It may not be X-ray vision, but humans’ ability to see polarised light seems to set us apart from other vertebrates.

The ability to sense this third property of light (beyond colour and brightness) is most commonly associated with and other insects, as well as octopuses and cuttlefish.

But few people realise they possess the skill.

What is polarised light?

Bumblebee
Bees are big into polarised light (Nick Ansell/PA)

Polarised light consists of light waves that are oscillating in a particular direction, much like a skipping rope being shaken up and down or from side-to-side.

Some animals, especially invertebrates, employ polarised light to navigate, find water, detect prey or predators, or for communication.

The new evidence suggests that humans use it too – or at least our ancestors did.

Why do humans need to see polarised light?

Sunrise people
Polarised light was useful for navigating by the sun (Malcolm Foster/AP)

Arguably we don’t any more, but it could have been much more useful for early humans.

An ability to identify the position of the sun in the sky by recognising polarised light might once have helped humans to navigate, the researchers speculated.

This would have been especially useful in northern latitudes where the twilight period, when the sun is below the horizon but the stars are still not visible, is particularly long.

How can we all tap into this super-vision?

Gordon Brown stares at computer screen
“Haidinger’s brushes” – as seen by staring at computer screens (Andrew Winning/PA)

Most of us have to be taught how to tap into our super-vision.

The key is to watch out for Haidinger’s brushes, a subtle short-lived visual effect that occurs when we look at polarised light.

Haidinger’s brushes, named after their 19th century discoverer Wilhelm Karl von Haidinger, appear as a faint pattern of yellow and blue bow-tie shapes that fade after less than five seconds.

Twlight
Looks like a good time to go a-hunting for “Haidinger’s brushes” (Neil Squires/PA)

They can be observed by looking at a region of blue sky approximately 90 degrees from the sun, especially around sunset or sunrise, or at a white region on a liquid crystal display.

“You can see Haidinger’s brushes if you look at a blank white portion of an LCD screen on a computer, tablet or phone,” said Dr Shelby Temple, from the Ecology of Vision Group at the University of Bristol. “Tilt your head from side to side and faint yellow brushes should become visible.”

Although the effect quickly fades, it can be maintained by rotating the eye around the primary visual axis relative to the light field – in other words, by tilting the head.

What did the study look at?

Eye close up
Remember: The lower polarisation sensitivity threshold, the better you are at seeing polarised light (John Stillwell/PA)

Tests carried out on 24 volunteers by the Bristol team found that participants had an average polarisation sensitivity threshold of 56%.

The study used special filters to vary the percentage of polarised light from zero to 100%.

The aim was to test the minimum percentage polarisation at which Haidinger’s brushes could be detected.

Scientists were also able to confirm that some individuals can detect a “flip-flop” Haidinger effect as the polarisation angle is rotated.

How do humans compare to other animals?

Trout
Trout come close, but no cigar… (Jamie Lusch/AP)

Some participants in the study could detect light that was less than 25% polarised. While this performance is not on a par with an invertebrate like a cuttlefish, it’s still better than that of any other vertebrate tested so far.

Studies of polarised light detection in other vertebrates have involved rainbow trout and other fish, and seals.

Rainbow trout were shown to have sensitivity threshold levels of between 63% and 75%. However, experiments using modified LCDs to test polarisation vision in seals and other fishes failed to show any behavioural responses.

It could be that, like humans, such animals detect subtle phenomena that cannot easily be tested, said the researchers.

 

 

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