption may not always hold true.

And the results suggest a powerful new tool for manipulating plant genes.
żěè¶ĚĘÓƵs recently discovered that the activity of a gene in one plant cell can
silence genes in distant cells. If, as suspected, the silencer is RNA, then
researchers could hijack the phloem network to silence genes in whole plants.
“This technique is going to sweep through the plant literature—in fact,
it’s already started to do that,” says Lucas. “It’s a whole new revolution.”

Most neuroscientists study vision from the way neurons respond to different
images. This strategy has provided the foundations of our understanding of
perception. But the ultimate test is to apply the opposite approach—to
reconstruct videos of the ouside world from measurements of the neurons’
behaviour.

To do this, Yang Dan and her colleagues from the University of California at
Berkeley anaesthetised cats and used electrodes to measure the activity of 177
neurons in their lateral geniculate nuclei—the first stop in the brain for
incoming signals from the eyes. The cats were shown pictures such as the view
from a camera panning across a forest.

The team reconstructed scenes the cats saw from measurements of neuron
activity, via mathematical equations describing the responses of each cell and
their interactions with other cells. Results from earlier studies suggested 20
to 30 neurons respond to each point in the field of vision. And sure enough, the
reconstructed scenes were clearest when each point was recreated from a similar
number of neurons (The Journal of Neuroscience, vol 19, p 8036).

“The majority of what we know about vision has been determined by looking at
what a single neuron does,” says team leader Garrett Stanley of Harvard
University. “This work looks at groups of neurons and how they might encode
information about the outside world.”

Ehud Kaplan, a neuroscientist at the Mount Sinai School of Medicine in New
York, says the experiment is technically impressive. “They crossed all the t’s
and dotted all the i’s.” He hopes that future studies will shed light on how the
brain makes sense of these scenes. “Our ultimate goal is to comprehend how we
understand the world, how we know that this is grandmother and the other is
Marilyn Monroe.”

Physicists Lev Pokhmelnykh of the Mexican company ElectrificaciĂłn
Artificial de la Atmosfera and Gianfranco Bisiacchi of the National Autonomous
University of Mexico are testing an antenna that is designed to ionise the
atmosphere, creating nuclei on which water vapour can condense. The researchers
expect the air to be warmed by the heat released during condensation, causing
the intensity and direction of air currents to change.

By using an array of antennae to either create or neutralise ions, they hope
to influence local wind patterns and rainfall. Since most of Mexico City lies
within a valley, wind and rain can help to disperse air contaminants that often
reach high concentrations within the confines of the surrounding mountains.

But Sherwood Rowland of the University of California at Irvine, who won a
Nobel prize for his work on atmospheric chemistry, says that natural radioactive
materials continually decay and ionise the air. Within a cubic centimetre of
atmosphere, about 3000 free electrons are generated every second. So any attempt
to alter the natural level of ionisation would require the addition or removal
of an enormous number of electrons.

Dionisio Moreno, a physicist in Mexico’s environment department, says a
government committee evaluating the project—of which he is a
member—remains sceptical. He says they are still unable to detect a clear
change in the intensity or direction of any winds produced by the antenna.
“Preliminary results are insufficient,” he says. More recent results, presented
by Bisiacchi and Pokhmelnykh on 15 January, are still being analysed.