BBC_Science_Focus_-_08.2019

ROGER BRITTON, LANNER

HOW CAN ONE EYE ALONE

PROVIDE DEPTH PERCEPTION?

One of the main ways our brains perceive depth is by

using a technique called ‘binocular disparity’, which

compares the slight di￿erences in view from each eye to

determine the distance to objects. If you close one eye,

however, you’ll notice that you can still perceive depth.

This is thanks to a number of extra visual cues that our

brains exploit. For example, we know the size of things

from memory, so if an object looks smaller than expected

we know it’s further away. Our brains also understand

perspective, where parallel lines (such as train tracks)

get closer together as they stretch into the distance.

There’s also a ni￿y mechanism called motion parallax:

when we’re moving, objects appear to shi￿by di￿erent

amounts, depending on their distance from us.HG

QESTION OF THE MONTH

It’s thought that snakes lost their legs 100 to 150 million years ago, but

debate is still raging as to whether their limbed ancestors were aquatic or

terrestrial. The evolution of a long, legless body could be beneficial to life

underwater as it would enable eel-like swimming. But it could also be

beneficial on land, making burrowing and hunting underground easier.

Either way, we can still see traces of their legs today: boas and pythons,

the most ancient surviving snakes, have tiny leg bones buried in the

muscles towards their tail. The more advanced snakes, however, have lost

them completely.CC

TOBY GRAHAM, SHREWSBURY

WHY CAN’T I TICKLE MYSELF?

EMAIL YOUR QUESTIONS TO [email protected]

Q&A

It’s all down to the lack of surprise. A 1998 study at University College

London found that, when someone tries to tickle themselves, the brain

reduces the intensity of the sensation by damping down signals in the

somatosensory cortex – the brain region that processes touch. If

someone else tickles us, however, the brain doesn’t know what’s

coming, and this mechanism doesn’t kick in – and we bear the full

brunt of the tickling. This probably evolved so that we are able to

dierentiate between our own touches and those of another animal,

such as a bug that could sting or transmit disease. LV

ROB BANINO, BRISTOL

WHAT EVOLUTIONARY

ADVANTAGE DID SNAKES GAIN BY

LOSING THEIR LEGS?

WINNER

Roger wins a 50th

anniversary edition of the

Apollo 11 Owners’ Workshop

Manual that t akes a new

look at the legacy of Apollo

11, and NASA Moon Missions,

which explores the five

successful lunar landings that

followed Apollo 11. Worth £46

in total. haynes.com

As with so much in finance, the

answer isn’t straightforward.

Economists distinguish between

various types of money, ranging

from individual savings deposits

to the reserves held by central

banks. But taking ‘money’ to be

coins and notes, statistics collated

by the Swiss-based Bank for

International Settlements

suggests there’s over $6,000bn

worth of the stuff out there. This

figure does not, however, include

the vast amount believed to

be lost down the back of the

world’s sofas. RM

ADAM KING, HUDDERSFIELD

HOW MUCH MONEY

IS ESTIMATED TO

EXIST ON EARTH?