regardless of how many neurons

were in the network (PLoS Computational Biology, DOI:

10.1371/journal.pcbi.0030141).

This implies that to store a

large amount of information,

the brain would have to use

multiple networks. This may be

problematic for something like

vocabulary, Latham says. “You

wouldn’t want to store 100 words

in each of [many] different

networks; you probably want to

store them more or less in one

place. Now we don’t know how

[the brain] does this.”

this model relied on the notion

that each neuron is connected

to every other neuron, whereas

a neuron is actually connected to

between 5000 and 10,000 others.

Neuroscientists then proposed

that the number of memories was

proportional to the number of

connections per neuron. Now

Yasser Roudi and Peter Latham at

University College London have

found a problem with this model

too. They calculated that even

with 10,000 connections per

neuron, a network could only

store about 100 memories –

AN EXOTIC molecule built from

electrons and antimatter is being

touted as a route to powerful

gamma-ray lasers.

An electron can hook up with

its antiparticle, the positron, to

form a hydrogen-like atom called

positronium (Ps). It survives for

less than 150 nanoseconds before

it is annihilated in a puff of

gamma radiation. It was known

that two positronium atoms

should be able to bind together to

form a molecule, called Ps2, and

now David Cassidy and Allen Mills

from the University of California,

Riverside, have made that happen.

First, they trapped positrons in a

thin film of porous silica. Those

positrons captured electrons to

form positronium atoms, and the

pattern of decay rates signalled

that some of these atoms had

teamed up to form Ps2 (Nature,

DOI: 10.1038/nature06094).

If positronium atoms could

be forced to merge into a kind

of “super-atom” condensate, it

would decay in bursts of identical

gamma rays, which could lead to

gamma-ray lasers a million times

more powerful than standard

lasers. “It’s like comparing a

chemical explosion with a

nuclear explosion,” Cassidy says.

Gamma-ray laser with extra punch

PERHAPS Winnie the Pooh knows

something we don’t. Honey could

soon be marketed as a way to combat

the effects of ageing.

Lynne Chepulis and Nicola Starkey

of the University of Waikato in

Hamilton, New Zealand, raised rats

on diets containing 10 per cent honey,

8 per cent sucrose, or no sugar at all

for 12 months. The rats were two

months old at the start of the trial,

and were assessed every three

months using tests designed to

measure anxiety and spatial memory.

Honey-fed rats spent almost twice

as much time in the open sections of

an assessment maze than sucrose-fed

rats, suggesting they were less

anxious. They were also were more

likely to enter novel sections of a

Y-shaped maze, suggesting they knew

where they had been previously and

had better spatial memory.

“Diets sweetened with honey may

be beneficial in decreasing anxiety and

improving memory during ageing,”

says Starkey, whose work was funded

by Fonterra, a dairy company interested

in sweetening yoghurt with honey.

She suggests the findings may be

due to the antioxidant properties of

honey, which have previously been

demonstrated in humans. The results

were presented at the Association for

the Study of Animal Behaviour meeting

at Newcastle University, UK, last week.

IT HAS a mere dribble of water

compared to Earth, yet over the

past few million years Mars has

experienced periodic ice ages that

have shaped layers of ice lurking

beneath its dusty surface.

The ice cycle arises from periodic

fluctuations in its orbit similar to

those that cause Earth’s climate to

vary from ice age to interglacial. There

are important differences, though,

because with no large moon to

stabilise it, the Red Planet’s tilt has

varied far more widely than Earth’s.

As recently as 5 million years ago,

Mars’s axis wobbled between 25 and

45 degrees from the perpendicular

to the plane of the solar system –

enough to evaporate polar ice caps

and precipitate snow on the equator .

It then shifted to a low-tilt mode,

tipping back and forth every 125,000

years in a range from 15 to 35 degrees.

To see how this affected Mars’s

ice, Norbert Schörghofer of the

University of Hawaii modelled its

sublimation and diffusion, starting

5 million years ago with an ice sheet

covered by a layer of 15 per cent dust

and 85 per cent ice. While warm

spells sublime the ice, each of the

40 cold intervals caused diffusion of

moisture into soil, where it freezes.

The model results match the

distribution of ice at mid-latitudes,

suggesting that it diffused there

within the past half-million years. Ice

within about 15 degrees of the poles

is much older, Schörghofer says

(Nature, vol 449, p 192).

HOW do we store so many

memories? It is a question that

has puzzled neuroscientists for

decades – and now it seems that

our concept of how memory

works may have been wrong

all along.

It was originally assumed that

the number of memories was

proportional to the number of

neurons in a network. Given that

even 1 cubic centimetre of the

brain’s cortex contains about

50 million neurons, it seemed

that the brain could indeed store

masses of information. However,

NASA

/JPL

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IA ST

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L/NA

TION

AL G

EOGR

APHI

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Thanks for the memories, but how do you keep them?The ebb and flow of Martian ice

Honey makes life a little bit sweeter

www.newscientist.com 15 September 2007 | NewScientist | 23

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