See this spiral-like shape?
It’s actually rings of concentric circles.
And these lines look like they’re leaning away from each other, but they’re actually
parallel.
These lines look curved, but really, they’re straight.
And the line that looks like it breaks as it goes underneath this rectangle?
That’s one, continuous line.
All of these are optical illusions, images that somehow trick our brains into perceiving
something different from what exists in reality.
These particular ones are called geometrical illusions.
Stuart Anstis: “A geometrical illusion arises when you put a geometrical figure, which could
be a straight line, or square, or circle, onto a background of other, often parallel,
lines.
And we call the background lines inducers if you like, and the front figure we call
the test.
And typically you find the inducers make the test line tilt away from the inducers.”
People have been squinting and staring at optical illusions for centuries, but scientists
still don’t fully understand why certain combinations of shapes and lines mess with
our minds.
What they do know is that these distortions probably come from how our brains interpret
the images, rather than how our eyes see them.
Each eye projects a slightly different view of the world that the brain then combines
to create a 3D perception.
In the middle of the 20th century, neuroscientist Bela Julesz invented a kind of illusion called
the random dot stereogram.
This is similar to the Magic Eye illusions you might remember from the 1990s.
These illusions are made up of two images of random dots.
If you unfocus your eyes just right or use a stereoscope, they come together form a picture.
When Julesz converted geometrical illusions like the Poggendorff illusion into a random
dot stereograms, people could still see the distortions.
That means that the illusion occurs in the brain, once the images have been combined
— not in our eyes.
One explanation has to do with a phenomenon called lateral inhibition.
In the visual cortex there are neurons that specifically respond to lines oriented in
different directions.
Stuart Anstis: “There are individual nerve cells that respond to the vertical and a different
set of nerve cells that respond to the tilted ones — and these tend to inhibit or turn
each other off.
Which means that the perceived direction of the two lines will diverge — so acute angles
will appear to get bigger.”
That could also explain the Hering illusion.
The long red lines seem to bend but they’re actually parallel.
The thing is, lateral inhibition can’t explain every geometrical illusion.
In the Poggendorff illusion, a line passes behind a big vertical rectangle and it looks
like the two ends don’t align — but really, they’re continuous.
If this illusion occurs because lateral inhibition makes the acute angles look larger than they
really are, then the illusion should disappear when you strip it down to just the obtuse
angles.
But you can still see it, and when you strip it down to just the acute angles, the illusion’s
gone.
Another theory is that our brains try to process these 2D images like 3D objects, adding the
illusion of depth and perspective where there is none.
That especially seems to be the case for the Ponzo illusion, Even though the top line looks
longer, it’s the same length as the bottom one.
This could be because of a process called size constancy.
To get a better sense for how this works, take your hand and hold it out at arm’s
length, then bring it to your nose.
Even though your hand takes up more and more of your visual field as you bring it closer
to your face, your brain doesn’t think your hand has grown, it tells you that your hand
is closer.
The same thing could be going on in the image: the two lines take up the same amount of space
in our visual fields, but because the top line looks farther away, our minds interpret
it as larger.
Another explanation could be that our minds judge an object’s size by what’s next
to it.
So because the top line intersects with the lines next to it, and the bottom one is surrounded
with white space, the top line looks longer.
What we do know for sure is that when the 3D world projects 2D information onto our
retinas — which the brain then has to turn back into a 3D perception — sometimes our
minds cut corners.
So you may have noticed that The Verge’s logo is an optical illusion, too!
It’s called an impossible triangle, first created by a Swedish artist in the 1930s.
Then in 1954, a British physicist was so inspired by the king of optical illusions M.C.
Escher, that he made and published this version with his father in a psychology journal, that
we’re most familiar with.